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Journal manual

Table of Contents

[in package JOURNAL with nicknames JRN]

1 The journal ASDF System

2 Links

Here is the official repository and the HTML documentation for the latest version.

3 Portability

Tested on ABCL, CCL, CLISP, CMUCL, ECL, and SBCL. AllegroCL Express edition runs out of heap while running the tests. On Lisps that seem to lack support for disabling and enabling of interrupts, such as ABCL and CLISP, durability is compromised, and any attempt to SYNC-JOURNAL (see Synchronization strategies and Safety) will be a runtime error.

4 Background

Logging, tracing, testing, and persistence are about what happened during code execution. Recording machine-readable logs and traces can be repurposed for white-box testing. More, when the code is rerun, selected frames may return their recorded values without executing the code, which could serve as a mock object framework for writing tests. This ability to isolate external interactions and to reexecute traces is sufficient to reconstruct the state of a program, achieving simple persistence not unlike a journaling filesystem or event sourcing.

Journal is the library to log, trace, test and persist. It has a single macro at its heart: JOURNALED, which does pretty much what was described. It can be thought of as generating two events around its body: one that records the name and an argument list (as in a function call), and another that records the return values. In Lisp-like pseudocode:

(defmacro journaled (name args &body body)
  `(progn
     (record-event `(:in ,name :args ,args))
     (let ((,return-values (multiple-value-list (progn ,@body))))
       (record-event `(:out ,name :values ,return-values))
       (values-list ,return-values))))

This is basically how recording works. When replaying events from a previous run, the return values of BODY can be checked against the recorded ones, or we may return the recorded values without even running BODY.

In summary, we can produce selective execution traces by wrapping code in JOURNALED and use those traces for various purposes. The Journal library is this idea taken to its logical conclusion.

5 Distinguishing features

As a logging facility

  • Nested contexts and single messages

  • Customizable content and format

  • Human- or machine-readable output

#68200.234: ("some-context")
#68200.234:   Informative log message
#68200.250: => NIL

See Logging for a complete example.

Compared to CL:TRACE

  • Ability to handle non-local exits

  • Customizable content and format

  • Optional timestamps, internal real- and run-time

(FOO 2.1)
  (1+ 2.1)
  => 3.1
=E "SIMPLE-ERROR" "The assertion (INTEGERP 3.1) failed."

See Tracing for a complete example.

As a test framework

  • White-box testing based on execution traces

  • Isolation of external dependencies

  • Record-and-replay testing

(define-file-bundle-test (test-user-registration :directory "registration")
  (let ((username (replayed ("ask-username")
                    (format t "Please type your username: ")
                    (read-line))))
    (add-user username)
    (assert (user-exists-p username))))

See Testing for a complete example.

As a solution for persistence

  • Event Sourcing: replay interactions with the external world

  • Unchanged control flow

  • Easy to implement history, undo

(defun my-resumable-autosaving-game-with-history ()
  (with-bundle (bundle)
    (play-guess-my-number)))

See Persistence for a complete example.

6 Basics

The JOURNALED macro does both recording and replaying of events, possibly at the same time. Recording is easy: events generated by JOURNALED are simply written to a journal, which is a sequence of events much like a file. What events are generated is described in JOURNALED. Replay is much more involved, thus it gets its own section. The journals used for recording and replaying are specified by WITH-JOURNALING or by WITH-BUNDLE.

The Journals reference is presented later, but for most purposes, creating them (e.g. with MAKE-IN-MEMORY-JOURNAL, MAKE-FILE-JOURNAL) and maybe querying their contents with LIST-EVENTS will suffice. Some common cases of journal creation are handled by the convenience function TO-JOURNAL.

Built on top of journals, Bundles juggle repeated replay-and-record cycles focussing on persistence.

  • [generic-function] TO-JOURNAL DESIGNATOR

    Return the journal designated by DESIGNATOR or signal an error. The default implementation:

  • [macro] WITH-JOURNALING (&KEY RECORD REPLAY REPLAY-EOJ-ERROR-P) &BODY BODY

    Turn recording and/or replaying of events on or off for the duration of BODY. Both RECORD and REPLAY should be a JOURNAL designator (in the sense of TO-JOURNAL) or NIL.

    If RECORD designates a JOURNAL, then events generated by enclosed JOURNALED blocks are written to that journal (with exceptions, see the LOG-RECORD argument of JOURNALED). If REPLAY designates a JOURNAL, then the generated events are matched against events from that journal according to the rules of Replay.

    A JOURNAL-ERROR is signalled if RECORD is a JOURNAL that has been previously recorded to by another WITH-JOURNALING (that is, if its JOURNAL-STATE is not :NEW) or if REPLAY is a JOURNAL that is not a complete recording of successful replay (i.e. its JOURNAL-STATE is not :COMPLETED). These checks are intended to catch mistakes that would render the new or existing records unusable for replay. When WITH-JOURNALING finishes, the RECORD journal is marked :COMPLETED or :FAILED in its JOURNAL-STATE.

    REPLAY-EOJ-ERROR-P controls whether END-OF-JOURNAL is signalled when a new event is being matched to the replay journal from which there are no more events to read. If there was a JOURNALING-FAILURE or a REPLAY-FAILURE during execution, then END-OF-JOURNAL is not signalled.

    If BODY completes successfully, but REPLAY has unprocessed events, then REPLAY-INCOMPLETE is signalled.

    WITH-JOURNALING for different RECORD journals can be nested and run independently.

  • [glossary-term] block

    A journaled block, or simply block, is a number of forms wrapped in JOURNALED. When a block is executed, a frame is created.

  • [glossary-term] frame

    A frame is an IN-EVENT, OUT-EVENT pair, which are created when a block is entered and left, respectively.

  • [macro] JOURNALED (NAME &KEY (LOG-RECORD :RECORD) VERSION ARGS VALUES CONDITION INSERTABLE REPLAY-VALUES REPLAY-CONDITION) &BODY BODY

    JOURNALED generates events upon entering and leaving the dynamic extent of BODY (also known as the journaled block), which we call the In-events and Out-events. Between generating the two events, BODY is typically executed normally (except for Replaying the outcome).

    Where the generated events are written is determined by the :RECORD argument of the enclosing WITH-JOURNALING. If there is no enclosing WITH-JOURNALING and LOG-RECORD is NIL, then event recording is turned off and JOURNALED imposes minimal overhead.

    • NAME can be of any type except NULL, not evaluated. For names, and for anything that gets written to a journal, a non-keyword symbol is a reasonable choice as it can be easily made unique. However, it also exposes the package structure, which might make reading stuff back more difficult. Keywords and strings do not have this problem.

    • ARGS can be of any type, but is typically a list.

    Also see :LOG-RECORD in the Logging section. For a description of VERSION, INSERTABLE, REPLAY-VALUES and REPLAY-CONDITION, see Journaled for replay.

6.1 In-events

Upon entering a block, JOURNALED generates an IN-EVENT, which conceptually opens a new frame. These in-events are created from the NAME, VERSION and ARGS arguments of JOURNALED. For example,

(journaled (name :version version :args args) ...)

creates an event like this:

`(:in ,name :version ,version :args ,args)

where :VERSION and :ARGS may be omitted if they are NIL. Versions are used for Replay.

6.2 Out-events

Upon leaving a block, JOURNALED generates an OUT-EVENT, closing the frame opened by the corresponding IN-EVENT. These out-events are property lists like this:

(:out foo :version 1 :values (42))

Their NAME and VERSION (FOO and 1 in the example) are the same as in the in-event: they come from the corresponding arguments of JOURNALED. EXIT and OUTCOME are filled in differently depending on how the block finished its execution.

  • [glossary-term] values outcome

    If the JOURNALED block returns normally, EVENT-EXIT is :VALUES, and the outcome is the list of values returned:

    (journaled (foo) (values 7 t))
;; generates the out-event
(:out foo :values (7 t))

    The list of return values of the block is transformed by the VALUES argument of JOURNALED, whose default is #'IDENTITY. Also see Working with unreadable values).

  • [glossary-term] condition outcome

    If the block unwound due to a condition, and JOURNALED's CONDITION argument (a function whose default is (CONSTANTLY NIL)) returns non-NIL when invoked on it, then EVENT-EXIT is :CONDITION, and the outcome is this return value:

    (journaled (foo :condition (lambda (c) (prin1-to-string c)))
  (error "xxx"))
;; generates the out-event
(:out foo :condition "xxx")

    Conditions thus recognized are those that can be considered part of normal execution. Just like return values, these expected conditions may be required to match what's in the replay journal. Furthermore, given a suitable REPLAY-CONDITION in JOURNALED, they may be replayed without running the block.

  • [glossary-term] error outcome

    If the JOURNALED block unwound due to a condition, but JOURNALED's CONDITION argument returns NIL when invoked on it, then EVENT-EXIT is :ERROR and the outcome the string representations of the type of the condition and the condition itself.

    (journaled (foo)
  (error "xxx"))
;; generates this out-event:
;; (:out foo :error ("simple-error" "xxx"))

    The conversion to string is performed with PRINC in WITH-STANDARD-IO-SYNTAX. This scheme is intended to avoid leaking random implementation details into the journal, which would make READing it back difficult.

    In contrast with condition outcomes, error outcomes are what the code is not prepared to handle or replay in a meaningful way.

  • [glossary-term] nlx outcome

    If the JOURNALED block performed a non-local exit that was not due to a condition, then EVENT-EXIT is :NLX and the outcome is NIL.

    (catch 'xxx
  (journaled (foo)
    (throw 'xxx nil)))
;; generates the out-event
(:out foo :nlx nil)

    Note that condition outcomes and error outcomes are also due to non-local exits but are distinct from nlx outcomes.

    Currently, nlx outcomes are detected rather heuristically as there is no portable way to detect what really caused the unwinding of the stack.

There is a further grouping of outcomes into expected and unexpected.

6.3 Working with unreadable values

The events recorded often need to be readable. This is always required with FILE-JOURNALs, often with IN-MEMORY-JOURNALs, but never with PPRINT-JOURNALs. By choosing an appropriate identifier or string representation of the unreadable object to journal, this is not a problem in practice. JOURNALED provides the VALUES hook for this purpose.

With EXTERNAL-EVENTs, whose outcome is replayed (see Replaying the outcome), we also need to be able to reverse the transformation of VALUES, and this is what the REPLAY-VALUES argument of JOURNALED is for.

Let's see a complete example.

(defclass user ()
  ((id :initarg :id :reader user-id)))

(defmethod print-object ((user user) stream)
  (print-unreadable-object (user stream :type t)
    (format stream "~S" (slot-value user 'id))))

(defvar *users* (make-hash-table))

(defun find-user (id)
  (gethash id *users*))

(defun add-user (id)
  (setf (gethash id *users*) (make-instance 'user :id id)))

(defvar *user7* (add-user 7))

(defun get-message ()
  (replayed (listen :values (values-> #'user-id)
                    :replay-values (values<- #'find-user))
    (values *user7* "hello")))

(jtrace user-id find-user get-message)

(let ((bundle (make-file-bundle "/tmp/user-example/")))
  (format t "Recording")
  (with-bundle (bundle)
    (get-message))
  (format t "~%Replaying")
  (with-bundle (bundle)
    (get-message)))
.. Recording
.. (GET-MESSAGE)
..   (USER-ID #<USER 7>)
..   => 7
.. => #<USER 7>, "hello"
.. Replaying
.. (GET-MESSAGE)
..   (FIND-USER 7)
..   => #<USER 7>, T
.. => #<USER 7>, "hello"
==> #<USER 7>
=> "hello"

To be able to journal the return values of GET-MESSAGE, the USER object must be transformed to something readable. On the Recording run, (VALUES-> #'USER-ID) replaces the user object with its id in the EVENT-OUTCOME recorded, but the original user object is returned.

When Replaying, the journaled OUT-EVENT is replayed (see Replaying the outcome):

(:OUT GET-MESSAGE :VERSION :INFINITY :VALUES (7 "hello"))

The user object is looked up according to :REPLAY-VALUES and is returned along with "hello".

  • [function] VALUES-> &REST FNS

    A utility to create a function suitable as the VALUES argument of JOURNALED. The VALUES function is called with the list of values returned by the block and returns a transformed set of values that may be recorded in a journal. While arbitrary transformations are allowed, VALUES-> handles the common case of transforming individual elements of the list independently by calling the functions in FN with the values of the list of the same position.

    (funcall (values-> #'1+) '(7 :something))
=> (8 :SOMETHING)

    Note how #'1+ is applied only to the first element of the values list. The list of functions is shorter than the values list, so :SOMETHING is not transformed. A value can be left explicitly untransformed by specifying #'IDENTITY or NIL as the function:

    (funcall (values-> #'1+ nil #'symbol-name)
         '(7 :something :another))
=> (8 :SOMETHING "ANOTHER")

  • [function] VALUES<- &REST FNS

    The inverse of VALUES->, this returns a function suitable as the REPLAY-VALUES argument of JOURNALED. It does pretty much what VALUES-> does, but the function returned returns the transformed list as multiple values instead of as a list.

    (funcall (values<- #'1-) '(8 :something))
=> 7
=> :SOMETHING

6.4 Utilities

  • [function] LIST-EVENTS &OPTIONAL (JOURNAL (RECORD-JOURNAL))

    Return a list of all the events in the journal designated by JOURNAL. Calls SYNC-JOURNAL first to make sure that all writes are taken into account.

  • [function] EVENTS-TO-FRAMES EVENTS

    Convert a flat list of events, such as those returned by LIST-EVENTS, to a nested list representing the frames. Each frame is a list of the form (<in-event> <nested-frames>* <out-event>?). Like in PRINT-EVENTS, EVENTS may be a JOURNAL.

    (events-to-frames '((:in foo :args (1 2))
                    (:in bar :args (7))
                    (:leaf "leaf")
                    (:out bar :values (8))
                    (:out foo :values (2))
                    (:in foo :args (3 4))
                    (:in bar :args (8))))
=> (((:IN FOO :ARGS (1 2))
     ((:IN BAR :ARGS (7))
      (:LEAF "leaf")
      (:OUT BAR :VALUES (8)))
     (:OUT FOO :VALUES (2)))
    ((:IN FOO :ARGS (3 4)) ((:IN BAR :ARGS (8)))))

    Note that, as in the above example, incomplete frames (those without an OUT-EVENT) are included in the output.

  • [function] EXPECTED-TYPE TYPE

    Return a function suitable as the CONDITION argument of JOURNALED, which returns the type of its single argument as a string if it is of TYPE, else NIL.

6.5 Pretty-printing

  • [function] PRINT-EVENTS EVENTS &KEY STREAM

    Print EVENTS to STREAM as lists, starting a new line for each event and indenting them according to their nesting structure. EVENTS may be a sequence or a JOURNAL, in which case LIST-EVENTS is called on it first.

    (print-events '((:in log :args ("first arg" 2))
                (:in versioned :version 1 :args (3))
                (:out versioned :version 1 :values (42 t))
                (:out log :condition "a :CONDITION outcome")
                (:in log-2)
                (:out log-2 :nlx nil)
                (:in external :version :infinity)
                (:out external :version :infinity
                 :error ("ERROR" "an :ERROR outcome"))))
..
.. (:IN LOG :ARGS ("first arg" 2))
..   (:IN VERSIONED :VERSION 1 :ARGS (3))
..   (:OUT VERSIONED :VERSION 1 :VALUES (42 T))
.. (:OUT LOG :CONDITION "a :CONDITION outcome")
.. (:IN LOG-2)
.. (:OUT LOG-2 :NLX NIL)
.. (:IN EXTERNAL :VERSION :INFINITY)
.. (:OUT EXTERNAL :VERSION :INFINITY :ERROR ("ERROR" "an :ERROR outcome"))
=> ; No value

  • [function] PPRINT-EVENTS EVENTS &KEY STREAM (PRETTIFIER 'PRETTIFY-EVENT)

    Like PRINT-EVENTS, but produces terser, more human readable output.

    (pprint-events '((:in log :args ("first arg" 2))
                 (:in versioned :version 1 :args (3))
                 (:leaf "This is a leaf, not a frame.")
                 (:out versioned :version 1 :values (42 t))
                 (:out log :condition "a :CONDITION outcome")
                 (:in log-2)
                 (:out log-2 :nlx nil)
                 (:in external :version :infinity)
                 (:out external :version :infinity
                  :error ("ERROR" "an :ERROR outcome"))))
..
.. (LOG "first arg" 2)
..   (VERSIONED 3) v1
..     This is a leaf, not a frame.
..   => 42, T
.. =C "a :CONDITION outcome"
.. (LOG-2)
.. =X
.. (EXTERNAL) ext
.. =E "ERROR" "an :ERROR outcome"
=> ; No value

    The function given as the PRETTIFIER argument formats individual events. The above output was produced with PRETTIFY-EVENT. For a description of PRETTIFIER's arguments see PRETTIFY-EVENT.

  • [function] PRETTIFY-EVENT EVENT DEPTH STREAM

    Write EVENT to STREAM in a somewhat human-friendly format. This is the function PPRINT-JOURNAL, PPRINT-EVENTS, and Tracing use by default. In addition to the basic example in PPRINT-EVENTS, decoration on events is printed before normal, indented output like this:

    (pprint-events '((:leaf "About to sleep" :time "19:57:00" :function "FOO")))
..
.. 19:57:00 FOO: About to sleep

    DEPTH is the nesting level of the EVENT. Top-level events have depth 0. PRETTIFY-EVENT prints indents the output after printing the decorations by 2 spaces per depth.

Instead of collecting events and then printing them, events can be pretty-printed to a stream as they generated. This is accomplished with Pretty-printing journals, discussed in detail later, in the following way:

(let ((journal (make-pprint-journal)))
  (with-journaling (:record journal)
    (journaled (foo) "Hello")))
..
.. (FOO)
.. => "Hello"

Note that Pretty-printing journals are not tied to WITH-JOURNALING and are most often used for Logging and Tracing.

6.6 Error handling

  • [condition] JOURNALING-FAILURE SERIOUS-CONDITION

    Signalled during the dynamic extent of WITH-JOURNALING when an error threatens to leave the journaling mechanism in an inconsistent state. These include I/O errors encountered reading or writing journals by WITH-JOURNALING, JOURNALED, LOGGED, WITH-REPLAY-FILTER, SYNC-JOURNAL, and also STORAGE-CONDITIONs, assertion failures, errors calling JOURNALED's VALUES and CONDITION function arguments. Crucially, this does not apply to non-local exits from other code, such as JOURNALED blocks, whose error handling is largely unaltered (see Out-events and Replay failures).

    In general, any non-local exit from critical parts of the code is turned into a JOURNALING-FAILURE to protect the integrity of the RECORD-JOURNAL. The condition that caused the unwinding is in JOURNALING-FAILURE-EMBEDDED-CONDITION, or NIL if it was a pure non-local exit like THROW. This is a SERIOUS-CONDITION, not to be handled within WITH-JOURNALING.

    After a JOURNALING-FAILURE, the journaling mechanism cannot be trusted anymore. The REPLAY-JOURNAL might have failed a read and be out-of-sync. The RECORD-JOURNAL may have missing events (or even half-written events with FILE-JOURNALs without SYNC, see Synchronization strategies), and further writes to it would risk replayability, which is equivalent to database corruption. Thus, upon signalling JOURNALING-FAILURE, JOURNAL-STATE is set to

    • :COMPLETED if the journal is in state :RECORDING or :LOGGING and the transition to :RECORDING was reflected in storage,

    • else it is set to :FAILED.

    After a JOURNALING-FAILURE, any further attempt within the affected WITH-JOURNALING to use the critical machinery mentioned above (JOURNALED, LOGGED, etc) resignals the same journal failure condition. As a consequence, the record journal cannot be changed, and the only way to recover is to leave WITH-JOURNALING. This does not affect processing in other threads, which by design cannot write to the record journal.

    Note that in contrast with JOURNALING-FAILURE and REPLAY-FAILURE, which necessitate leaving WITH-JOURNALING to recover from, the other conditions – JOURNAL-ERROR, and STREAMLET-ERROR – are subclasses of ERROR as the their handling need not be so heavy-handed.

  • [reader] JOURNALING-FAILURE-EMBEDDED-CONDITION JOURNALING-FAILURE (:EMBEDDED-CONDITION)

  • [condition] RECORD-UNEXPECTED-OUTCOME

    Signalled (with SIGNAL: this is not an ERROR) by JOURNALED when a VERSIONED-EVENT or an EXTERNAL-EVENT had an UNEXPECTED-OUTCOME while in JOURNAL-STATE :RECORDING. Upon signalling this condition, JOURNAL-STATE is set to :LOGGING, thus no more events can be recorded that will affect replay of the journal being recorded. The event that triggered this condition is recorded in state :LOGGING, with its version downgraded. Since Replay (except invoked) is built on the assumption that control flow is deterministic, an unexpected outcome is significant because it makes this assumption to hold unlikely.

    Also see REPLAY-UNEXPECTED-OUTCOME.

  • [condition] DATA-EVENT-LOSSAGE JOURNALING-FAILURE

    Signalled when a data event is about to be recorded in JOURNAL-STATE :MISMATCHED or :LOGGING. Since the data event will not be replayed that constitutes data loss.

7 Logging

Before we get into the details, here is a self-contained example that demonstrates typical use.

(defvar *communication-log* nil)
(defvar *logic-log* nil)
(defvar *logic-log-level* 0)

(defun call-with-connection (port fn)
  (framed (call-with-connection :log-record *communication-log*
                                :args `(,port))
    (funcall fn)))

(defun fetch-data (key)
  (let ((value 42))
    (logged ((and (<= 1 *logic-log-level*) *logic-log*))
      "The value of ~S is ~S." key value)
    value))

(defun init-logging (&key (logic-log-level 1))
  (let* ((stream (open "/tmp/xxx.log"
                       :direction :output
                       :if-does-not-exist :create
                       :if-exists :append))
         (journal (make-pprint-journal
                   :stream (make-broadcast-stream
                            (make-synonym-stream '*standard-output*)
                            stream))))
    (setq *communication-log* journal)
    (setq *logic-log* journal)
    (setq *logic-log-level* logic-log-level)))

(init-logging)

(call-with-connection 8080 (lambda () (fetch-data :foo)))
..
.. (CALL-WITH-CONNECTION 8080)
..   The value of :FOO is 42.
.. => 42
=> 42

(setq *logic-log-level* 0)
(call-with-connection 8080 (lambda () (fetch-data :foo)))
..
.. (CALL-WITH-CONNECTION 8080)
.. => 42
=> 42

(ignore-errors
  (call-with-connection 8080 (lambda () (error "Something unexpected."))))
..
.. (CALL-WITH-CONNECTION 8080)
.. =E "SIMPLE-ERROR" "Something unexpected."
Default to muffling

Imagine a utility library called glib.

(defvar *glib-log* nil)
(defvar *patience* 1)

(defun sl33p (seconds)
  (logged (*glib-log*) "Sleeping for ~As." seconds)
  (sleep (* *patience* seconds)))

Glib follows the recommendation to have a special variable globally bound to NIL by default. The value of *GLIB-LOG* is the journal to which glib log messages will be routed. Since it's NIL, the log messages are muffled, and to record any log message, we need to change its value.

Routing logs to a journal

Let's send the logs to a PPRINT-JOURNAL:

(setq *glib-log* (make-pprint-journal
                  :log-decorator (make-log-decorator :time t)))
(sl33p 0.01)
..
.. 2020-08-31T12:45:23.827172+02:00: Sleeping for 0.01s.

That's a bit too wordy. For this tutorial, let's stick to less verbose output:

(setq *glib-log* (make-pprint-journal))
(sl33p 0.01)
..
.. Sleeping for 0.01s.

To log to a file:

(setq *glib-log* (make-pprint-journal
                  :stream (open "/tmp/glib.log"
                                :direction :output
                                :if-does-not-exist :create
                                :if-exists :append)))
Capturing logs in WITH-JOURNALING's RECORD-JOURNAL

If we were recording a journal for replay and wanted to include glib logs in the journal, we would do something like this:

(with-journaling (:record t)
  (let ((*glib-log* :record))
    (sl33p 0.01)
    (journaled (non-glib-stuff :version 1)))
  (list-events))
=> ((:LEAF "Sleeping for 0.01s.")
    (:IN NON-GLIB-STUFF :VERSION 1)
    (:OUT NON-GLIB-STUFF :VERSION 1 :VALUES (NIL)))

We could even (SETQ *GLIB-LOG* :RECORD) to make it so that glib messages are included by default in the RECORD-JOURNAL. In this example, the special *GLIB-LOG* acts like a log category for all the log messages of the glib library (currently one).

Rerouting a category

Next, we route *GLIB-LOG* to wherever *APP-LOG* is pointing by binding *GLIB-LOG* to the symbol *APP-LOG* (see :LOG-RECORD).

(defvar *app-log* nil)

(let ((*glib-log* '*app-log*))
  (setq *app-log* nil)
  (logged (*glib-log*) "This is not written anywhere.")
  (setq *app-log* (make-pprint-journal :pretty nil))
  (sl33p 0.01))
..
.. (:LEAF "Sleeping for 0.01s.")

Note how pretty-printing was turned off, and we see the LEAF-EVENT generated by LOGGED in its raw plist form.

Conditional routing

Finally, to make routing decisions conditional we need to change SL33P:

(defvar *glib-log-level* 1)

(defun sl33p (seconds)
  (logged ((and (<= 2 *glib-log-level*) *glib-log*))
          "Sleeping for ~As." (* *patience* seconds))
  (sleep seconds))

;;; Check that it all works:
(let ((*glib-log-level* 1)
      (*glib-log* (make-pprint-journal)))
  (format t "~%With log-level ~A" *glib-log-level*)
  (sl33p 0.01)
  (setq *glib-log-level* 2)
  (format t "~%With log-level ~A" *glib-log-level*)
  (sl33p 0.01))
..
.. With log-level 1
.. With log-level 2
.. Sleeping for 0.01s.
Nested log contexts

LOGGED is for single messages. JOURNALED, or in this example FRAMED, can provide nested context:

(defun callv (var value symbol &rest args)
  "Call SYMBOL-FUNCTION of SYMBOL with VAR dynamically bound to VALUE."
  (framed ("glib:callv" :log-record *glib-log*
                        :args `(,var ,value ,symbol ,@args))
    (progv (list var) (list value)
      (apply (symbol-function symbol) args))))

(callv '*print-base* 2 'print 10)
..
.. ("glib:callv" *PRINT-BASE* 2 PRINT 10)
.. 1010 
.. => 10
=> 10

(let ((*glib-log-level* 2))
  (callv '*patience* 7 'sl33p 0.01))
..
.. ("glib:callv" *PATIENCE* 7 SL33P 0.01)
..   Sleeping for 0.07s.
.. => NIL

7.1 Customizing logs

Customizing the output format is possible if we don't necessarily expect to be able to read the logs back programmatically. There is an example in Tracing, which is built on Pretty-printing journals.

Here, we discuss how to make logs more informative.

  • [glossary-term] decoration

    JOURNAL-LOG-DECORATOR adds additional data to LOG-EVENTs as they are written to the journal. This data is called decoration, and it is to capture the context in which the event was triggered. See MAKE-LOG-DECORATOR for a typical example. Decorations, since they can be on LOG-EVENTs only, do not affect Replay. Decorations are most often used with Pretty-printing.

  • [accessor] JOURNAL-LOG-DECORATOR JOURNAL (:LOG-DECORATOR = NIL)

    If non-NIL, this is a function to add decoration to LOG-EVENTs before they are written to a journal. The only allowed transformation is to append a plist to the event, which is a plist itself. The keys can be anything.

  • [function] MAKE-LOG-DECORATOR &KEY TIME REAL-TIME RUN-TIME THREAD DEPTH OUT-NAME

    Return a function suitable as JOURNAL-LOG-DECORATOR that may add a string timestamp, the internal real-time or run-time (both in seconds), the name of the thread, to events, which will be handled by PRETTIFY-EVENT. If DEPTH, then PRETTIFY-EVENT will the nesting level of the event being printed. If OUT-NAME, the PRETTIFY-EVENT will print the name of Out-events.

    All arguments are boolean-valued symbols.

    (funcall (make-log-decorator :depth t :out-name t :thread t
                             :time t :real-time t :run-time t)
         (make-leaf-event :foo))
=> (:LEAF :FOO :DEPTH T :OUT-NAME T :THREAD "worker"
               :TIME "2023-05-26T12:27:44.172614+01:00"
               :REAL-TIME 2531.3254 :RUN-TIME 28.972797)

7.2 :LOG-RECORD

WITH-JOURNALING and WITH-BUNDLE control replaying and recording within their dynamic extent, which is rather a necessity because Replay needs to read the events in the same order as the JOURNALED blocks are being executed. However, LOG-EVENTs do not affect replay, so we can allow more flexibility in routing them.

The LOG-RECORD argument of JOURNALED and LOGGED controls where LOG-EVENTs are written both within WITH-JOURNALING and without. The algorithm to determine the target journal is this:

  1. If LOG-RECORD is :RECORD, then the RECORD-JOURNAL is returned.

  2. If LOG-RECORD is NIL, then it is returned.

  3. If LOG-RECORD is a JOURNAL, then it is returned.

  4. If LOG-RECORD is a symbol (other than NIL), then the SYMBOL-VALUE of that symbol is assigned to LOG-RECORD, and we go to step 1.

If the return value is NIL, then the event will not be written anywhere, else it is written to the journal returned.

This is reminiscent of SYNONYM-STREAMs, also in that it is possible end up in cycles in the resolution. For this reason, the algorithm stop with a JOURNAL-ERROR after 100 iterations.

Interactions

Events may be written to LOG-RECORD even without an enclosing WITH-JOURNALING, and it does not affect the JOURNAL-STATE. However, it is a JOURNAL-ERROR to write to a :COMPLETED journal (see JOURNAL-STATE).

When multiple threads log to the same journal, it is guaranteed that individual events are written atomically, but frames from different threads do not necessarily nest. To keep the log informative, the name of thread may be added to the events as decoration.

Also, see notes on thread Safety.

7.3 Logging with LEAF-EVENTs

  • [macro] LOGGED (&OPTIONAL (LOG-RECORD :RECORD)) FORMAT-CONTROL &REST FORMAT-ARGS

    LOGGED creates a single LEAF-EVENT, whose name is the string constructed by FORMAT. For example:

    (with-journaling (:record t)
  (logged () "Hello, ~A." "world")
  (list-events))
=> ((:LEAF "Hello, world."))

    LEAF-EVENTs are LOG-EVENTs with no separate in- and out-events. They have an EVENT-NAME and no other properties. Use LOGGED for point-in-time textual log messages, and JOURNALED with VERSION NIL (i.e. FRAMED) to provide context.

    Also, see :LOG-RECORD.

8 Tracing

JTRACE behaves similarly to CL:TRACE but deals with non-local exits gracefully.

Basic tracing
(defun foo (x)
  (sleep 0.12)
  (1+ x))

(defun bar (x)
  (foo (+ x 2))
  (error "xxx"))

(jtrace foo bar)

(ignore-errors (bar 1))
..
.. 0: (BAR 1)
..   1: (FOO 3)
..   1: FOO => 4
.. 0: BAR =E "SIMPLE-ERROR" "xxx"
Log-like output

It can also include the name of the originating thread and timestamps in the output:

(let ((*trace-thread* t)
      (*trace-time* t)
      (*trace-depth* nil)
      (*trace-out-name* nil))
  (ignore-errors (bar 1)))
..
.. 2020-09-02T19:58:19.415204+02:00 worker: (BAR 1)
.. 2020-09-02T19:58:19.415547+02:00 worker:   (FOO 3)
.. 2020-09-02T19:58:19.535766+02:00 worker:   => 4
.. 2020-09-02T19:58:19.535908+02:00 worker: =E "SIMPLE-ERROR" "xxx"
Profiler-like output
(let ((*trace-real-time* t)
      (*trace-run-time* t)
      (*trace-depth* nil)
      (*trace-out-name* nil))
  (ignore-errors (bar 1)))
..
.. #16735.736 !68.368: (BAR 1)
.. #16735.736 !68.369:   (FOO 3)
.. #16735.857 !68.369:   => 4
.. #16735.857 !68.369: =E "SIMPLE-ERROR" "xxx"
Customizing the content and the format

If these options are insufficient, the content and the format of the trace can be customized:

(let ((*trace-journal*
       (make-pprint-journal :pretty '*trace-pretty*
                     :prettifier (lambda (event depth stream)
                                   (format stream "~%Depth: ~A, event: ~S"
                                           depth event))
                     :stream (make-synonym-stream '*error-output*)
                     :log-decorator (lambda (event)
                                      (append event '(:custom 7))))))
  (ignore-errors (bar 1)))
..
.. Depth: 0, event: (:IN BAR :ARGS (1) :CUSTOM 7)
.. Depth: 1, event: (:IN FOO :ARGS (3) :CUSTOM 7)
.. Depth: 1, event: (:OUT FOO :VALUES (4) :CUSTOM 7)
.. Depth: 0, event: (:OUT BAR :ERROR ("SIMPLE-ERROR" "xxx") :CUSTOM 7)

In the above, *TRACE-JOURNAL* was bound locally to keep the example from wrecking the global default, but the same effect could be achieved by SETFing PPRINT-JOURNAL-PRETTIFIER, PPRINT-JOURNAL-STREAM and JOURNAL-LOG-DECORATOR.

  • [macro] JTRACE &REST NAMES

    Like CL:TRACE, JTRACE takes a list of symbols. When functions denoted by those NAMES are invoked, their names, arguments and outcomes are printed in human readable form to *TRACE-OUTPUT*. These values may not be readable, JTRACE does not care.

    The format of the output is the same as that of PPRINT-EVENTS. Behind the scenes, JTRACE encapsulates the global functions with NAMES in wrapper that behaves as if FOO in the example above was defined like this:

    (defun foo (x)
  (framed (foo :args `(,x) :log-record *trace-journal*)
    (1+ x)))

    If JTRACE is invoked with no arguments, it returns the list of symbols currently traced.

    On Lisps other than SBCL, where a function encapsulation facility is not available or it is not used by Journal, JTRACE simply sets SYMBOL-FUNCTION. This solution loses the tracing encapsulation when the function is recompiled. On these platforms, (JTRACE) also retraces all functions that should be traced but aren't.

    The main advantage of JTRACE over CL:TRACE is the ability to trace errors, not just normal return values. As it is built on JOURNALED, it can also detect – somewhat heuristically – THROWs and similar.

  • [macro] JUNTRACE &REST NAMES

    Like CL:UNTRACE, JUNTRACE makes it so that the global functions denoted by the symbols NAMES are no longer traced by JTRACE. When invoked with no arguments, it untraces all traced functions.

  • [variable] *TRACE-DEPTH* T

    Controls whether to decorate the trace with the depth of event. See MAKE-LOG-DECORATOR.

  • [variable] *TRACE-THREAD* NIL

    Controls whether to decorate the trace with the name of the originating thread. See MAKE-LOG-DECORATOR.

  • [variable] *TRACE-TIME* NIL

    Controls whether to decorate the trace with a timestamp. See MAKE-LOG-DECORATOR.

  • [variable] *TRACE-REAL-TIME* NIL

    Controls whether to decorate the trace with the internal real-time. See MAKE-LOG-DECORATOR.

  • [variable] *TRACE-RUN-TIME* NIL

    Controls whether to decorate the trace with the internal run-time. See MAKE-LOG-DECORATOR.

8.1 Slime integration

Slime, by default, binds C-c C-t to toggling CL:TRACE. To integrate JTRACE into Slime, load src/mgl-jrn.el into Emacs.

  • If you installed Journal with Quicklisp, the location of mgl-jrn.el may change with updates, and you may want to copy the current version to a stable location:

    (journal:install-journal-elisp "~/quicklisp/")

Then, assuming the Elisp file is in the quicklisp directory, add this to your .emacs:

(load "~/quicklisp/mgl-jrn.el")

Since JTRACE lacks some features of CL:TRACE, most notably that of tracing non-global functions, it is assigned a separate binding, C-c C-j.

  • [function] INSTALL-JOURNAL-ELISP TARGET-DIR

    Copy mgl-jrn.el distributed with this package to TARGET-DIR.

9 Replay

During replay, code is executed normally with special rules for blocks. There are two modes for dealing with blocks: replaying the code and replaying the outcome. When code is replayed, upon entering and leaving a block, the events generated are matched to events read from the journal being replayed. If the events don't match, REPLAY-FAILURE is signalled, which marks the record journal as having failed the replay. This is intended to make sure that the state of the program during the replay matches the state at the time of recording. In the other mode, when the outcome is replayed, a block may not be executed at all, but its recorded outcome is reproduced (i.e. the recorded return values are simply returned).

Replay can be only be initiated with WITH-JOURNALING (or its close kin WITH-BUNDLE). After the per-event processing described below, when WITH-JOURNALING finishes, it might signal REPLAY-INCOMPLETE if there are unprocessed non-log events left in the replay journal.

Replay is deemed successful or failed depending on whether all events are replayed from the replay journal without a REPLAY-FAILURE. A journal that records events from a successful replay can be used in place of the journal that was replayed, and so on. The logic of replacing journals with their successful replays is automated by Bundles. WITH-JOURNALING does not allow replay from journals that were failed replays themselves. The mechanism, in terms of which tracking success and failure of replays is implemented, revolves around JOURNAL-STATE and EVENT-VERSIONs, which we discuss next.

  • [type] JOURNAL-STATE

    JOURNAL's state with respect to replay is updated during WITH-JOURNALING. The possible states are:

    • :NEW: This journal was just created but never recorded to.

    • :REPLAYING: Replaying events has started, some events may have been replayed successfully, but there are more non-log events to replay.

    • :MISMATCHED: There was a REPLAY-FAILURE. In this state, VERSIONED-EVENTs generated are downgraded to LOG-EVENTs, EXTERNAL-EVENTs and invoked trigger DATA-EVENT-LOSSAGE.

    • :RECORDING: All events from the replay journal were successfully replayed, and now new events are being recorded without being matched to the replay journal.

    • :LOGGING: There was a RECORD-UNEXPECTED-OUTCOME. In this state, VERSIONED-EVENTs generated are downgraded to LOG-EVENTs, EXTERNAL-EVENTs and invoked trigger DATA-EVENT-LOSSAGE.

    • :FAILED: The journal is to be discarded. It encountered a JOURNALING-FAILURE or a REPLAY-FAILURE without completing the replay and reaching :RECORDING.

    • :COMPLETED: All events were successfully replayed and WITH-JOURNALING finished or a JOURNALING-FAILURE occurred while :RECORDING or :LOGGING.

    The state transitions are:

      :NEW                -> :REPLAYING  (on entering WITH-JOURNALING)
  :REPLAYING          -> :MISMATCHED (on REPLAY-FAILURE)
  :REPLAYING          -> :FAILED     (on REPLAY-INCOMPLETE)
  :REPLAYING          -> :FAILED     (on JOURNALING-FAILURE)
  :REPLAYING          -> :RECORDING  (on successfully replaying all events)
  :MISMATCHED         -> :FAILED     (on leaving WITH-JOURNALING)
  :RECORDING          -> :LOGGING    (on RECORD-UNEXPECTED-OUTCOME)
  :RECORDING/:LOGGING -> :COMPLETED  (on leaving WITH-JOURNALING)
  :RECORDING/:LOGGING -> :COMPLETED  (on JOURNALING-FAILURE)

    :NEW is the starting state. It is a JOURNAL-ERROR to attempt to write to journals in :COMPLETED. Note that once in :RECORDING, the only possible terminal state is :COMPLETED.

9.1 Journaled for replay

The following arguments of JOURNALED control behaviour under replay.

  • VERSION: see EVENT-VERSION below.

  • INSERTABLE controls whether VERSIONED-EVENTs and EXTERNAL-EVENTs may be replayed with the insert replay strategy (see The replay strategy). Does not affect LOG-EVENTs, which are always _insert_ed. Note that inserting EXTERNAL-EVENTs while :REPLAYING is often not meaningful (e.g. asking the user for input may lead to a REPLAY-FAILURE). See PEEK-REPLAY-EVENT for an example on how to properly insert these kinds of EXTERNAL-EVENTs.

  • REPLAY-VALUES, a function or NIL, may be called with EVENT-OUTCOME when replaying and :VERSION :INFINITY. NIL is equivalent to VALUES-LIST. See VALUES<- for an example.

  • REPLAY-CONDITION, a function or NIL, may be called with EVENT-OUTCOME (the return value of the function provided as :CONDITION) when replaying and :VERSION is :INFINITY. NIL is equivalent to the ERROR function. Replaying conditions is cumbersome and best avoided.

  • [type] EVENT-VERSION

    An event's version is either NIL, a positive FIXNUM, or :INFINITY, which correspond to LOG-EVENTs, VERSIONED-EVENTs, and EXTERNAL-EVENTs, respectively, and have an increasingly strict behaviour with regards to Replay. All EVENTs have versions. The versions of the in- and out-events belonging to the same frame are the same.

  • [type] LOG-EVENT

    Events with EVENT-VERSION NIL called log events. During Replay, they are never matched to events from the replay journal, and log events in the replay do not affect events being recorded either. These properties allow log events to be recorded in arbitrary journals with JOURNALED's LOG-RECORD argument. The convenience macro FRAMED is creating frames of log-events, while the LOGGED generates a log-event that's a LEAF-EVENT.

Built on top of JOURNALED, the macros below record a pair of In-events and Out-events but differ in how they are replayed and the requirements on their blocks. The following table names the type of EVENT produced (Event), how In-events are replayed (In-e.), whether the block is always run (Run), how Out-events are replayed (Out-e.), whether the block must be deterministic (Det) or side-effect free (SEF).

|          | Event     | In-e.  | Run | Out-e. | Det | SEF |
|----------+-----------+--------+-----+--------+-----+-----|
| FRAMED   | log       | skip   | y   | skip   | n   | n   |
| CHECKED  | versioned | match  | y   | match  | y   | n   |
| REPLAYED | external  | match  | n   | replay | n   | y   |
| INVOKED  | versioned | replay | y   | match  | y   | n   |

Note that the replay-replay combination is not implemented because there is nowhere to return values from replay-triggered functions.

  • [macro] FRAMED (NAME &KEY LOG-RECORD ARGS VALUES CONDITION) &BODY BODY

    A wrapper around JOURNALED to produce frames of LOG-EVENTs. That is, VERSION is always NIL, and some irrelevant arguments are omitted. The related LOGGED creates a single LEAF-EVENT.

    With FRAMED, BODY is always run and no REPLAY-FAILUREs are triggered. BODY is not required to be deterministic, and it may have side-effects.

  • [macro] CHECKED (NAME &KEY (VERSION 1) ARGS VALUES CONDITION INSERTABLE) &BODY BODY

    A wrapper around JOURNALED to produce frames of VERSIONED-EVENTs. VERSION defaults to 1. CHECKED is for ensuring that supposedly deterministic processing does not veer off the replay.

    With CHECKED, BODY – which must be deterministic – is always run and REPLAY-FAILUREs are triggered when the events generated do not match the events in the replay journal. BODY may have side-effects.

    For further discussion of determinism, see REPLAYED.

  • [macro] REPLAYED (NAME &KEY ARGS VALUES CONDITION INSERTABLE REPLAY-VALUES REPLAY-CONDITION) &BODY BODY

    A wrapper around JOURNALED to produce frames of EXTERNAL-EVENTs. VERSION is :INFINITY. REPLAYED is for primarily for marking and isolating non-deterministic processing.

    With REPLAYED, the IN-EVENT is checked for consistency with the replay (as with CHECKED), but BODY is not run (assuming it has a recorded expected outcome), and the outcome in the OUT-EVENT is reproduced (see Replaying the outcome). For this scheme to work, REPLAYED requires its BODY to be side-effect free, but it may be non-deterministic.

  • [glossary-term] invoked

    Invoked refers to functions and blocks defined by DEFINE-INVOKED or FLET-INVOKED. Invoked frames may be recorded in response to asynchronous events, and at replay the presence of its in-event triggers the execution of the function associated with the name of the event.

    On the one hand, FRAMED, CHECKED, REPLAYED or plain JOURNALED have In-events that are always predictable from the code and the preceding events. The control flow – on the level of recorded frames – is deterministic in this sense. On the other hand, Invoked encodes in its IN-EVENT what function to call next, introducing non-deterministic control flow.

    By letting events choose the code to run, Invoked resembles typical event sourcing frameworks. When Invoked is used exclusively, the journal becomes a sequence of events. In contrast, JOURNALED and its wrappers put code first, and the journal will be a projection of the call tree.

  • [macro] DEFINE-INVOKED FUNCTION-NAME ARGS (NAME &KEY (VERSION 1) INSERTABLE) &BODY BODY

    DEFINE-INVOKED is intended for recording asynchronous function invocations like event or signal handlers. It defines a function that records VERSIONED-EVENTs with ARGS set to the actual arguments. At replay, it is invoked whenever the recorded IN-EVENT becomes the replay event.

    DEFUN and CHECKED rolled into one, DEFINE-INVOKED defines a top-level function with FUNCTION-NAME and ARGS (only simple positional arguments are allowed) and wraps CHECKED with NAME, the same ARGS and INSERTABLE around BODY. Whenever an IN-EVENT becomes the replay event, and it has a DEFINE-INVOKED defined with the name of the event, FUNCTION-NAME is invoked with EVENT-ARGS.

    While BODY's return values are recorded as usual, the defined function returns no values to make it less likely to affect control flow in a way that's not possible to reproduce when the function is called by the replay mechanism.

    (defvar *state*)

(define-invoked foo (x) ("foo")
  (setq *state* (1+ x)))

(define-invoked bar (x) ("bar")
  (setq *state* (+ 2 x)))

(if (zerop (random 2))
    (foo 0)
    (bar 1))

    The above can be alternatively implemented with REPLAYED explicitly encapsulating the non-determinism:

    (let ((x (replayed (choose) (random 2))))
  (if (zerop x)
      (checked (foo :args `(,x))
        (setq *state* (1+ x)))
      (checked (bar :args `(,x))
        (setq *state* (+ 2 x)))))

  • [macro] FLET-INVOKED DEFINITIONS &BODY BODY

    Like DEFINE-INVOKED, but with FLET instead of DEFUN. The event name and the function are associated in the dynamic extent of BODY. WITH-JOURNALING does not change the bindings. The example in DEFINE-INVOKED can be rewritten as:

    (let ((state nil))
  (flet-invoked ((foo (x) ("foo")
                   (setq state (1+ x)))
                 (bar (x) ("bar")
                   (setq state (+ 2 x))))
    (if (zerop (random 2))
      (foo 0)
      (bar 1))))

9.2 Bundles

Consider replaying the same code repeatedly, hoping to make progress in the processing. Maybe based on the availability of external input, the code may error out. After each run, one has to decide whether to keep the journal just recorded or stick with the replay journal. A typical solution to this would look like this:

(let ((record nil))
  (loop
    (setq record (make-in-memory-journal))
    (with-journaling (:record record :replay replay)
      ...)
    (when (and
           ;; RECORD is a valid replay of REPLAY ...
           (eq (journal-state record) :completed)
           ;; ... and is also significantly different from it ...
           (journal-diverged-p record))
      ;; so use it for future replays.
      (setq replay record))))

This is pretty much what bundles automate. The above becomes:

(let ((bundle (make-in-memory-bundle)))
  (loop
    (with-bundle (bundle)
      ...)))

With FILE-JOURNALs, the motivating example above would be even more complicated, but FILE-BUNDLEs work the same way as IN-MEMORY-BUNDLEs.

  • [macro] WITH-BUNDLE (BUNDLE) &BODY BODY

    This is like WITH-JOURNALING where the REPLAY-JOURNAL is the last successfully completed one in BUNDLE, and the RECORD-JOURNAL is a new one created in BUNDLE. When WITH-BUNDLE finishes, the record journal is in JOURNAL-STATE :FAILED or :COMPLETED.

    To avoid accumulating useless data, the new record is immediately deleted when WITH-BUNDLE finishes if it has not diverged from the replay journal (see JOURNAL-DIVERGENT-P). Because :FAILED journals are always divergent in this sense, they are deleted instead based on whether there is already a previous failed journal in the bundle and the new record is identical to that journal (see IDENTICAL-JOURNALS-P).

    It is a JOURNAL-ERROR to have concurrent or nested WITH-BUNDLEs on the same bundle.

9.3 The replay strategy

The replay process for both In-events and Out-events starts by determining how the generated event (the new event from now on) shall be replayed. Roughly, the decision is based on the NAME and VERSION of the new event and the replay event (the next event to be read from the replay). There are four possible strategies:

  • match: A new in-event must match the replay event in its ARGS. See Matching in-events for details. A new out-event must match the replay event's EXIT and OUTCOME, see Matching out-events.

  • upgrade: The new event is not matched to any replay event, but an event is consumed from the replay journal. This happens if the next new event has the same name as the replay event, but its version is higher.

  • insert: The new event is not matched to any replay event, and no events are consumed from the replay journal, which may be empty. This is always the case for new LOG-EVENTs and when there are no more events to read from the replay journal (unless REPLAY-EOJ-ERROR-P). For VERSIONED-EVENTs, it is affected by setting JOURNALED's INSERTABLE to true (see Journaled for replay).

    The out-event's strategy is always insert if the strategy for the corresponding in-event was insert.

  • Also, END-OF-JOURNAL, REPLAY-NAME-MISMATCH and REPLAY-VERSION-DOWNGRADE may be signalled. See the algorithm below details.

The strategy is determined by the following algorithm, invoked whenever an event is generated by a journaled block:

  1. Log events are not matched to the replay. If the new event is a log event or a REPLAY-FAILURE has been signalled before (i.e. the record journal's JOURNAL-STATE is :MISMATCHED), then insert is returned.

  2. Else, log events to be read in the replay journal are skipped, and the next unread, non-log event is peeked at (without advancing the replay journal).

    • end of replay: If there are no replay events left, then:

      • If REPLAY-EOJ-ERROR-P is NIL in WITH-JOURNALING (the default), insert is returned.

      • If REPLAY-EOJ-ERROR-P is true, then END-OF-JOURNAL is signalled.

    • mismatched name: Else, if the next unread replay event's name is not EQUAL to the name of the new event, then:

      • For VERSIONED-EVENTs, REPLAY-NAME-MISMATCH is signalled if INSERTABLE is NIL, else insert is returned.

      • For EXTERNAL-EVENTs, REPLAY-NAME-MISMATCH is signalled.

    • matching name: Else, if the name of the next unread event in the replay journal is EQUAL to the name of new event, then it is chosen as the replay event.

      • If the replay event's version is higher than the new event's version, then REPLAY-VERSION-DOWNGRADE is signalled.

      • If the two versions are equal, then match is returned.

      • If the new event's version is higher, then upgrade is returned.

      Where :INFINITY is considered higher than any integer and equal to itself.

In summary:

 | new event | end-of-replay     | mismatched name   | matching name |
 |-----------+-------------------+-------------------+---------------|
 | Log       | insert            | insert            | insert        |
 | Versioned | insert/eoj-error  | insert/name-error | match-version |
 | External  | insert/eoj-error  | insert/name-error | match-version |

Version matching (match-version above) is based on which event has a higher version:

 | replay event    | =     | new event |
 |-----------------+-------+-----------|
 | downgrade-error | match | upgrade   |

  • [glossary-term] replay event

    The replay event is the next event to be read from REPLAY-JOURNAL which is not to be skipped. There may be no replay event if there are no more unread events in the replay journal.

    An event in the replay journal is skipped if it is a LOG-EVENT or there is a WITH-REPLAY-FILTER with a matching :SKIP. If :SKIP is in effect, the replay event may be indeterminate.

    Events from the replay journal are read when they are :MATCHed or :UPGRADEd (see The replay strategy), when nested events are echoed while Replaying the outcome, or when there is an invoked defined with the same name as the replay event.

    The replay event is available via PEEK-REPLAY-EVENT.

9.4 Matching in-events

If the replay strategy is match, then, for in-events, the matching process continues like this:

9.4.1 Replaying the outcome

So, if an in-event is triggered that matches the replay, EVENT-VERSION(0 1) is :INFINITY, then normal execution is altered in the following manner:

  • The journaled block is not executed.

  • To keep execution and the replay journal in sync, events of frames nested in the current one are skipped over in the replay journal.

  • All events (including LOG-EVENTs) skipped over are echoed to the record journal. This serves to keep a trail of what happened during the original recording. Note that functions corresponding to invoked frames are called when their IN-EVENT is skipped over.

  • The out-event corresponding to the in-event being processed is then read from the replay journal and is recorded again (to allow recording to function properly).

To be able to reproduce the outcome in the replay journal, some assistance may be required from REPLAY-VALUES and REPLAY-CONDITION:

  • If the replay event has a normal return (i.e. EVENT-EXIT(0 1) :VALUES), then the recorded return values (in EVENT-OUTCOME) are returned immediately as in (VALUES-LIST (EVENT-OUTCOME REPLAY-EVENT)). If REPLAY-VALUES is specified, it is called instead of VALUES-LIST. See Working with unreadable values for an example.

  • Similarly, if the replay event has unwound with an expected condition (has EVENT-EXIT :CONDITION), then the recorded condition (in EVENT-OUTCOME) is signalled as IN (ERROR (EVENT-OUTCOME REPLAY-EVENT)). If REPLAY-CONDITION is specified, it is called instead of ERROR(0 1). REPLAY-CONDITION must not return normally, and it's a JOURNAL-ERROR if it does.

WITH-REPLAY-FILTER's NO-REPLAY-OUTCOME can selectively turn off replaying the outcome. See Testing on multiple levels, for an example.

9.5 Matching out-events

If there were no Replay failures during the matching of the IN-EVENT, and the conditions for Replaying the outcome were not met, then the block is executed. When the outcome of the block is determined, an OUT-EVENT is triggered and is matched to the replay journal. The matching of out-events starts out as in The replay strategy with checks for EVENT-NAME and EVENT-VERSION.

If the replay strategy is insert or upgrade, then the out-event is written to RECORD-JOURNAL, consuming an event with a matching name from the REPLAY-JOURNAL in the latter case. If the strategy is match, then:

Note that The replay strategy for the in-event and the out-event of the same frame may differ if the corresponding out-event is not present in REPLAY-JOURNAL, which may be the case when the recording process failed hard without unwinding properly, or when an unexpected outcome triggered the transition to JOURNAL-STATE :LOGGING.

9.6 Replay failures

  • [condition] REPLAY-FAILURE SERIOUS-CONDITION

    A abstract superclass (never itself signalled) for all kinds of mismatches between the events produced and the replay journal. Signalled only in JOURNAL-STATE :REPLAYING and only once per WITH-JOURNALING. If a REPLAY-FAILURE is signalled for an EVENT, then the event will be recorded, but RECORD-JOURNAL will transition to JOURNAL-STATE :MISMATCHED. Like JOURNALING-FAILURE, this is a serious condition because it is to be handled outside the enclosing WITH-JOURNALING. If a REPLAY-FAILURE were to be handled inside the WITH-JOURNALING, keep in mind that in :MISMATCHED, replay always uses the insert replay strategy (see The replay strategy).

  • [reader] REPLAY-FAILURE-NEW-EVENT REPLAY-FAILURE (:NEW-EVENT)

  • [reader] REPLAY-FAILURE-REPLAY-EVENT REPLAY-FAILURE (:REPLAY-EVENT)

  • [reader] REPLAY-FAILURE-REPLAY-JOURNAL REPLAY-FAILURE (= '(REPLAY-JOURNAL))

  • [condition] REPLAY-INCOMPLETE REPLAY-FAILURE

    Signalled if there are unprocessed non-log events in REPLAY-JOURNAL when WITH-JOURNALING finishes and the body of WITH-JOURNALING returned normally, which is to prevent this condition to cancel an ongoing unwinding. No restarts are provided.

  • [restart] REPLAY-FORCE-INSERT

    This restart forces The replay strategy to be :INSERT, overriding REPLAY-NAME-MISMATCH. This is intended for upgrades, and extreme care must be taken not to lose data.

9.7 Upgrades and replay

The replay mechanism is built on the assumption that the tree of frames is the same when the code is replayed as it was when the replay journal was originally recorded. Thus, non-deterministic control flow poses a challenge, but non-determinism can be isolated with EXTERNAL-EVENTs. However, when the code changes, we might find the structure of frames in previous recordings hard to accommodate. In this case, we might decide to alter the structure, giving up some of the safety provided by the replay mechanism. There are various tools at our disposal to control this tradeoff between safety and flexibility:

  • We can insert individual frames with JOURNALED's INSERTABLE, upgrade frames by bumping JOURNALED's VERSION, and filter frames with WITH-REPLAY-FILTER. This option allows for the most consistency checks.

  • The REPLAY-FORCE-UPGRADE and REPLAY-FORCE-INSERT restarts allow overriding The replay strategy, but their use requires great care to be taken.

  • Or we may decide to keep the bare minimum of the replay journal around and discard everything except for EXTERNAL-EVENTs. This option is equivalent to

      (let ((*force-insertable* t))
    (with-replay-filter (:skip '((:name nil)))
      42))

  • Rerecording the journal without replay might be another option if there are no EXTERNAL-EVENTs to worry about.

  • Finally, we can rewrite the replay journal using the low-level interface (see Streamlets reference). In this case, extreme care must be taken not to corrupt the journal (and lose data) as there are no consistency checks to save us.

With that, let's see how WITH-REPLAY-FILTER works.

  • [function] PEEK-REPLAY-EVENT

    Return the replay event to be read from REPLAY-JOURNAL. This is roughly equivalent to

    (when (replay-journal)
  (with-replay-streamlet (streamlet)
    (peek-event streamlet))

    except PEEK-REPLAY-EVENT takes into account WITH-REPLAY-FILTER :MAP, and it may return (:INDETERMINATE) if WITH-REPLAY-FILTER :SKIP is in effect and what events are to be skipped cannot be decided until the next in-event generated by the code.

    Imagine a business process for paying an invoice. In the first version of this process, we just pay the invoice:

    (replayed (pay))

    We have left the implementation of PAY blank. In the second version, we need to get an approval first:

    (when (replayed (get-approval)
        (= (random 2) 0))
  (replayed (pay)))

    Replaying a journal produced by the first version of the code with the second version would run into difficulties because inserting EXTERNAL-EVENTs is tricky.

    We have to first decide how to handle the lack of approval in the first version. Here, we just assume the processes started by the first version get approval automatically. The implementation is based on a dummy PROCESS block whose version is bumped when the payment process changes and is inspected at the start of journaling.

    When v1 is replayed with v2, we introduce an INSERTABLE, versioned GET-APPROVAL block that just returns T. When replaying the code again, still with v2, the GET-APPROVAL block will be upgraded to :INFINITY.

    (let ((bundle (make-in-memory-bundle)))
  ;; First version of the payment process. Just pay.
  (with-bundle (bundle)
    (checked (process :version 1))
    (replayed (pay)))
  ;; Second version of the payment process. Only pay if approved.
  (loop repeat 2 do
    (with-bundle (bundle)
      (let ((replay-process-event (peek-replay-event)))
        (checked (process :version 2))
        (when (if (and replay-process-event
                       (< (event-version replay-process-event) 2))
                  ;; This will be upgraded to :INFINITY the second
                  ;; time around the LOOP.
                  (checked (get-approval :insertable t)
                    t)
                  (replayed (get-approval)
                    (= (random 2) 0)))
          (replayed (pay)))))))

  • [macro] WITH-REPLAY-FILTER (&KEY MAP SKIP NO-REPLAY-OUTCOME) &BODY BODY

    WITH-REPLAY-FILTER performs journal upgrade during replay by allowing events to be transformed as they are read from the replay journal or skipped if they match some patterns. For how to add new blocks in a code upgrade, see JOURNALED's :INSERTABLE argument. In addition, it also allows some control over Replaying the outcome.

    • MAP: A function called with an event read from the replay journal which returns a transformed event. See Events reference. MAP takes effect before before SKIP.

    • SKIP: In addition to filtering out LOG-EVENTs (which always happens during replay), filter out all events that belong to frames that match any of its SKIP patterns. Filtered out events are never seen by JOURNALED as it replays events. SKIP patterns are of the format (&KEY NAME VERSION<), where VERSION< is a valid EVENT-VERSION, and NAME may be NIL, which acts as a wildcard.

      SKIP is for when JOURNALED blocks are removed from the code, which would render replaying previously recorded journals impossible. Note that, for reasons of safety, it is not possible to filter EXTERNAL-EVENTs.

    • NO-REPLAY-OUTCOME is a list of EVENT-NAMEs. Replaying the outcome is prevented for frames with EQUAL names. See Testing on multiple levels for an example.

    WITH-REPLAY-FILTER affects only the immediately enclosing WITH-JOURNALING. A WITH-REPLAY-FILTER nested within another in the same WITH-JOURNALING inherits the SKIP patterns of its parent, to which it adds its own. The MAP function is applied to before the parent's MAP.

    Examples of SKIP patterns:

    ;; Match events with name FOO and version 1, 2, 3 or 4
(:name foo :version< 5)
;; Match events with name BAR and any version
(:name bar :version< :infinity)
;; Same as the previous
(:name bar)
;; Match all names
(:name nil)
;; Same as the previous
()

    Skipping can be thought of as removing nodes of the tree of frames, connecting its children to its parent. The following example removes frames J1 and J2 from around J3, the J1 frame from within J3, and the third J1 frame.

    (let ((journal (make-in-memory-journal)))
  ;; Record trees J1 -> J2 -> J3 -> J1, and J1.
  (with-journaling (:record journal)
    (checked (j1)
      (checked (j2)
        (checked (j3)
          (checked (j1)
            42))))
    (checked (j1)
      7))
  ;; Filter out all occurrences of VERSIONED-EVENTs named J1 and
  ;; J2 from the replay, leaving only J3 to match.
  (with-journaling (:replay journal :record t :replay-eoj-error-p t)
    (with-replay-filter (:skip '((:name j1) (:name j2)))
      (checked (j3)
        42))))

10 Testing

Having discussed the Replay mechanism, next are Testing and Persistence, which rely heavily on replay. Suppose we want to unit test user registration. Unfortunately, the code communicates with a database service and also takes input from the user. A natural solution is to create mock objects for these external systems to unshackle the test from the cumbersome database dependency and to allow it to run without user interaction.

We do this below by wrapping external interaction in JOURNALED with :VERSION :INFINITY (see Replaying the outcome).

(defparameter *db* (make-hash-table))

(defun set-key (key value)
  (replayed ("set-key" :args `(,key ,value))
    (format t "Updating db~%")
    (setf (gethash key *db*) value)
    nil))

(defun get-key (key)
  (replayed ("get-key" :args `(,key))
    (format t "Query db~%")
    (gethash key *db*)))

(defun ask-username ()
  (replayed ("ask-username")
    (format t "Please type your username: ")
    (read-line)))

(defun maybe-win-the-grand-prize ()
  (checked ("maybe-win-the-grand-prize")
    (when (= 1000000 (hash-table-count *db*))
      (format t "You are the lucky one!"))))

(defun register-user (username)
  (unless (get-key username)
    (set-key username `(:user-object :username ,username))
    (maybe-win-the-grand-prize)))

Now, we write a test that records these interactions in a file when it's run for the first time.

(define-file-bundle-test (test-user-registration
                          :directory (asdf:system-relative-pathname
                                      :journal "test/registration/"))
  (let ((username (ask-username)))
    (register-user username)
    (assert (get-key username))
    (register-user username)
    (assert (get-key username))))

;; Original recording: everything is executed
JRN> (test-user-registration)
Please type your username: joe
Query db
Updating db
Query db
Query db
Query db
=> NIL

On reruns, none of the external stuff is executed. The return values of the external JOURNALED blocks are replayed from the journal:

;; Replay: all external interactions are mocked.
JRN> (test-user-registration)
=> NIL

Should the code change, we might want to upgrade carefully (see Upgrades and replay) or just rerecord from scratch:

JRN> (test-user-registration :rerecord t)
Please type your username: joe
Query db
Updating db
Query db
Query db
Query db
=> NIL

Thus satisfied that our test runs, we can commit the journal file in the bundle into version control. Its contents are:


(:IN "ask-username" :VERSION :INFINITY)
(:OUT "ask-username" :VERSION :INFINITY :VALUES ("joe" NIL))
(:IN "get-key" :VERSION :INFINITY :ARGS ("joe"))
(:OUT "get-key" :VERSION :INFINITY :VALUES (NIL NIL))
(:IN "set-key" :VERSION :INFINITY :ARGS ("joe" (:USER-OBJECT :USERNAME "joe")))
(:OUT "set-key" :VERSION :INFINITY :VALUES (NIL))
(:IN "maybe-win-the-grand-prize" :VERSION 1)
(:OUT "maybe-win-the-grand-prize" :VERSION 1 :VALUES (NIL))
(:IN "get-key" :VERSION :INFINITY :ARGS ("joe"))
(:OUT "get-key" :VERSION :INFINITY :VALUES ((:USER-OBJECT :USERNAME "joe") T))
(:IN "get-key" :VERSION :INFINITY :ARGS ("joe"))
(:OUT "get-key" :VERSION :INFINITY :VALUES ((:USER-OBJECT :USERNAME "joe") T))
(:IN "get-key" :VERSION :INFINITY :ARGS ("joe"))
(:OUT "get-key" :VERSION :INFINITY :VALUES ((:USER-OBJECT :USERNAME "joe") T))

Note that when this journal is replayed, new VERSIONED-EVENTs are required to match the replay. So, after the original recording, we can check by eyeballing that the record represents a correct execution. Then on subsequent replays, even though MAYBE-WIN-THE-GRAND-PRIZE sits behind REGISTER-USER and is hard to test with ASSERTs, the replay mechanism verifies that it is called only for new users.

This record-and-replay style of testing is not the only possibility: direct inspection of a journal with the low-level events api (see Events reference) can facilitate checking non-local invariants.

  • [macro] DEFINE-FILE-BUNDLE-TEST (NAME &KEY DIRECTORY (EQUIVALENTP T)) &BODY BODY

    Define a function with NAME for record-and-replay testing. The function's BODY is executed in a WITH-BUNDLE to guarantee replayability. The bundle in question is a FILE-BUNDLE created in DIRECTORY. The function has a single keyword argument, RERECORD. If RERECORD is true, the bundle is deleted with DELETE-FILE-BUNDLE to start afresh.

    Furthermore, if BODY returns normally, and it is a replay of a previous run, and EQUIVALENTP, then it is ASSERTed that the record and replay journals are EQUIVALENT-REPLAY-JOURNALS-P. If this check fails, RECORD-JOURNAL is discarded when the function returns. In addition to the replay consistency, this checks that no inserts or upgrades were performed (see The replay strategy).

10.1 Testing on multiple levels

Nesting REPLAYEDs (that is, frames of EXTERNAL-EVENTs) is not obviously useful since the outer REPLAYED will be replayed by outcome, and the inner one will be just echoed to the record journal. However, if we turn off Replaying the outcome for the outer, the inner will be replayed.

This is useful for testing layered communication. For example, we might have written code that takes input from an external system (READ-LINE) and does some complicated processing (READ-FROM-STRING) before returning the input in a form suitable for further processing. Suppose we wrap REPLAYED around READ-FROM-STRING for Persistence because putting it around READ-LINE would expose low-level protocol details in the journal, making protocol changes difficult.

However, upon realizing that READ-FROM-STRING was not the best tool for the job and switching to PARSE-INTEGER, we want to test by replaying all previously recorded journals. For this, we prevent the outer REPLAYED from being replayed by outcome with WITH-REPLAY-FILTER:

(let ((bundle (make-in-memory-bundle)))
  ;; Original with READ-FROM-STRING
  (with-bundle (bundle)
    (replayed ("accept-number")
      (values (read-from-string (replayed ("input-number")
                                  (read-line))))))
  ;; Switch to PARSE-INTEGER and test by replay.
  (with-bundle (bundle)
    (with-replay-filter (:no-replay-outcome '("accept-number"))
      (replayed ("accept-number")
        ;; 1+ is our bug.
        (values (1+ (parse-integer (replayed ("input-number")
                                     (read-line)))))))))

The inner input-number block is replayed by outcome, and PARSE-INTEGER is called with the string READ-LINE returned in the original invocation. The outcome of the outer accept-number block checked as if it was a VERSIONED-EVENT and we get a REPLAY-OUTCOME-MISMATCH due to the bug.

11 Persistence

11.1 Persistence tutorial

Let's write a simple game.

(defun play-guess-my-number ()
  (let ((my-number (replayed (think-of-a-number)
                     (random 10))))
    (format t "~%I thought of a number.~%")
    (loop for i upfrom 0 do
      (write-line "Guess my number:")
      (let ((guess (replayed (read-guess)
                     (values (parse-integer (read-line))))))
        (format t "You guessed ~D.~%" guess)
        (when (= guess my-number)
          (checked (game-won :args `(,(1+ i))))
          (format t "You guessed it in ~D tries!" (1+ i))
          (return))))))

(defparameter *the-evergreen-game* (make-in-memory-bundle))
Original recording

Unfortunately, the implementation is lacking in the input validation department. In the transcript below, PARSE-INTEGER fails with junk in string when the user enters not a number:

CL-USER> (handler-case
             (with-bundle (*the-evergreen-game*)
               (play-guess-my-number))
           (error (e)
             (format t "Oops. ~A~%" e)))
I thought of a number.
Guess my number:
7 ; real user input
You guessed 7.
Guess my number:
not a number ; real user input
Oops. junk in string "not a number"
Replay and extension

Instead of fixing this bug, we just restart the game from the beginning, Replaying the outcome of external interactions marked with REPLAYED:

CL-USER> (with-bundle (*the-evergreen-game*)
           (play-guess-my-number))
I thought of a number.
Guess my number:
You guessed 7.
Guess my number: ; New recording starts here
5 ; real user input
You guessed 5.
Guess my number:
4 ; real user input
You guessed 4.
Guess my number:
2 ; real user input
You guessed 2.
You guessed it in 4 tries!
It's evergreen

We can now replay this game many times without any user interaction:

CL-USER> (with-bundle (*the-evergreen-game*)
           (play-guess-my-number))
I thought of a number.
Guess my number:
You guessed 7.
Guess my number:
You guessed 5.
Guess my number:
You guessed 4.
Guess my number:
You guessed 2.
You guessed it in 4 tries!
The generated events

This simple mechanism allows us to isolate external interactions and write tests in record-and-replay style based on the events produced:

CL-USER> (list-events *the-evergreen-game*)
((:IN THINK-OF-A-NUMBER :VERSION :INFINITY)
 (:OUT THINK-OF-A-NUMBER :VERSION :INFINITY :VALUES (2))
 (:IN READ-GUESS :VERSION :INFINITY)
 (:OUT READ-GUESS :VERSION :INFINITY :VALUES (7))
 (:IN READ-GUESS :VERSION :INFINITY :ARGS NIL)
 (:OUT READ-GUESS :VERSION :INFINITY :VALUES (5))
 (:IN READ-GUESS :VERSION :INFINITY :ARGS NIL)
 (:OUT READ-GUESS :VERSION :INFINITY :VALUES (4))
 (:IN READ-GUESS :VERSION :INFINITY :ARGS NIL)
 (:OUT READ-GUESS :VERSION :INFINITY :VALUES (2))
 (:IN GAME-WON :VERSION 1 :ARGS (4))
 (:OUT GAME-WON :VERSION 1 :VALUES (NIL)))

In fact, being able to replay this game at all already checks it through the GAME-WON event that the number of tries calculation is correct.

In addition, thus being able to reconstruct the internal state of the program gives us persistence by replay. If instead of a IN-MEMORY-BUNDLE, we used a FILE-BUNDLE, the game would have been saved on disk without having to write any code for saving and loading the game state.

Discussion

Persistence by replay, also known as event sourcing, is appropriate when the external interactions are well-defined and stable. Storing events shines in comparison to persisting state when the control flow is too complicated to be interrupted and resumed easily. Resuming execution in deeply nested function calls is fraught with such peril that it is often easier to flatten the program into a state machine, which is as pleasant as manually managing continuations.

In contrast, the Journal library does not favour certain styles of control flow and only requires that non-determinism is packaged up in REPLAYED, which allows it to reconstruct the state of the program from the recorded events at any point during its execution and resume from there.

11.2 Synchronization to storage

In the following, we explore how journals can serve as a persistence mechanism and the guarantees they offer. The high-level summary is that journals with SYNC can serve as a durable and consistent storage medium. The other two ACID properties, atomicity and isolation, do not apply because Journal is single-client and does not need transactions.

  • [glossary-term] aborted execution

    Aborted execution is when the operating system or the application crashes, calls abort(), is killed by a SIGKILL signal or there is a power outage. Synchronization guarantees are defined in the face of aborted execution and do not apply to hardware errors, Lisp or OS bugs.

  • [glossary-term] data event

    Data events are the only events that may be non-deterministic. They record information that could change if the same code were run multiple times. Data events typically correspond to interactions with the user, servers or even the random number generator. Due to their non-determinism, they are the only parts of the journal not reproducible by rerunning the code. In this sense, only the data events are not redundant with the code, and whether other events are persisted does not affect durability. There are two kinds of data events:

11.2.1 Synchronization strategies

When a journal or bundle is created (see MAKE-IN-MEMORY-JOURNAL, MAKE-FILE-JOURNAL, MAKE-IN-MEMORY-BUNDLE, MAKE-FILE-BUNDLE), the SYNC option determines when – as a RECORD-JOURNAL – the recorded events and JOURNAL-STATE changes are persisted durably. For FILE-JOURNALs, persisting means calling something like fsync, while for IN-MEMORY-JOURNALs, a user defined function is called to persist the data.

  • NIL: Never synchronize. A FILE-JOURNAL's file may be corrupted on aborted execution. In IN-MEMORY-JOURNALs, SYNC-FN is never called.

  • T: This is the no data loss setting with minimal synchronization. It guarantees consistency (i.e. no corruption) and durability up to the most recent data event written in JOURNAL-STATE :RECORDING or for the entire record journal in states :FAILED and :COMPLETED. :FAILED or :COMPLETED is guaranteed when leaving WITH-JOURNALING at the latest.

  • Values other than NIL and T are reserved for future extensions. Using them triggers a JOURNAL-ERROR.

11.2.2 Synchronization with in-memory journals

Unlike FILE-JOURNALs, IN-MEMORY-JOURNALs do not have any built-in persistent storage backing them, but with SYNC-FN, persistence can be tacked on. If non-NIL, SYNC-FN must be a function of a single argument, an IN-MEMORY-JOURNAL. SYNC-FN is called according to Synchronization strategies, and upon normal return the journal must be stored durably.

The following example saves the entire journal history when a new data event is recorded. Note how SYNC-TO-DB is careful to overwrite *DB* only if it is called with a journal that has not failed the replay (as in Replay failures) and is sufficiently different from the replay journal as determined by JOURNAL-DIVERGENT-P.

(defparameter *db* ())

(defun sync-to-db (journal)
  (when (and (member (journal-state journal)
                     '(:recording :logging :completed))
             (journal-divergent-p journal))
    (setq *db* (journal-events journal))
    (format t "Saved ~S~%New events from position ~S~%" *db*
            (journal-previous-sync-position journal))))

(defun make-db-backed-record-journal ()
  (make-in-memory-journal :sync-fn 'sync-to-db))

(defun make-db-backed-replay-journal ()
  (make-in-memory-journal :events *db*))

(with-journaling (:record (make-db-backed-record-journal)
                  :replay (make-db-backed-replay-journal))
  (replayed (a)
    2)
  (ignore-errors
    (replayed (b)
      (error "Whoops"))))
.. Saved #((:IN A :VERSION :INFINITY)
..         (:OUT A :VERSION :INFINITY :VALUES (2)))
.. New events from position 0
.. Saved #((:IN A :VERSION :INFINITY)
..         (:OUT A :VERSION :INFINITY :VALUES (2))
..         (:IN B :VERSION :INFINITY)
..         (:OUT B :ERROR ("SIMPLE-ERROR" "Whoops")))
.. New events from position 2
..

In a real application, external events often involve unreliable or high-latency communication. In the above example, block B signals an error, say, to simulate some kind of network condition. Now, a new journal for replay is created and initialized with the saved events, and the whole process is restarted.

(defun run-with-db ()
  (with-journaling (:record (make-db-backed-record-journal)
                    :replay (make-db-backed-replay-journal))
    (replayed (a)
      (format t "A~%")
      2)
    (replayed (b)
      (format t "B~%")
      3)))

(run-with-db)
.. B
.. Saved #((:IN A :VERSION :INFINITY)
..         (:OUT A :VERSION :INFINITY :VALUES (2))
..         (:IN B :VERSION :INFINITY)
..         (:OUT B :VERSION :INFINITY :VALUES (3)))
.. New events from position 0
..
=> 3

Note that on the rerun, block A is not executed because external events are replayed simply by reproducing their outcome, in this case returning 2. See Replaying the outcome. Block B, on the other hand, was rerun because it had an unexpected outcome the first time around. This time it ran without error, a data event was triggered, and SYNC-FN was invoked.

If we were to invoke the now completed RUN-WITH-DB again, it would simply return 3 without ever invoking SYNC-FN:

(run-with-db)
=> 3

With JOURNAL-REPLAY-MISMATCH, SYNC-FN can be optimized to to reuse the sequence of events in the replay journal up until the point of divergence.

11.2.3 Synchronization with file journals

For FILE-JOURNALs, SYNC determines when the events written to the RECORD-JOURNAL and its JOURNAL-STATE will be persisted durably in the file. Syncing to the file involves two calls to fsync and is not cheap.

Syncing events to files is implemented as follows.

  • When the journal file is created, its parent directory is immediately fsynced to make sure that the file will not be lost on aborted execution.

  • When an event is about to be written the first time after file creation or after a sync, a transaction start marker is written to the file.

  • Any number of events may be subsequently written until syncing is deemed necessary (see Synchronization strategies).

  • At this point, fsync is called to flush all event data and state changes to the file, and the transaction start marker is overwritten with a transaction completed marker and another fsync is performed.

  • When reading back this file (e.g. for replay), an open transaction marker is treated as the end of file.

Note that this implementation assumes that after writing the start transaction marker, a crash cannot leave any kind of garbage bytes around: it must leave zeros. This is not true for all filesytems. For example, ext3/ext4 with data=writeback can leave garbage around.

12 Safety

Thread safety

Changes to journals come in two varieties: adding an event and changing the JOURNAL-STATE. Both are performed by JOURNALED only unless the low-level streamlet interface is used (see Streamlets reference). Using JOURNALED wrapped in a WITH-JOURNALING, WITH-BUNDLE, or :LOG-RECORD without WITH-JOURNALING is thread-safe.

  • Every journal is guaranteed to have at most a single writer active at any time. Writers are mainly WITH-JOURNALING and WITH-BUNDLE, but any journals directly logged to have a log writer stored in the journal object. See Logging.

  • WITH-JOURNALING and WITH-BUNDLE have dynamic extent as writers, but log writers of journals have indefinite extent: once a journal is used as a LOG-RECORD, there remains a writer.

  • Attempting to create a second writer triggers a JOURNAL-ERROR.

  • Writing to the same journal via :LOG-RECORD from multiple threads concurrently is possible since this doesn't create multiple writers. It is ensured with locking that events are written atomically. Frames can be interleaved, but these are LOG-EVENTs, so this does not affect replay.

  • The juggling of replay and record journals performed by WITH-BUNDLE is also thread-safe.

  • It is ensured that there is at most one FILE-JOURNAL object in the same Lisp image is backed by the same file.

  • Similarly, there is at most FILE-BUNDLE object for a directory.

Process safety

Currently, there is no protection against multiple OS processes writing the same FILE-JOURNAL or FILE-BUNDLE.

Signal safety

Journal is designed to be async-unwind safe but not reentrant. Interrupts are disabled only for the most critical cleanup forms. If a thread is killed without unwinding, that constitutes aborted execution, so guarantees about Synchronization to storage apply, but JOURNAL objects written by the thread are not safe to access, and the Lisp should probably be restarted.

13 Events reference

Events are normally triggered upon entering and leaving the dynamic extent of a JOURNALED block (see In-events and Out-events) and also by LOGGED. Apart from being part of the low-level substrate of the Journal library, working with events directly is sometimes useful when writing tests that inspect recorded events. Otherwise, skip this entire section.

All EVENTs have EVENT-NAME and EVENT-VERSION(0 1), which feature prominently in The replay strategy. After the examples in In-events and Out-events, the following example is a reminder of how events look in the simplest case.

(with-journaling (:record t)
  (journaled (foo :version 1 :args '(1 2))
    (+ 1 2))
  (logged () "Oops")
  (list-events))
=> ((:IN FOO :VERSION 1 :ARGS (1 2))
    (:OUT FOO :VERSION 1 :VALUES (3))
    (:LEAF "Oops"))

So, a JOURNALED block generates an IN-EVENT and an OUT-EVENT, which are simple property lists. The following reference lists these properties, their semantics and the functions to read them.

  • [function] EVENT= EVENT-1 EVENT-2

    Return whether EVENT-1 and EVENT-2 represent the same event. In- and out-events belonging to the same frame are not the same event. EVENT-OUTCOMEs are not compared when EVENT-EXIT(0 1) is :ERROR to avoid undue dependence on implementation specific string representations. This function is useful in conjunction with MAKE-IN-EVENT and MAKE-OUT-EVENT to write tests.

  • [function] EVENT-NAME EVENT

    The name of an event can be of any type. It is often a symbol or a string. When replaying, names are compared with EQUAL. All EVENTs have names. The names of the in- and out-events belonging to the same frame are the same.

13.1 Event versions

  • [function] EVENT-VERSION EVENT

    Return the version of EVENT of type EVENT-VERSION.

  • [function] LOG-EVENT-P EVENT

    See if EVENT is a LOG-EVENT.

  • [function] EXTERNAL-EVENT-P EVENT

    See if EVENT is an EXTERNAL-EVENT.

13.2 In-events

  • [function] IN-EVENT-P EVENT

    See if EVENT is a IN-EVENT.

  • [function] EVENT-ARGS IN-EVENT

    Return the arguments of IN-EVENT, normally populated using the ARGS form in JOURNALED.

13.3 Out-events

  • [function] OUT-EVENT-P EVENT

    See if EVENT is an OUT-EVENT.

  • [function] EVENT-EXIT OUT-EVENT

    Return how the journaled block finished. See EVENT-EXIT for the possible types.

  • [function] EXPECTED-OUTCOME-P OUT-EVENT

    See if OUT-EVENT has an expected outcome.

  • [function] EVENT-OUTCOME OUT-EVENT

    Return the outcome of the frame (or loosely speaking of a block) to which OUT-EVENT belongs.

13.4 Leaf-events

  • [type] LEAF-EVENT

    Leaf events are triggered by LOGGED. Unlike IN-EVENTs and OUT-EVENTs, which represent a frame, leaf events represent a point in execution thus cannot have children. They are also the poorest of their kind: they only have an EVENT-NAME. Their VERSION is always NIL, which makes them LOG-EVENTs.

  • [function] LEAF-EVENT-P EVENT

    See if EVENT is a LEAF-EVENT.

  • [function] MAKE-LEAF-EVENT NAME

    Create a LEAF-EVENT with NAME.

14 Journals reference

In Basics, we covered the bare minimum needed to work with journals. Here, we go into the details.

  • [class] JOURNAL

    JOURNAL is an abstract base class for a sequence of events. In case of FILE-JOURNALs, the events are stored in a file, while for IN-MEMORY-JOURNALs, they are in a Lisp array. When a journal is opened, it is possible to perform I/O on it (see Streamlets reference), which is normally taken care of by WITH-JOURNALING. For this reason, the user's involvement with journals normally only consists of creating and using them in WITH-JOURNALING.

  • [reader] JOURNAL-STATE JOURNAL (:STATE)

    Return the state of JOURNAL, which is of type JOURNAL-STATE.

  • [function] SYNC-JOURNAL &OPTIONAL (JOURNAL (RECORD-JOURNAL))

    Durably persist changes made to JOURNAL if JOURNAL-SYNC is T. The changes that are persisted are

    In particular, writes made in a WITH-JOURNALING in another thread are not persisted. SYNC-JOURNAL is a noop if JOURNAL-SYNC is NIL. It is safe to call from any thread.

14.1 Comparing journals

After replay finished (i.e. WITH-JOURNALING completed), we can ask whether there were any changes produced. This is answered in the strictest sense by IDENTICAL-JOURNALS-P and somewhat more functionally by EQUIVALENT-REPLAY-JOURNALS-P.

Also see JOURNAL-DIVERGENT-P.

  • [generic-function] IDENTICAL-JOURNALS-P JOURNAL-1 JOURNAL-2

    Compare two journals in a strict sense: whether they have the same JOURNAL-STATE and the lists of their events (as in LIST-EVENTS) are EQUAL.

  • [generic-function] EQUIVALENT-REPLAY-JOURNALS-P JOURNAL-1 JOURNAL-2

    See if two journals are equivalent when used the for REPLAY in WITH-JOURNALING. EQUIVALENT-REPLAY-JOURNALS-P is like IDENTICAL-JOURNALS-P, but it ignores LOG-EVENTs and allows events with EVENT-EXIT(0 1) :ERROR to differ in their outcomes, which may very well be implementation specific, anyway. Also, it considers two groups of states as different :NEW, :REPLAYING, :MISMATCHED, :FAILED vs :RECORDING, :LOGGING, COMPLETED.

The rest of section is about concrete subclasses of JOURNAL.

14.2 In-memory journals

  • [class] IN-MEMORY-JOURNAL JOURNAL

    IN-MEMORY-JOURNALs are backed by a non-persistent Lisp array of events. Much quicker than FILE-JOURNALs, they are ideal for smallish journals persisted manually (see Synchronization with in-memory journals for an example).

    They are also useful for writing tests based on what events were generated. They differ from FILE-JOURNALs in that events written to IN-MEMORY-JOURNALs are not serialized (and deserialized on replay) with the following consequences for the objects recorded by JOURNALED (i.e. its NAME, ARGS arguments, and also the return VALUES(0 1) of the block, or the value returned by CONDITION):

    • These objects need not be readable.

    • Their identity (EQness) is not lost.

    • They must must not be mutated in any way.

  • [function] MAKE-IN-MEMORY-JOURNAL &KEY (EVENTS NIL EVENTSP) STATE (SYNC NIL SYNCP) SYNC-FN

    Create an IN-MEMORY-JOURNAL.

    The returned journal's JOURNAL-STATE will be set to STATE. If STATE is NIL, then it is replaced by a default value, which is :COMPLETED if the EVENTS argument is provided, else it is :NEW.

    Thus, (make-in-memory-journal) creates a journal suitable for recording, and to make a replay journal, use :STATE :COMPLETED with some sequence of EVENTS:

    (make-in-memory-journal :events '((:in foo :version 1)) :state :completed)

    SYNC determines when SYNC-FN will be invoked on the RECORD-JOURNAL. SYNC defaults to T if SYNC-FN, else to NIL. For a description of possible values, see Synchronization strategies. For more discussion, see Synchronization with in-memory journals.

  • [reader] JOURNAL-EVENTS IN-MEMORY-JOURNAL (:EVENTS)

    A sequence of events in the journal. Not to be mutated by client code.

  • [reader] JOURNAL-PREVIOUS-SYNC-POSITION IN-MEMORY-JOURNAL (= 0)

    The length of JOURNAL-EVENTS at the time of the most recent invocation of SYNC-FN.

14.3 File journals

  • [class] FILE-JOURNAL JOURNAL

    A FILE-JOURNAL is a journal whose contents and JOURNAL-STATE are persisted in a file. This is the JOURNAL subclass with out-of-the-box persistence, but see File bundles for a more full-featured solution for repeated Replays.

    Since serialization in FILE-JOURNALs is built on top of Lisp READ and WRITE, everything that JOURNALED records in events (i.e. its NAME, ARGS arguments, and also the return VALUES(0 1) of the block, or the value returned by CONDITION) must be readable.

    File journals are human-readable and editable by hand with some care. When editing, the following needs to be remembered:

    • The first character of the file represents its JOURNAL-STATE. It is a #\Space (for state :NEW, :REPLAYING, :MISMATCHED and :FAILED), or a #\Newline (for state :RECORDING, :LOGGING and :COMPLETED).

    • If the journal has SYNC (see Synchronization strategies), then between two events, there may be #\Del (also called #\Rubout) or #\Ack characters (CHAR-CODE 127 and 6). #\Del marks the end of the journal contents that may be read back: it's kind of an uncommitted-transaction marker for the events that follow it. #\Ack characters, of which there may be many in the file, mark the sequence of events until the next marker of either kind as valid (or committed). #\Ack characters are ignored when reading the journal.

    Thus, when editing a file, don't change the first character and leave the #\Del character, if any, where it is. Also see Synchronization with file journals.

  • [function] MAKE-FILE-JOURNAL PATHNAME &KEY SYNC

    Return a FILE-JOURNAL backed by the file with PATHNAME. The file is created when the journal is opened for writing. For a description of SYNC, see Synchronization strategies.

    If there is already an existing FILE-JOURNAL backed by the same file, then that object is returned. If the existing object has different options (e.g. it has SYNC T while the SYNC argument is NIL here), then a JOURNAL-ERROR is signalled.

    If there is already an existing FILE-JOURNAL backed by the same file, the JOURNAL-STATE is not :NEW, but the file doesn't exist, then the existing object is invalidated: attempts to write will fail with JOURNAL-ERROR. If the existing journal object is being written, then invalidation fails with a JOURNAL-ERROR. After invalidation, a new FILE-JOURNAL object is created.

  • [reader] PATHNAME-OF FILE-JOURNAL (:PATHNAME)

    The pathname of the file backing the journal.

14.4 Pretty-printing journals

  • [class] PPRINT-JOURNAL JOURNAL

    Events written to a PPRINT-JOURNAL have a customizable output format. PPRINT-JOURNALs are intended for producing prettier output for Logging and Tracing, but they do not support reads, so they cannot be used as a REPLAY-JOURNAL or in LIST-EVENTS, for example. On the other hand, events written to PPRINT-JOURNALs need not be readable.

  • [function] MAKE-PPRINT-JOURNAL &KEY (STREAM (MAKE-SYNONYM-STREAM '*STANDARD-OUTPUT*)) (PRETTY T) (PRETTIFIER 'PRETTIFY-EVENT) LOG-DECORATOR

    Creates a PPRINT-JOURNAL.

  • [accessor] PPRINT-JOURNAL-STREAM PPRINT-JOURNAL (:STREAM = *STANDARD-OUTPUT*)

    The stream where events are dumped. May be set any time to another STREAM.

  • [accessor] PPRINT-JOURNAL-PRETTIFIER PPRINT-JOURNAL (:PRETTIFIER = 'PRETTIFY-EVENT)

    A function like PRETTIFY-EVENT that writes an event to a stream. Only used when PPRINT-JOURNAL-PRETTY, this is the output format customization knob. Also see decorations.

15 Bundles reference

In Bundles, we covered the repeated replay problem that WITH-BUNDLE automates. Here, we provide a reference for the bundle classes.

  • [class] BUNDLE

    A BUNDLE consists of a sequence of journals which are all reruns of the same code, hopefully making more and more progress towards completion. These journals are Replays of the previous successful one, extending it with new events. Upon replay (see WITH-BUNDLE), the latest journal in the bundle in JOURNAL-STATE :COMPLETED plays the role of the replay journal, and a new journal is added to the bundle for recording. If the replay succeeds, this new journal eventually becomes :COMPLETED and takes over the role of the replay journal for future replays until another replay succeeds. When the bundle is created and it has no journals yet, the replay journal is an empty, completed one.

    This is an abstract base class. Direct subclasses are IN-MEMORY-BUNDLE and FILE-BUNDLE.

  • [accessor] MAX-N-FAILED BUNDLE (:MAX-N-FAILED = 1)

    If MAX-N-FAILED is non-NIL, and the number of journals of JOURNAL-STATE :FAILED in the bundle exceeds its value, then some journals (starting with the oldest) are deleted.

  • [accessor] MAX-N-COMPLETED BUNDLE (:MAX-N-COMPLETED = 1)

    If MAX-N-COMPLETED is non-NIL, and the number of journals of JOURNAL-STATE :COMPLETED in the bundle exceeds its value, then some journals (starting with the oldest) are deleted.

15.1 In-memory bundles

15.2 File bundles

  • [class] FILE-BUNDLE BUNDLE

    A FILE-BUNDLE is a BUNDLE that is built on FILE-JOURNALs. It provides easy replay-based persistence.

  • [reader] DIRECTORY-OF FILE-BUNDLE (:DIRECTORY)

    The directory where the files backing the FILE-JOURNALs in the FILE-BUNDLE are kept.

  • [function] DELETE-FILE-BUNDLE DIRECTORY

    Delete all journal files (*.jrn) from DIRECTORY. Delete the directory if empty after the journal files were deleted, else signal an error. Existing FILE-BUNDLE objects are not updated, so MAKE-FILE-JOURNAL with FORCE-RELOAD may be required.

16 Streamlets reference

This section is relevant mostly for implementing new kinds of JOURNALs in addition to FILE-JOURNALs and IN-MEMORY-JOURNALs. In normal operation, STREAMLETs are not worked with directly.

16.1 Opening and closing

  • [class] STREAMLET

    A STREAMLET is a handle to perform I/O on a JOURNAL. The high-level stuff (WITH-JOURNALING, JOURNALED, etc) is built on top of streamlets.

  • [reader] JOURNAL STREAMLET (:JOURNAL)

    The JOURNAL that was passed to OPEN-STREAMLET. This is the journal STREAMLET operates on.

  • [generic-function] OPEN-STREAMLET JOURNAL &KEY DIRECTION

    Return a STREAMLET suitable for performing I/O on JOURNAL. DIRECTION (defaults to :INPUT) is one of :INPUT, :OUTPUT, :IO, and it has the same purpose as the similarly named argument of CL:OPEN.

  • [generic-function] CLOSE-STREAMLET STREAMLET

    Close STREAMLET, which was returned by OPEN-STREAMLET. After closing, STREAMLET may not longer be used for IO.

  • [generic-function] MAKE-STREAMLET-FINALIZER STREAMLET

    Return NIL or a function of no arguments suitable as a finalizer for STREAMLET. That is, a function that closes STREAMLET but holds no reference to it. This is intended for streamlets that are not dynamic-extent, so using WITH-OPEN-JOURNAL is not appropriate.

  • [generic-function] OPEN-STREAMLET-P STREAMLET

    Return true if STREAMLET is open. STREAMLETs are open until they have been explicitly closed with CLOSE-STREAMLET.

  • [function] INPUT-STREAMLET-P STREAMLET

    See if STREAMLET was opened for input (the DIRECTION argument of OPEN-STREAMLET was :INPUT or :IO).

  • [function] OUTPUT-STREAMLET-P STREAMLET

    See if STREAMLET was opened for input (the DIRECTION argument of OPEN-STREAMLET was :OUTPUT or :IO).

  • [macro] WITH-OPEN-JOURNAL (VAR JOURNAL &KEY (DIRECTION :INPUT)) &BODY BODY

    This is like WITH-OPEN-FILE but for JOURNALs. Open the journal designated by JOURNAL (see TO-JOURNAL) with OPEN-STREAMLET, passing DIRECTION along, and bind VAR to the resulting STREAMLET. Call CLOSE-STREAMLET after BODY finishes. If JOURNAL is NIL, then VAR is bound to NIL and no streamlet is created.

  • [condition] STREAMLET-ERROR ERROR

    Like CL:STREAM-ERROR: failures pertaining to I/O on a closed STREAMLET or of the wrong DIRECTION. Actual I/O errors are not encapsulated in STREAMLET-ERROR.

16.2 Reading from streamlets

  • [generic-function] READ-EVENT STREAMLET &OPTIONAL EOJ-ERROR-P

    Read the event at the current read position from STREAMLET, and move the read position to the event after. If there are no more events, signal END-OF-JOURNAL or return NIL depending on EOJ-ERROR-P. Signals STREAMLET-ERROR if STREAMLET is not INPUT-STREAMLET-P or not OPEN-STREAMLET-P.

  • [generic-function] READ-POSITION STREAMLET

    Return an integer that identifies the position of the next event to be read from STREAMLET. SETFable, see SET-READ-POSITION.

  • [generic-function] SET-READ-POSITION STREAMLET POSITION

    Set the read position of STREAMLET to POSITION, which must have been acquired from READ-POSITION.

  • [macro] SAVE-EXCURSION (STREAMLET) &BODY BODY

    Save READ-POSITION of STREAMLET, execute BODY, and make sure to restore the saved read position.

  • [generic-function] PEEK-EVENT STREAMLET

    Read the next event from STREAMLET without changing the read position, or return NIL if there is no event to be read.

  • [method] PEEK-EVENT (STREAMLET STREAMLET)

    This is a slow default implementation, which relies on SAVE-EXCURSION and READ-EVENT.

16.3 Writing to streamlets

  • [method] WRITE-EVENT EVENT (JOURNAL JOURNAL)

    For convenience, it is possible to write directly to a JOURNAL, in which case the journal's internal output streamlet is used. This internal streamlet is opened for :OUTPUT and may be used by :LOG-RECORD.

  • [generic-function] WRITE-POSITION STREAMLET

    Return an integer that identifies the position of the next event to be written to STREAMLET.

  • [generic-function] REQUEST-COMPLETED-ON-ABORT STREAMLET

    Make it so that upon aborted execution, STREAMLET's JOURNAL will be in JOURNAL-STATE :COMPLETED when loaded fresh (e.g. when creating a FILE-JOURNAL with an existing file). Any previously written events must be persisted before making this change. Before REQUEST-COMPLETED-ON-ABORT is called, a journal must be reloaded in state :FAILED.

    It is permissible to defer carrying out this request until the next SYNC-STREAMLET call. If the request was carried out, return true. If it was deferred, return NIL.

  • [generic-function] SYNC-STREAMLET STREAMLET

    Durably persist the effects of all preceding WRITE-EVENT calls made via STREAMLET to its journal and any deferred REQUEST-COMPLETED-ON-ABORT in this order.

17 Glossary

  • [glossary-term] async-unwind

    If an asynchronous event, say a SIGINT triggered by C-c, is delivered to a thread running Lisp or foreign code called from Lisp, a Lisp condition is typically signalled. If the handler for this condition unwinds the stack, then we have an asynchronous unwind. Another example is BT:INTERRUPT-THREAD, which, as it can execute arbitrary code, may unwind the stack in the target thread.

  • [glossary-term] boolean-valued symbol

    Imagine writing two STREAMs with a spaghetti of functions and wanting to have pretty-printed output on one of them. Unfortunately, binding *PRINT-PRETTY* to T will affect writes to both streams.

    One solution would be to have streams look up their own print-pretty flag with (SYMBOL-VALUE (STREAM-PRETTY-PRINT STREAM)) and have the caller specify the dynamic variable they want:

    (defvar *print-pretty-1* nil)
(setf (stream-print-pretty stream-1) '*print-pretty-1*)
(let ((*print-pretty-1* t))
  (spaghetti stream-1 stream-2))

    Note that if the default STREAM-PRINT-PRETTY is '*PRINT-PRETTY*, then we have the normal Common Lisp behaviour. Setting STREAM-PRINT-PRETTY to NIL or T also works, because they are self-evaluating.

    The above hypothetical example demonstrates the concept of boolean-valued symbols on CL:STREAMs. In Journal, they are used by MAKE-LOG-DECORATOR and PPRINT-JOURNALs.

  • [glossary-term] readable

    In Common Lisp, readable objects are those that can be printed readably. Anything written to stream-based journals needs to be readable.

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A Common Lisp library for logging, tracing, testing and persistence.

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