doc: add AsyncResource + Worker pool example

doc/api/async_hooks.md

@@ -682,6 +682,126 @@ never be called.
 * Returns: {number} The same `triggerAsyncId` that is passed to the
 `AsyncResource` constructor.
 
+<a id="async-resource-worker-pool"></a>
+### Using `AsyncResource` for a `Worker` thread pool
+
+The following example shows how to use the `AsyncResource` class to properly
+provide async tracking for a [`Worker`][] pool. Other resource pools, such as
+database connection pools, can follow a similar model.
+
+Assuming that the task is adding two numbers, using a file named
+`task_processor.js` with the following content:
+
+```js
+const { parentPort } = require('worker_threads');
+parentPort.on('message', (task) => {
+  parentPort.postMessage(task.a + task.b);
+});
+```
+
+a Worker pool around it could use the following structure:
+
+```js
+const { AsyncResource } = require('async_hooks');
+const { EventEmitter } = require('events');
+const path = require('path');
+const { Worker } = require('worker_threads');
+
+const kTaskInfo = Symbol('kTaskInfo');
+const kWorkerFreedEvent = Symbol('kWorkerFreedEvent');
+
+class WorkerPoolTaskInfo extends AsyncResource {
+  constructor(callback) {
+    super('WorkerPoolTaskInfo');
+    this.callback = callback;
+  }
+
+  done(err, result) {
+    this.runInAsyncScope(this.callback, null, err, result);
+    this.emitDestroy();  // `TaskInfo`s are used only once.
+  }
+}
+
+class WorkerPool extends EventEmitter {
+  constructor(numThreads) {
+    super();
+    this.numThreads = numThreads;
+    this.workers = [];
+    this.freeWorkers = [];
+
+    for (let i = 0; i < numThreads; i++)
+      this.addNewWorker();
+  }
+
+  addNewWorker() {
+    const worker = new Worker(path.resolve(__dirname, 'task_processor.js'));
+    worker.on('message', (result) => {
+      // In case of success: Call the callback that was passed to `runTask`,
+      // remove the `TaskInfo` associated with the Worker, and mark it as free
+      // again.
+      worker[kTaskInfo].done(null, result);
+      worker[kTaskInfo] = null;
+      this.freeWorkers.push(worker);
+      this.emit(kWorkerFreedEvent);
+    });
+    worker.on('error', (err) => {
+      // In case of an uncaught exception: Call the callback that was passed to
+      // `runTask` with the error, and start a new Worker to replace the
+      // current one.
+      if (worker[kTaskInfo])
+        worker[kTaskInfo].done(err, null);
+      else
+        this.emit('error', err);
+      this.addNewWorker();
+    });
+    this.workers.push(worker);
+    this.freeWorkers.push(worker);
+  }
+
+  runTask(task, callback) {
+    if (this.freeWorkers.length === 0) {
+      // No free threads, wait until a worker thread becomes free.
+      this.once(kWorkerFreedEvent, () => this.runTask(task, callback));
+      return;
+    }
+
+    const worker = this.freeWorkers.pop();
+    worker[kTaskInfo] = new WorkerPoolTaskInfo(callback);
+    worker.postMessage(task);
+  }
+
+  close() {
+    for (const worker of this.workers) worker.terminate();
+  }
+}
+
+module.exports = WorkerPool;
+```
+
+Without the explicit tracking added by the `WorkerPoolTaskInfo` objects,
+it would appear that the callbacks are associated with the individual `Worker`
+objects. However, the creation of the `Worker`s is not associated with the
+creation of the tasks and does not provide information about when tasks
+were scheduled.
+
+This pool could be used as follows:
+
+```js
+const WorkerPool = require('./worker_pool.js');
+const os = require('os');
+
+const pool = new WorkerPool(os.cpus().length);
+
+let finished = 0;
+for (let i = 0; i < 10; i++) {
+  pool.runTask({ a: 42, b: 100 }, (err, result) => {
+    console.log(i, err, result);
+    if (++finished === 10)
+      pool.close();
+  });
+}
+```
+
 [`after` callback]: #async_hooks_after_asyncid
 [`before` callback]: #async_hooks_before_asyncid
 [`destroy` callback]: #async_hooks_destroy_asyncid

doc/api/worker_threads.md

@@ -51,7 +51,8 @@ overhead of creating Workers would likely exceed their benefit.
 
 When implementing a worker pool, use the [`AsyncResource`][] API to inform
 diagnostic tools (e.g. in order to provide asynchronous stack traces) about the
-correlation between tasks and their outcomes.
+correlation between tasks and their outcomes. See
+[here][async-resource-worker-pool] for an example implementation.
 
 ## `worker.isMainThread`
 <!-- YAML
@@ -750,6 +751,7 @@ active handle in the event system. If the worker is already `unref()`ed calling
 [`worker.terminate()`]: #worker_threads_worker_terminate
 [`worker.threadId`]: #worker_threads_worker_threadid_1
 [Addons worker support]: addons.html#addons_worker_support
+[async-resource-worker-pool]: async_hooks.html#async-resource-worker-pool
 [HTML structured clone algorithm]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Structured_clone_algorithm
 [Signals events]: process.html#process_signal_events
 [Web Workers]: https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API