This is the Rust implementation of the waPC host protocol. This crate defines the WapcHost struct and the WebAssemblyEngineProvider trait to provide dynamic execution of WebAssembly modules. For information on the implementations with WebAssembly engines, check out the provider crates below:
The wapc crate provides a high-level WebAssembly host runtime that conforms to an RPC mechanism called waPC (WebAssembly Procedure Calls). waPC is designed to be a fixed, lightweight standard allowing both sides of the guest/host boundary to make method calls containing arbitrary binary payloads. Neither side
of the contract is ever required to perform explicit allocation, ensuring maximum portability for wasm targets that might behave differently in the presence of garbage collectors and memory
relocation, compaction, etc.
To use wapc, first you'll need a waPC-compliant WebAssembly module (referred to as the guest) to load and execute. You can find a number of these samples available in the GitHub repository.
Next, you will need to chose a runtime engine. waPC describes the function call flow required for wasm-RPC, but it does not dictate how the low-level WebAssembly function calls are made. This allows you to select whatever engine best suits your needs, whether it's a JIT-based engine or an interpreter-based one. Simply instantiate anything that implements the WebAssemblyEngineProvider trait and pass it to the WapcHost constructor and the WapcHost will facilitate all RPCs.
To make function calls, ensure that you provided a suitable host callback function (or closure) when you created your WapcHost. Then invoke the call function to initiate the RPC flow.
The following is an example of synchronous, bi-directional procedure calls between a WebAssembly host runtime and the guest module.
```rust
use wasmtime_provider::WasmtimeEngineProvider; // Or Wasm3EngineProvider
use wapc::WapcHost;
use std::error::Error;
pub fn main() -> Result<(), Box
// Sample host callback that prints the operation a WASM module requested. let host_callback = |id: u64, bd: &str, ns: &str, op: &str, payload: &[u8]| { println!("Guest {} invoked '{}->{}:{}' with a {} byte payload", id, bd, ns, op, payload.len()); // Return success with zero-byte payload. Ok(vec![]) };
let file = "../../wasm/crates/wasm-basic/build/wasmbasic.wasm"; let modulebytes = std::fs::read(file)?;
let engine = WasmtimeEngineProvider::new(&modulebytes, None)?; let host = WapcHost::new(Box::new(engine), Some(Box::new(hostcallback)))?;
let res = host.call("ping", b"payload bytes")?; assert_eq!(res, b"payload bytes");
Ok(()) } ```
For running examples, take a look at the examples available in the individual engine provider repositories:
waPC is reactive. Hosts make requests and guests respond. During a request, guests can initiate calls back to the host and interact with the environment (via WASI). When a request is done the guest should be considered parked until the next request.
The following is a detailed outline of which functions are invoked and in which order to support
a waPC exchange flow, which is always triggered by a consumer invoking the call function. Invoking
and handling these low-level functions is the responsibility of the engine provider, while
orchestrating the high-level control flow is the job of the WapcHost.
__guest_call on the WebAssembly module (via the engine provider)__guest_request function to instruct the host to write the request parameters to linear memoryop_len and msg_len parameters long with the pointer values it generated in step 2 to retrieve the operation (UTF-8 string) and payload (opaque byte array)__host_call on host with pointers and lengths indicating the binding, namespace, operation, and payload.__host_response and host_response_len functions to obtain and interpret results__host_error_len and __host_error to obtain the host error if indicated (__host_call returns 0)
guest_error to indicate if an error occurred during processingguest_response to store the opaque response payload__guest_callList of functions that must be exported by the host (imported by the guest)
| Module | Function | Parameters | Description |
| ------ | --------------------- | -------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------ |
| wapc | __hostcall | brptr: i32
bdlen: i32
nsptr: i32
nslen: i32
opptr: i32
oplen: i32
ptr: i32
len: i32
-> i32 | Invoked to initiate a host call |
| wapc | __consolelog | ptr: i32, len: i32 | Allows guest to log to stdout |
| wapc | __guestrequest | opptr: i32
ptr: i32 | Writes the guest request payload and operation name to linear memory at the designated locations |
| wapc | __hostresponse | ptr: i32 | Instructs host to write the host response payload to the given location in linear memory |
| wapc | __hostresponselen | -> i32 | Obtains the length of the current host response |
| wapc | __guestresponse | ptr: i32
len: i32 | Tells the host the size and location of the current guest response payload |
| wapc | __guesterror | ptr: i32
len: i32 | Tells the host the size and location of the current guest error payload |
| wapc | __hosterror | ptr: i32 | Instructs the host to write the host error payload to the given location |
| wapc | __hosterrorlen | -> i32 | Queries the host for the length of the current host error (0 if none) |
List of functions that must be exported by the guest (invoked by the host)
| Function | Parameters | Description |
| -------------- | ---------------------------- | ------------------------------------------------------------------ |
| __guestcall | oplen: i32
msg_len: i32 | Invoked by the host to start an RPC exchange with the guest module |