Gproc Publish/Subscribe patterns
This module implements a few convenient functions for publish/subscribe.
Authors: Ulf Wiger (ulf@wiger.net).
Publish/subscribe with Gproc relies entirely on gproc properties and counters. This makes for a very concise implementation, as the monitoring of subscribers and removal of subscriptions comes for free with Gproc.
Using this module instead of rolling your own (which is easy enough) brings the benefit of consistency, in tracing and debugging. The implementation can also serve to illustrate how to use gproc properties and counters to good effect.
event() = any()
msg() = any()
scope() = l | g
status() = 1 | 0
| change_cond/3 | Change the condition specification of an existing subscription. |
| create_single/2 | Creates a single-shot subscription entry for Event. |
| delete_single/2 | Deletes the single-shot subscription for Event. |
| disable_single/2 | Disables the single-shot subscription for Event. |
| enable_single/2 | Enables the single-shot subscription for Event. |
| list_singles/2 | Lists all single-shot subscribers of Event, together with their status. |
| list_subs/2 | List the pids of all processes subscribing to Event |
| notify_single_if_true/4 | Create/enable a single subscription for event; notify at once if F() -> true. |
| publish/3 | Publish the message Msg to all subscribers of Event |
| publish_cond/3 | Publishes the message Msg to conditional subscribers of Event |
| subscribe/2 | Subscribe to events of type Event |
| subscribe_cond/3 | Subscribe conditionally to events of type Event |
| tell_singles/3 | Publish Msg to all single-shot subscribers of Event |
| unsubscribe/2 | Remove subscribtion created using subscribe(Scope, Event) |
change_cond(Scope::scope(), Event::event(), Spec::undefined | ets:match_spec()) -> true
Change the condition specification of an existing subscription.
This function atomically changes the condition spec of an existing
subscription (see subscribe_cond/3). An exception is raised if
the subscription doesn't already exist.
Note that this function can also be used to change a conditional subscription
to an unconditional one (by setting Spec = undefined), or a 'normal'
subscription to a conditional one.
Creates a single-shot subscription entry for Event
Single-shot subscriptions behave similarly to the {active,once} property of sockets.
Once a message has been published, the subscription is disabled, and no more messages
will be delivered to the subscriber unless the subscription is re-enabled using
enable_single/2.
The function creates a gproc counter entry, {c,Scope,{gproc_ps_event,Event}}, which
will have either of the values 0 (disabled) or 1 (enabled). Initially, the value
is 1, meaning the subscription is enabled.
Counters are used in this case, since they can be atomically updated by both the
subscriber (owner) and publisher. The publisher sets the counter value to 0 as soon
as it has delivered a message.
Deletes the single-shot subscription for Event
This function deletes the counter entry representing the single-shot description. An exception will be raised if there is no such subscription.
Disables the single-shot subscription for Event
This function changes the value of the corresponding gproc counter to 0 (disabled).
The subscription remains (e.g. for debugging purposes), but with a 'disabled' status. This function is insensitive to concurrency, using 'wrapping' ets counter update ops. This guarantees that the counter will have either the value 1 or 0, depending on which update happened last.
The return value indicates the previous status.
Enables the single-shot subscription for Event
This function changes the value of the corresponding gproc counter to 1 (enabled).
After enabling, the subscriber will receive the next message published for Event,
after which the subscription is automatically disabled.
This function is insensitive to concurrency, using 'wrapping' ets counter update ops. This guarantees that the counter will have either the value 1 or 0, depending on which update happened last.
The return value indicates the previous status.
Lists all single-shot subscribers of Event, together with their status
List the pids of all processes subscribing to Event
This function uses gproc:select/2 to find all properties indicating a subscription.
Create/enable a single subscription for event; notify at once if F() -> true
This function is a convenience function, wrapping a single-shot pub/sub around a
user-provided boolean test. Msg should be what the publisher will send later, if the
immediate test returns false.
Publish the message Msg to all subscribers of Event
The message delivered to each subscriber will be of the form:
{gproc_ps_event, Event, Msg}
The function uses gproc:send/2 to send a message to all processes which have a
property {p,Scope,{gproc_ps_event,Event}}.
Publishes the message Msg to conditional subscribers of Event
The message will be delivered to each subscriber provided their respective condition tests succeed.
See also: subscribe_cond/3.
Subscribe to events of type Event
Any messages published with gproc_ps:publish(Scope, Event, Msg) will be
delivered to the current process, along with all other subscribers.
This function creates a property, {p,Scope,{gproc_ps_event,Event}}, which
can be searched and displayed for debugging purposes.
Note that, as with gproc:reg/1, this function will raise an
exception if you try to subscribe to the same event twice from the same
process.
subscribe_cond(Scope::scope(), Event::event(), Spec::undefined | ets:match_spec()) -> true
Subscribe conditionally to events of type Event
This function is similar to subscribe/2, but adds a condition
in the form of a match specification.
The condition is tested by the publish_cond/3 function
and a message is delivered only if the condition is true. Specifically,
the test is:
ets:match_spec_run([Msg], ets:match_spec_compile(Cond)) == [true]
In other words, if the match_spec returns true for a message, that message
is sent to the subscriber. For any other result from the match_spec, the
message is not sent. Cond == undefined means that all messages will be
delivered (that is, publish_cond/3 will treat 'normal' subscribers just
like publish/3 does, except that publish/3 strictly speaking
ignores the Value part of the property completely, whereas publish_cond/3
expects it to be either undefined or a valid match spec).
This means that Cond=undefined and Cond=[{'_',[],[true]}] are
equivalent.
Note that, as with gproc:reg/1, this function will raise an
exception if you try to subscribe to the same event twice from the same
process.
Publish Msg to all single-shot subscribers of Event
The subscriber status of each active subscriber is changed to 0 (disabled) before
delivering the message. This reduces the risk that two different processes will be able
to both deliver a message before disabling the subscribers. This could happen if the
context switch happens just after the select operation (finding the active subscribers)
and before the process is able to update the counters. In this case, it is possible
that more than one can be delivered.
The way to prevent this from happening is to ensure that only one process publishes
for Event.
Remove subscribtion created using subscribe(Scope, Event)
This removes the property created through subscribe/2.