Port Agent

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

Features

• Port Agent will marshall the return value or Error back to the caller.
• Registered functions (i.e., Agent.register) are persistent.
• Late binding registrants will be called with previously awaited invocations.

Table of Contents

1. API
2. Examples

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>>

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>

Examples

An Example

In this example you will:

1. Instantiate a Worker thread.
2. Instantiate an Agent in the Main thread.
3. Use the Agent to call the hello_world function.
   • At this point the hello_world function has not yet been registered in the Worker thread.
4. Wait for the Worker to come online.
5. Instantiate an Agent in the Worker thread.
6. Use the Agent to register the hello_world function in the Worker.
7. Use the Agent to register the a_reasonable_assertion function in the Worker.
8. Use the Agent to call the function registered as hello_world.
9. Await (3) and log the return value.
10. Await (8) and log the return value.
11. Use the Agent to call the function registered as a_reasonable_assertion.
12. Await (11).
13. Catch the Error and log the stack trace in the Main thread.
    • The Error was marshalled from the Error produced by the reasonable assertion that was made in the nowThrowAnError function in the Worker thread.
14. Examine the output.

./tests/test/index.ts

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

if (isMainThread) { // This is the Main Thread.
    void (async () => {

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

        worker.on('online', async () => {
            try {
                const greeting = await agent.call<string>('hello_world', 'again, another');

                console.log(greeting);

                await agent.call('a_reasonable_assertion', 'To err is Human.');
            }
            catch (err) {
                console.error(`Now, back in the Main Thread, we will handle the`, err);
            }
            finally {
                void worker.terminate();
            }
        });

        // This call will be invoked once the `hello_world` function has been bound in the Worker.
        const greeting = await agent.call<string>('hello_world', 'another');
        console.log(greeting);
    })();
} else { // This is a Worker Thread.

    function nowThrowAnError(message: string) {
        // This seems reasonable...
        assert.notEqual(typeof new Object(), typeof null, message);
    }

    function callAFunction(message: string) {
        nowThrowAnError(message);
    }

    if (parentPort) {
        const agent = new Agent(parentPort);

        agent.register('hello_world', (value: string): string => `Hello ${value} world!`);
        
        agent.register('a_reasonable_assertion', callAFunction); // This will throw in the Main thread.
    }
} 

This example should log to the console something that looks similar to this:

Hello another world!
Hello again, another world!
Now, back in the Main Thread, we will handle the AssertionError [ERR_ASSERTION]: To err is Human.
    at nowThrowAnError (file:///port_agent/tests/test/dist/index.js:31:16)
    at callAFunction (file:///port_agent/tests/test/dist/index.js:34:9)
    at Agent.tryPost (/port_agent/dist/index.js:92:33)
    at MessagePort.<anonymous> (/port_agent/dist/index.js:62:36)
    at [nodejs.internal.kHybridDispatch] (node:internal/event_target:762:20)
    at exports.emitMessage (node:internal/per_context/messageport:23:28) {
  generatedMessage: false,
  code: 'ERR_ASSERTION',
  actual: 'object',
  expected: 'object',
  operator: 'notStrictEqual'
}

Run the test.

You can run the test using:

npm run test