Design Documentation Update

doc/design.md

@@ -13,7 +13,7 @@ There is also optional support for thread mapping, so each thread making interpr
 
 Libmultiprocess acts as a pure wrapper or layer over the underlying protocol. Clients and servers written in other languages, but using a shared capnproto schema can communicate with interprocess counterparties using libmultiprocess without having to use libmultiprocess themselves or having to know about the implementation details of libmultiprocess.
 
-### Internals
+## Core Architecture
 
 The `ProxyClient` and `ProxyServer` generated classes are not directly exposed to the user, as described in [usage.md](usage.md). Instead, they wrap C++ interfaces and appear to the user as pointers to an interface. They are first instantiated when calling `ConnectStream` and `ServeStream` respectively for creating the `InitInterface`. These methods establish connections through sockets, internally creating `Connection` objects wrapping a `capnp::RpcSystem` configured for client and server mode respectively.
 
@@ -25,7 +25,190 @@ When a generated method on the `ProxyClient` is called, it calls `clientInvoke`
 
 On the server side, the `capnp::RpcSystem` receives the capnp request and invokes the corresponding C++ method through the corresponding `ProxyServer` and the heavily templated `serverInvoke` triggering a `ServerCall`. The return values from the actual C++ methods are copied into capnp responses by `ServerRet` and exceptions are caught and copied by `ServerExcept`. The two are connected through `ServerField`. The main method driving execution of a request is `PassField`, which is invoked through `ServerField`. Instantiated interfaces, or capabilities in capnp speak, are tracked and owned by the server's `capnp::RpcSystem`.
 
-## Interface descriptions
+## Request and Response Flow
+
+Method parameters and return values are serialized using Cap'n Proto's Builder objects (for sending) and Reader objects (for receiving). Input parameters flow from the client to the server, while output parameters (return values) flow back from the server to the client.
+
+```mermaid
+sequenceDiagram
+    participant clientInvoke
+    participant BuildField as BuildField<br/>(Client)
+    participant ReadField_C as ReadField<br/>(Client)
+    participant Request as Request<br/>message
+    participant serverInvoke
+    participant ReadField as ReadField<br/>(Server)
+    participant BuildField_S as BuildField<br/>(Server)
+    participant Response as Response<br/>message
+
+    Note over clientInvoke,ReadField: Input Parameter Flow
+    clientInvoke->>BuildField: BuildField(input_arg)
+    BuildField->>Request: Serialize input
+    Request->>serverInvoke: Cap'n Proto message
+    serverInvoke->>ReadField: Deserialize input
+
+    Note over clientInvoke,Response: Output Parameter Flow
+    serverInvoke-->>BuildField_S: BuildField(output)
+    BuildField_S-->Response: Serialize output
+    Response-->>ReadField_C: Cap'n Proto message
+    ReadField_C-->>clientInvoke: Deserialize output
+```
+
+### Detailed Serialization Mechanism
+
+Parameters are represented as Fields that must be set on Cap'n Proto Builder objects (for sending) and read from Reader objects (for receiving).
+
+#### Building Fields
+
+`BuildField` uses a generated parameter `Accessor` to set the appropriate field in the Cap'n Proto Builder object.
+
+```mermaid
+sequenceDiagram
+    participant clientInvoke as clientInvoke or<br/>serverInvoke
+    participant BuildField
+    participant Accessor
+    participant Builder as Params::Builder
+
+    Note over clientInvoke,Builder: Serializing Parameters
+    clientInvoke->>BuildField: BuildField(param1)
+    BuildField->>Accessor: Use generated field accessor
+    Accessor->>Builder: builder.setField1(param1)
+
+    clientInvoke->>BuildField: BuildField(param2)
+    BuildField->>Accessor: Use generated field Accessor
+    Accessor->>Builder: builder.setField2(param2)
+```
+
+#### Reading Fields
+
+`ReadField` uses a generated parameter `Accessor` to read the appropriate field from the Cap'n Proto Reader object and reconstruct C++ parameters.
+
+```mermaid
+sequenceDiagram
+    participant serverInvoke as clientInvoke or<br/>serverInvoke
+    participant ReadField
+    participant Accessor
+    participant Reader as Params::Reader
+    participant ServerCall
+
+    Note over serverInvoke,ServerCall: Deserializing Parameters
+    serverInvoke->>ReadField: Read param1
+    ReadField->>Accessor: Use generated field accessor
+    Accessor->>Reader: reader.getField1()
+    Reader-->>ServerCall: call function with param1
+```
+
+## Server-Side Request Processing
+
+The generated server code uses a Russian nesting doll structure to process method fields. Each `ServerField` wraps another `ServerField` (for the next parameter), or wraps `ServerRet` (for the return value), which finally wraps `ServerCall` (which invokes the actual C++ method).
+
+Each `ServerField` invokes `PassField`, which:
+1. Calls `ReadField` to deserialize the parameter from the `Params::Reader`
+2. Calls the next nested layer's `invoke()` with the accumulated parameters
+3. Calls `BuildField` to serialize the parameter back if it's an output parameter
+
+`ServerRet` invokes the next layer (typically `ServerCall`), stores the result, and calls `BuildField` to serialize it into the `Results::Builder`.
+
+`ServerCall` uses the generated `ProxyMethod<MethodParams>::impl` pointer-to-member to invoke the actual C++ method on the wrapped implementation object.
+
+```mermaid
+sequenceDiagram
+    participant serverInvoke
+    participant SF1 as ServerField<br/>(param 1)
+    participant SF2 as ServerField<br/>(param 2)
+    participant SR as ServerRet<br/>(return value)
+    participant SC as ServerCall
+    participant PMT as ProxyMethodTraits
+    participant Impl as Actual C++ Method
+
+    serverInvoke->>SF1: SF1::invoke 
+    SF1->>SF2: SF2::invoke
+    SF2->>SR: SR::invoke
+    SR->>SC: SC::invoke
+    SC->>PMT: PMT::invoke
+    PMT->>Impl: Call impl method
+    Impl->>PMT: return
+    PMT->>SC: return
+    SC->>SR: return
+    SR->>SF2: return
+    SF2->>SF1: return
+    SF1->>serverInvoke: return
+```
+
+## Advanced Features
+
+### Callbacks
+
+Callbacks (passed as `std::function` arguments) are intercepted by `CustomBuildField` and converted into Cap'n Proto capabilities that can be invoked across process boundaries. On the receiving end, `CustomReadField` intercepts the capability and constructs a `ProxyCallFn` object with an `operator()` that sends function calls back over the socket to invoke the original callback.
+
+```mermaid
+sequenceDiagram
+    participant CT as Client Thread
+    participant C as clientInvoke
+    participant CBF1 as CustomBuildField (Client)
+    participant S as Socket
+    participant CRF1 as CustomReadField (Server)
+    participant Srv as Server Code
+    participant PCF as ProxyCallFn
+
+    C->>CBF1: send function parameter
+    CBF1->>S: creates a Server for the function and sends a capability
+    S->>CRF1: receives a capability and creates ProxyCallFn
+    CRF1->>Srv:
+    Srv->>PCF: call the callback
+    PCF-->>CT: sends request to Client
+```
+
+### Thread Mapping
+
+Thread mapping enables each client thread to have a dedicated server thread processing its requests, preserving thread-local state and allowing recursive mutex usage across process boundaries.
+
+Thread mapping is initialized by defining an interface method with a `ThreadMap` parameter and/or response. The example below adds `ThreadMap` to the `construct` method because libmultiprocess calls the `construct` method automatically.
+
+```capnp
+interface InitInterface $Proxy.wrap("Init") { 
+    construct @0 (threadMap: Proxy.ThreadMap) -> (threadMap :Proxy.ThreadMap);
+}
+```
+
+- **ThreadMap in parameter**: The client's `CustomBuildField` creates a `ThreadMap::Server` capability and sends it to the server, where `CustomReadField` stores the `ThreadMap::Client` in `connection.m_thread_map`
+- **ThreadMap in response**: The server's `CustomBuildField` creates a `ThreadMap::Server` capability and sends it to the client, where `CustomReadField` stores the `ThreadMap::Client` in `connection.m_thread_map`
+
+You can specify ThreadMap in the parameter only, response only, or both:
+- **Parameter only**: Server can create threads on the client
+- **Response only**: Client can create threads on the server
+- **Both (as shown)**: Bidirectional thread creation
+
+When both parameter and response include ThreadMap, both processes end up with `ThreadMap::Client` capabilities pointing to each other's `ThreadMap::Server`, allowing both sides to create threads on the other process.
+
+### Async Processing with Context
+
+By adding a `Context` parameter to a method in the capnp interface file, you enable async processing where the client tells the server to execute the request in a separate worker thread. For example:
+
+```capnp
+processData @5 (context :Proxy.Context, data :Data) -> (result :Result);
+```
+
+If a method does not have a `Context` parameter, then libmultiprocess will execute IPC requests invoking that method on the I/O event loop thread. This is fine for fast and non-blocking methods, but should be avoided for any methods that are slow or blocking or make any IPC calls(including callbacks to the client), since as long as the method is executing, the Cap'n Proto event loop will not be able to perform any I/O.
+
+When a method has a `Context` parameter:
+
+**Client side** (`CustomBuildField`):
+If this is the first asynchronous request made from the current client thread, `CustomBuildField` will:
+1. Call `connection.m_thread_map.makeThreadRequest()` to request a dedicated worker thread on the server (stored in `request_threads` map)
+2. Set the remote thread capability in `Context.thread`
+3. Create a local `Thread::Server` object for the current thread (stored in `callback_threads` map)
+4. Set the local thread capability in `Context.callbackThread`
+
+Subsequent requests will resuse the existing thread capabilites held in `callback_threads` and `request_threads`.
+
+**Server side** (`PassField`):
+1. Looks up the local `Thread::Server` object specified by `context.thread`
+2. The worker thread:
+   - Stores `context.callbackThread` in its `request_threads` map (so callbacks go to the right client thread)
+   - Posts the work lambda to that thread's queue via `waiter->post(invoke)`
+   - Cleans up the `request_threads` entry
+
+## Interface Definitions
 
 As explained in the [usage](usage.md) document, interface descriptions need to be consumed both by the _libmultiprocess_ code generator, and by C++ code that calls and implements the interfaces. The C++ code only needs to know about C++ arguments and return types, while the code generator only needs to know about capnp arguments and return types, but both need to know class and method names, so the corresponding `.h` and `.capnp` source files contain some of the same information, and have to be kept in sync manually when methods or parameters change. Despite the redundancy, reconciling the interface definitions is designed to be _straightforward_ and _safe_. _Straightforward_ because there is no need to write manual serialization code or use awkward intermediate types like [`UniValue`](https://github.com/bitcoin/bitcoin/blob/master/src/univalue/include/univalue.h) instead of native types. _Safe_ because if there are any inconsistencies between API and data definitions (even minor ones like using a narrow int data type for a wider int API input), there are errors at build time instead of errors or bugs at runtime.