Port Agent

A RPC-like facility for making inter-thread function calls.

Introduction

Port Agent provides a simple and intuitive interface that makes inter-thread function calls easy. Please see the Usage or Examples for instructions on how to use Port Agent in your application.

Features

• Port Agent will marshal the return value or Error from the other thread back to the caller.
• The other thread may be the main thread or a worker thread.
• Registered functions (i.e., Agent.register) persist until deregistered (i.e., Agent.deregister) .
• Late binding registrants will be called with previously awaited invocations.

Table of Contents

• Concepts
  • Agent
• API
• Usage
  • How to create an Agent instance.
• Examples
  • A simple example.
  • A comprehensive example.
• Notes
  • Support for BroadcastChannels.
  • Support for other communication channels.

Concepts

Agent

An instance of an Agent facilitates communication across threads. The Agent can be used in order to register a function in one thread and call it from another thread. Calls may be made from the main thread to a worker thread, and conversely from a worker thread to the main thread.

Late binding registrants will be called with previously awaited invocations; thus preventing a race condition. This means that you may await a call to a function that has not yet been registered. Once the function is registered in the other thread it will be called and its return value or Error will be marshalled back to the caller.

Please see the Examples for variations on the Agent's usage.

API

The Agent class.

port_agent.Agent(port)

• port <threads.MessagePort> or <threads.Worker> The message port.

agent.call<T>(name, ...args)

• name <string> The name of the registered function.

• ...args <Array<unknown>> Arguments to be passed to the registered function.

• Returns: <Promise<T>>

• Errors:

  • If the registered function in the other thread throws an Error, the Error will be marshalled back from the other thread to this thread and the Promise will reject with the Error as its failure reason.
  • If a worker thread throws an unhandled exception while a call is awaited, the Error will be marshalled back from the other thread to this thread and the Promise will reject with the unhandled exception as its failure reason.
  • If a worker exits while a call is awaited, the Error will be marshalled back from the other thread to this thread and the Promise will reject with the exit code as its failure reason.

agent.register(name, fn)

• name <string> The name of the registered function.

• fn <(...args: Array<any>) => any> The registered function.

• Returns: <void>

agent.deregister(name)

• name <string> The name of the registered function.

• Returns: <void>

Usage

How to create an Agent instance.

Import the Agent class from Port Agent.

import { Agent } from 'port_agent';

You can create a new Agent by passing a parentPort or a Worker instance to the Agent constructor:

In the main thread,

const worker = new Worker(fileURLToPath(import.meta.url));
const agent = new Agent(worker);

or, in a worker thread,

const agent = new Agent(worker_threads.parentPort);

How to use an Agent instance.

You can register a function in the main thread or in a worker thread using the Agent.register method:

agent.register('hello_world', (value: string): string => `Hello, ${value} world!`);

You can call a function registered in another thread (i.e., the main thread or a worker thread) using the Agent.call method:

const greeting = await agent.call<string>('hello_world', 'happy');
console.log(greeting); // Hello, happy world!

Examples

A simple example. ^(example)

In this example you will:

1. Instantiate a worker thread.
2. Instantiate an Agent in the main thread.
3. Use the Agent to await a call to the hello_world function.
4. Instantiate an Agent in the worker thread.
5. Use the Agent in order to register a function named abend that will throw an Error when it is called.
6. Use the Agent in order to register a function named hello_word to handle calls to the hello_world function.
7. Use the Agent in order to register a function named add that will return the sum of two operands.
8. Resolve (3) and log the greeting to the console.
9. Use the Agent to await a call to the function named add.
10. Resolve (9) and log the result to the console.
11. Use the Agent to await a call to the function named abend.
12. Catch the Error from (11) and log the stack trace to the console.
13. Terminate the thread.

examples/simple/index.js

import { Worker, isMainThread, parentPort } from 'node:worker_threads';
import { fileURLToPath } from 'node:url';
import { Agent } from 'port_agent';

if (isMainThread) { // This is the main thread.
    void (async () => {
        const worker = new Worker(fileURLToPath(import.meta.url)); // (1)
        const agent = new Agent(worker); // (2)
        try {
            const greeting = await agent.call('hello_world', 'another'); // (3)
            console.log(greeting); // (8)

            const result = await agent.call('add', 1, 1); // (9)
            console.log(result); // (10)

            await agent.call('abend', "This Error is expected, indeed.") // (11)
        }
        catch (err) {
            console.error(err); // (12)
        }
        finally {
            worker.terminate(); // (13)
        }
    })();
}
else { // This is a worker thread.
    if (parentPort) {
        const agent = new Agent(parentPort); // (4)
        agent.register('abend', (message) => { throw new Error(message); }); // (5)
        agent.register('hello_world', (value) => `Hello, ${value} world!`); // (6)
        agent.register('add', (a, b) => a + b); // (7)
    }
}

The example will log to the console something similar to this:

Hello, another world!
2
Error: This Error is expected, indeed.
    at file:///port_agent/examples/simple/index.js:29:54
    at Agent.tryPost (/port_agent/examples/simple/node_modules/port_agent/dist/index.js:145:33)
    at MessagePort.<anonymous> (/port_agent/examples/simple/node_modules/port_agent/dist/index.js:114:36)
    at [nodejs.internal.kHybridDispatch] (node:internal/event_target:762:20)
    at exports.emitMessage (node:internal/per_context/messageport:23:28)

Please see the Simple example for a working implementation.

A comprehensive example. ^(example)

Please see the Comprehensive example for a working implementation.

Notes

Support for BroadcastChannels.

Port Agent supports one to one communication over a MessagePort. BroadcastChannels are not presently supported.

Support for other communication channels.

Port Agent is strictly focused on efficient communication over MessagePorts. Port Agent will not support communication over other communication channels e.g., Sockets, IPC, etc.