Crate wapc

waPC

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:

• wasmtime-provider
• wasm3-provider

wapc

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.

Example

The following is an example of synchronous, bi-directional procedure calls between a WebAssembly host runtime and the guest module.

use wasmtime_provider::WasmtimeEngineProvider; // Or Wasm3EngineProvider
use wapc::WapcHost;
use std::error::Error;
pub fn main() -> Result<(), Box<dyn Error>> {

  // 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/wasm_basic.wasm";
  let module_bytes = std::fs::read(file)?;

  let engine = WasmtimeEngineProvider::new(&module_bytes, None)?;
  let host = WapcHost::new(Box::new(engine), Some(Box::new(host_callback)))?;

  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:

• wasmtime-provider - Utilizes the Bytecode Alliance runtime wasmtime for WebAssembly JIT compilation and execution.
• wasm3-provider - Uses the wasm3 C interpreter runtime (with a Rust wrapper)

Notes

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.

RPC Exchange Flow

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.

1. Host invokes __guest_call on the WebAssembly module (via the engine provider)
2. Guest calls the __guest_request function to instruct the host to write the request parameters to linear memory
3. Guest uses the op_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)
4. Guest performs work
5. (Optional) Guest invokes __host_call on host with pointers and lengths indicating the binding, namespace, operation, and payload.
6. (Optional) Guest can use __host_response and host_response_len functions to obtain and interpret results
7. (Optional) Guest can use __host_error_len and __host_error to obtain the host error if indicated (__host_call returns 0)
   1. Steps 5-7 can repeat with as many different host calls as the guest needs
8. Guest will call guest_error to indicate if an error occurred during processing
9. Guest will call guest_response to store the opaque response payload
10. Guest will return 0 (error) or 1 (success) at the end of __guest_call

Required Host Exports

List of functions that must be exported by the host (imported by the guest)

Module	Function	Parameters	Description
wapc	__host_call	br_ptr: i32 bd_len: i32 ns_ptr: i32 ns_len: i32 op_ptr: i32 op_len: i32 ptr: i32 len: i32 -> i32	Invoked to initiate a host call
wapc	__console_log	ptr: i32, len: i32	Allows guest to log to stdout
wapc	__guest_request	op_ptr: i32 ptr: i32	Writes the guest request payload and operation name to linear memory at the designated locations
wapc	__host_response	ptr: i32	Instructs host to write the host response payload to the given location in linear memory
wapc	__host_response_len	-> i32	Obtains the length of the current host response
wapc	__guest_response	ptr: i32 len: i32	Tells the host the size and location of the current guest response payload
wapc	__guest_error	ptr: i32 len: i32	Tells the host the size and location of the current guest error payload
wapc	__host_error	ptr: i32	Instructs the host to write the host error payload to the given location
wapc	__host_error_len	-> i32	Queries the host for the length of the current host error (0 if none)

Required Guest Exports

List of functions that must be exported by the guest (invoked by the host)

Function	Parameters	Description
__guest_call	op_len: i32 msg_len: i32	Invoked by the host to start an RPC exchange with the guest module

Modules

• errors
  Library-specific error types and utility functions
• wapc_functions
  A list of the function names that are part of each waPC conversation

Structs

• Invocation
  Represents a waPC invocation, which is a combination of an operation string and the corresponding binary payload
• ModuleState
  Module state is essentially a ‘handle’ that is passed to a runtime engine to allow it to read and write relevant data as different low-level functions are executed during a waPC conversation
• WapcHost
  A WebAssembly host runtime for waPC-compliant modules
• WasiParams
  Parameters defining the options for enabling WASI on a module (if applicable)

Constants

• HOST_NAMESPACE
  The host module name / namespace that guest modules must use for imports

Traits

• ModuleHost
  The module host (waPC) must provide an implementation of this trait to the engine provider to enable waPC function calls.
• WebAssemblyEngineProvider
  An engine provider is any code that encapsulates low-level WebAssembly interactions such as reading from and writing to linear memory, executing functions, and mapping imports in a way that conforms to the waPC conversation protocol.

Type Definitions

• HostCallback
  The signature of a Host Callback function.