wRPC

wRPC is a component-native transport-agnostic RPC protocol and framework based on WebAssembly Interface Types (WIT). If facilitates execution of arbitrary functionality defined in WIT over network or other means of communication.

Main use cases for wRPC are:

• out-of-tree WebAssembly runtime plugins
• distributed WebAssembly component communication

Even though wRPC is designed for Wasm components first and foremost, it is fully usable outside of WebAssembly context and can serve as a general-purpose RPC framework.

wRPC uses component model value encoding (fully defined and pending spec merge at WebAssembly/component-model#336) on the wire.

wRPC supports both dynamic (based on e.g. runtime WebAssembly component type introspection) and static use cases.

For static use cases, wRPC provides WIT binding generators for:

• Rust
• Go

wRPC fully supports the unreleased native WIT stream and future data types along with all currently released WIT functionality.

Design

Transport

wRPC transport is the core abstraction on top of which all the other functionality is built.

A transport represents a multiplexed bidirectional communication channel, over which wRPC invocations are transmitted.

wRPC operates under assumption that transport communication channels can be "indexed" by a sequence of unsigned 32-bit integers, which represent a reflective structural path.

Invocation

As part of every wRPC invocation at least 2 independent, directional byte streams will be established by the chosen transport:

• parameters (client -> server)
• results (server -> client)

wRPC transport implementations MAY (and are encouraged to) provide two more directional communication channels:

• client error (client -> server)
• server error (server -> client)

Error channels are the only channels that are typed, in particular, values sent on these channels are strings.

If async values are being transmitted as parameters or results of an invocation, wRPC MAY send those values on an indexed path asynchronously.

Consider the invocation of WIT function foo from instance wrpc-example:doc/example@0.1.0:

package wrpc-example:doc@0.1.0;

interface example {
    record rec {
        a: stream<u8>,
        b: u32,
    }

    foo: func(v: rec) -> stream<u8>;
}

1. Since foo parameter 0 is a record, which contains an async type (stream) as the first field, wRPC will communicate to the transport that apart from the "root" parameter channel, it may need to receive results at index path 0 (first return value).
2. wRPC will encode the parameters as a single-element tuple in a non-blocking fashion. If full contents of rec.a are not available at the time of encoding, the stream will be encoded as option::none.
3. (concurrently, if in 2. stream was not fully available) wRPC will transfer the contents of the stream<u8> on parameter byte stream at index 0->0 (first field of the record, which is the first parameter) as they become available.
4. wRPC will attempt to decode stream<u8> from the "root" result byte stream.
5. (if 4. decoded an option::none for the stream value) wRPC will attempt to decode stream<u8> from result byte stream at index 0

Note, that the handler of foo (server) MAY:

• receive rec.b value before rec.a is sent or even available
• send a result back to the invoker of foo (client) before it has received rec.a

Repository structure

This repository contains (for all supported languages):

• core libraries and abstractions
• binding generators
• WebAssembly runtime integrations
• wRPC transport implementations

wit-bindgen-wrpc aims to closely match UX of wit-bindgen and therefore includes a subtree merge of the project, which is occasionally merged into this tree.

• wRPC binding generators among other tests, are tested using the wit-bindgen test suite
• wit-bindgen documentation is reused where applicable