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</div> <div id="gtoc"> <ul> <li class="pinned-header">Node.js v16.20.2</li> <li class="picker-header"> <a href="#"> <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> Table of contents </a> <div class="picker"><div class="toc"><ul> <li><span class="stability_2"><a href="#asynchronous-context-tracking">Asynchronous context tracking</a></span> <ul> <li><a href="#introduction">Introduction</a></li> <li><a href="#class-asynclocalstorage">Class: <code>AsyncLocalStorage</code></a> <ul> <li><a href="#new-asynclocalstorage"><code>new AsyncLocalStorage()</code></a></li> <li><span class="stability_1"><a href="#asynclocalstoragedisable"><code>asyncLocalStorage.disable()</code></a></span></li> <li><a href="#asynclocalstoragegetstore"><code>asyncLocalStorage.getStore()</code></a></li> <li><span class="stability_1"><a href="#asynclocalstorageenterwithstore"><code>asyncLocalStorage.enterWith(store)</code></a></span></li> <li><a href="#asynclocalstoragerunstore-callback-args"><code>asyncLocalStorage.run(store, callback[, ...args])</code></a></li> <li><span class="stability_1"><a href="#asynclocalstorageexitcallback-args"><code>asyncLocalStorage.exit(callback[, ...args])</code></a></span></li> <li><a href="#usage-with-asyncawait">Usage with <code>async/await</code></a></li> <li><a href="#troubleshooting-context-loss">Troubleshooting: Context loss</a></li> </ul> </li> <li><a href="#class-asyncresource">Class: <code>AsyncResource</code></a> <ul> <li><a href="#new-asyncresourcetype-options"><code>new AsyncResource(type[, options])</code></a></li> <li><a href="#static-method-asyncresourcebindfn-type-thisarg">Static method: <code>AsyncResource.bind(fn[, type[, thisArg]])</code></a></li> <li><a href="#asyncresourcebindfn-thisarg"><code>asyncResource.bind(fn[, thisArg])</code></a></li> <li><a href="#asyncresourceruninasyncscopefn-thisarg-args"><code>asyncResource.runInAsyncScope(fn[, thisArg, ...args])</code></a></li> <li><a href="#asyncresourceemitdestroy"><code>asyncResource.emitDestroy()</code></a></li> <li><a href="#asyncresourceasyncid"><code>asyncResource.asyncId()</code></a></li> <li><a href="#asyncresourcetriggerasyncid"><code>asyncResource.triggerAsyncId()</code></a></li> <li><a href="#using-asyncresource-for-a-worker-thread-pool">Using <code>AsyncResource</code> for a <code>Worker</code> thread pool</a></li> <li><a href="#integrating-asyncresource-with-eventemitter">Integrating <code>AsyncResource</code> with <code>EventEmitter</code></a></li> </ul> </li> </ul> </li> </ul></div></div> </li> <li class="picker-header"> <a href="#"> <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> Index </a> <div class="picker"><ul> <li><a href="documentation.html" class="nav-documentation">About this documentation</a></li> <li><a href="synopsis.html" class="nav-synopsis">Usage and example</a></li> <li> <a href="index.html">Index</a> </li> </ul> <hr class="line"> <ul> <li><a 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href="all.html">View on single page</a> </li> <li> <a href="async_context.json">View as JSON</a> </li> <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/main/doc/api/async_context.md">Edit on GitHub</a></li> </ul> </div> </li> </ul> </div> <hr> </header> <details id="toc" open><summary>Table of contents</summary><ul> <li><span class="stability_2"><a href="#asynchronous-context-tracking">Asynchronous context tracking</a></span> <ul> <li><a href="#introduction">Introduction</a></li> <li><a href="#class-asynclocalstorage">Class: <code>AsyncLocalStorage</code></a> <ul> <li><a href="#new-asynclocalstorage"><code>new AsyncLocalStorage()</code></a></li> <li><span class="stability_1"><a href="#asynclocalstoragedisable"><code>asyncLocalStorage.disable()</code></a></span></li> <li><a href="#asynclocalstoragegetstore"><code>asyncLocalStorage.getStore()</code></a></li> <li><span class="stability_1"><a href="#asynclocalstorageenterwithstore"><code>asyncLocalStorage.enterWith(store)</code></a></span></li> <li><a href="#asynclocalstoragerunstore-callback-args"><code>asyncLocalStorage.run(store, callback[, ...args])</code></a></li> <li><span class="stability_1"><a href="#asynclocalstorageexitcallback-args"><code>asyncLocalStorage.exit(callback[, ...args])</code></a></span></li> <li><a href="#usage-with-asyncawait">Usage with <code>async/await</code></a></li> <li><a href="#troubleshooting-context-loss">Troubleshooting: Context loss</a></li> </ul> </li> <li><a href="#class-asyncresource">Class: <code>AsyncResource</code></a> <ul> <li><a href="#new-asyncresourcetype-options"><code>new AsyncResource(type[, options])</code></a></li> <li><a href="#static-method-asyncresourcebindfn-type-thisarg">Static method: <code>AsyncResource.bind(fn[, type[, thisArg]])</code></a></li> <li><a href="#asyncresourcebindfn-thisarg"><code>asyncResource.bind(fn[, thisArg])</code></a></li> <li><a href="#asyncresourceruninasyncscopefn-thisarg-args"><code>asyncResource.runInAsyncScope(fn[, thisArg, ...args])</code></a></li> <li><a href="#asyncresourceemitdestroy"><code>asyncResource.emitDestroy()</code></a></li> <li><a href="#asyncresourceasyncid"><code>asyncResource.asyncId()</code></a></li> <li><a href="#asyncresourcetriggerasyncid"><code>asyncResource.triggerAsyncId()</code></a></li> <li><a href="#using-asyncresource-for-a-worker-thread-pool">Using <code>AsyncResource</code> for a <code>Worker</code> thread pool</a></li> <li><a href="#integrating-asyncresource-with-eventemitter">Integrating <code>AsyncResource</code> with <code>EventEmitter</code></a></li> </ul> </li> </ul> </li> </ul></details> <div id="apicontent"> <h2>Asynchronous context tracking<span><a class="mark" href="#asynchronous-context-tracking" id="asynchronous-context-tracking">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynchronous_context_tracking"></a></h2> <p></p><div class="api_stability api_stability_2"><a href="documentation.html#stability-index">Stability: 2</a> - Stable</div><p></p> <p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v16.20.2/lib/async_hooks.js">lib/async_hooks.js</a></p> <section><h3>Introduction<span><a class="mark" href="#introduction" id="introduction">#</a></span><a aria-hidden="true" class="legacy" id="async_context_introduction"></a></h3> <p>These classes are used to associate state and propagate it throughout callbacks and promise chains. They allow storing data throughout the lifetime of a web request or any other asynchronous duration. It is similar to thread-local storage in other languages.</p> <p>The <code>AsyncLocalStorage</code> and <code>AsyncResource</code> classes are part of the <code>node:async_hooks</code> module:</p> <pre class="with-73-chars"><input class="js-flavor-selector" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { <span class="hljs-title class_">AsyncLocalStorage</span>, <span class="hljs-title class_">AsyncResource</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>;</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncLocalStorage</span>, <span class="hljs-title class_">AsyncResource</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>);</code></pre> </section><section><h3>Class: <code>AsyncLocalStorage</code><span><a class="mark" href="#class-asynclocalstorage" id="class-asynclocalstorage">#</a></span><a aria-hidden="true" class="legacy" id="async_context_class_asynclocalstorage"></a></h3> <div class="api_metadata"> <details class="changelog"><summary>History</summary> <table> <tbody><tr><th>Version</th><th>Changes</th></tr> <tr><td>v16.4.0</td> <td><p>AsyncLocalStorage is now Stable. Previously, it had been Experimental.</p></td></tr> <tr><td>v13.10.0, v12.17.0</td> <td><p><span>Added in: v13.10.0, v12.17.0</span></p></td></tr> </tbody></table> </details> </div> <p>This class creates stores that stay coherent through asynchronous operations.</p> <p>While you can create your own implementation on top of the <code>node:async_hooks</code> module, <code>AsyncLocalStorage</code> should be preferred as it is a performant and memory safe implementation that involves significant optimizations that are non-obvious to implement.</p> <p>The following example uses <code>AsyncLocalStorage</code> to build a simple logger that assigns IDs to incoming HTTP requests and includes them in messages logged within each request.</p> <pre class="with-58-chars"><input class="js-flavor-selector" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> http <span class="hljs-keyword">from</span> <span class="hljs-string">'node:http'</span>; <span class="hljs-keyword">import</span> { <span class="hljs-title class_">AsyncLocalStorage</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; <span class="hljs-keyword">const</span> asyncLocalStorage = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>(); <span class="hljs-keyword">function</span> <span class="hljs-title function_">logWithId</span>(<span class="hljs-params">msg</span>) { <span class="hljs-keyword">const</span> id = asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`<span class="hljs-subst">${id !== <span class="hljs-literal">undefined</span> ? id : <span class="hljs-string">'-'</span>}</span>:`</span>, msg); } <span class="hljs-keyword">let</span> idSeq = <span class="hljs-number">0</span>; http.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { asyncLocalStorage.<span class="hljs-title function_">run</span>(idSeq++, <span class="hljs-function">() =></span> { <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'start'</span>); <span class="hljs-comment">// Imagine any chain of async operations here</span> <span class="hljs-title function_">setImmediate</span>(<span class="hljs-function">() =></span> { <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'finish'</span>); res.<span class="hljs-title function_">end</span>(); }); }); }).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>); http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>); http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>); <span class="hljs-comment">// Prints:</span> <span class="hljs-comment">// 0: start</span> <span class="hljs-comment">// 1: start</span> <span class="hljs-comment">// 0: finish</span> <span class="hljs-comment">// 1: finish</span></code><code class="language-js cjs"><span class="hljs-keyword">const</span> http = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:http'</span>); <span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncLocalStorage</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); <span class="hljs-keyword">const</span> asyncLocalStorage = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>(); <span class="hljs-keyword">function</span> <span class="hljs-title function_">logWithId</span>(<span class="hljs-params">msg</span>) { <span class="hljs-keyword">const</span> id = asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(<span class="hljs-string">`<span class="hljs-subst">${id !== <span class="hljs-literal">undefined</span> ? id : <span class="hljs-string">'-'</span>}</span>:`</span>, msg); } <span class="hljs-keyword">let</span> idSeq = <span class="hljs-number">0</span>; http.<span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { asyncLocalStorage.<span class="hljs-title function_">run</span>(idSeq++, <span class="hljs-function">() =></span> { <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'start'</span>); <span class="hljs-comment">// Imagine any chain of async operations here</span> <span class="hljs-title function_">setImmediate</span>(<span class="hljs-function">() =></span> { <span class="hljs-title function_">logWithId</span>(<span class="hljs-string">'finish'</span>); res.<span class="hljs-title function_">end</span>(); }); }); }).<span class="hljs-title function_">listen</span>(<span class="hljs-number">8080</span>); http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>); http.<span class="hljs-title function_">get</span>(<span class="hljs-string">'http://localhost:8080'</span>); <span class="hljs-comment">// Prints:</span> <span class="hljs-comment">// 0: start</span> <span class="hljs-comment">// 1: start</span> <span class="hljs-comment">// 0: finish</span> <span class="hljs-comment">// 1: finish</span></code></pre> <p>Each instance of <code>AsyncLocalStorage</code> maintains an independent storage context. Multiple instances can safely exist simultaneously without risk of interfering with each other's data.</p> <h4><code>new AsyncLocalStorage()</code><span><a class="mark" href="#new-asynclocalstorage" id="new-asynclocalstorage">#</a></span><a aria-hidden="true" class="legacy" id="async_context_new_asynclocalstorage"></a></h4> <div class="api_metadata"> <span>Added in: v13.10.0, v12.17.0</span> </div> <p>Creates a new instance of <code>AsyncLocalStorage</code>. Store is only provided within a <code>run()</code> call or after an <code>enterWith()</code> call.</p> <h4><code>asyncLocalStorage.disable()</code><span><a class="mark" href="#asynclocalstoragedisable" id="asynclocalstoragedisable">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynclocalstorage_disable"></a></h4> <div class="api_metadata"> <span>Added in: v13.10.0, v12.17.0</span> </div> <p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p> <p>Disables the instance of <code>AsyncLocalStorage</code>. All subsequent calls to <code>asyncLocalStorage.getStore()</code> will return <code>undefined</code> until <code>asyncLocalStorage.run()</code> or <code>asyncLocalStorage.enterWith()</code> is called again.</p> <p>When calling <code>asyncLocalStorage.disable()</code>, all current contexts linked to the instance will be exited.</p> <p>Calling <code>asyncLocalStorage.disable()</code> is required before the <code>asyncLocalStorage</code> can be garbage collected. This does not apply to stores provided by the <code>asyncLocalStorage</code>, as those objects are garbage collected along with the corresponding async resources.</p> <p>Use this method when the <code>asyncLocalStorage</code> is not in use anymore in the current process.</p> <h4><code>asyncLocalStorage.getStore()</code><span><a class="mark" href="#asynclocalstoragegetstore" id="asynclocalstoragegetstore">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynclocalstorage_getstore"></a></h4> <div class="api_metadata"> <span>Added in: v13.10.0, v12.17.0</span> </div> <ul> <li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> </ul> <p>Returns the current store. If called outside of an asynchronous context initialized by calling <code>asyncLocalStorage.run()</code> or <code>asyncLocalStorage.enterWith()</code>, it returns <code>undefined</code>.</p> <h4><code>asyncLocalStorage.enterWith(store)</code><span><a class="mark" href="#asynclocalstorageenterwithstore" id="asynclocalstorageenterwithstore">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynclocalstorage_enterwith_store"></a></h4> <div class="api_metadata"> <span>Added in: v13.11.0, v12.17.0</span> </div> <p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p> <ul> <li><code>store</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> </ul> <p>Transitions into the context for the remainder of the current synchronous execution and then persists the store through any following asynchronous calls.</p> <p>Example:</p> <pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">1</span> }; <span class="hljs-comment">// Replaces previous store with the given store object</span> asyncLocalStorage.<span class="hljs-title function_">enterWith</span>(store); asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span> <span class="hljs-title function_">someAsyncOperation</span>(<span class="hljs-function">() =></span> { asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span> });</code></pre> <p>This transition will continue for the <em>entire</em> synchronous execution. This means that if, for example, the context is entered within an event handler subsequent event handlers will also run within that context unless specifically bound to another context with an <code>AsyncResource</code>. That is why <code>run()</code> should be preferred over <code>enterWith()</code> unless there are strong reasons to use the latter method.</p> <pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">1</span> }; emitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'my-event'</span>, <span class="hljs-function">() =></span> { asyncLocalStorage.<span class="hljs-title function_">enterWith</span>(store); }); emitter.<span class="hljs-title function_">on</span>(<span class="hljs-string">'my-event'</span>, <span class="hljs-function">() =></span> { asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span> }); asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span> emitter.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'my-event'</span>); asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object</span></code></pre> <h4><code>asyncLocalStorage.run(store, callback[, ...args])</code><span><a class="mark" href="#asynclocalstoragerunstore-callback-args" id="asynclocalstoragerunstore-callback-args">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynclocalstorage_run_store_callback_args"></a></h4> <div class="api_metadata"> <span>Added in: v13.10.0, v12.17.0</span> </div> <ul> <li><code>store</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> <li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> <li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> </ul> <p>Runs a function synchronously within a context and returns its return value. The store is not accessible outside of the callback function. The store is accessible to any asynchronous operations created within the callback.</p> <p>The optional <code>args</code> are passed to the callback function.</p> <p>If the callback function throws an error, the error is thrown by <code>run()</code> too. The stacktrace is not impacted by this call and the context is exited.</p> <p>Example:</p> <pre><code class="language-js"><span class="hljs-keyword">const</span> store = { <span class="hljs-attr">id</span>: <span class="hljs-number">2</span> }; <span class="hljs-keyword">try</span> { asyncLocalStorage.<span class="hljs-title function_">run</span>(store, <span class="hljs-function">() =></span> { asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span> <span class="hljs-built_in">setTimeout</span>(<span class="hljs-function">() =></span> { asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object</span> }, <span class="hljs-number">200</span>); <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(); }); } <span class="hljs-keyword">catch</span> (e) { asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span> <span class="hljs-comment">// The error will be caught here</span> }</code></pre> <h4><code>asyncLocalStorage.exit(callback[, ...args])</code><span><a class="mark" href="#asynclocalstorageexitcallback-args" id="asynclocalstorageexitcallback-args">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asynclocalstorage_exit_callback_args"></a></h4> <div class="api_metadata"> <span>Added in: v13.10.0, v12.17.0</span> </div> <p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p> <ul> <li><code>callback</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a></li> <li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> </ul> <p>Runs a function synchronously outside of a context and returns its return value. The store is not accessible within the callback function or the asynchronous operations created within the callback. Any <code>getStore()</code> call done within the callback function will always return <code>undefined</code>.</p> <p>The optional <code>args</code> are passed to the callback function.</p> <p>If the callback function throws an error, the error is thrown by <code>exit()</code> too. The stacktrace is not impacted by this call and the context is re-entered.</p> <p>Example:</p> <pre><code class="language-js"><span class="hljs-comment">// Within a call to run</span> <span class="hljs-keyword">try</span> { asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the store object or value</span> asyncLocalStorage.<span class="hljs-title function_">exit</span>(<span class="hljs-function">() =></span> { asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns undefined</span> <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Error</span>(); }); } <span class="hljs-keyword">catch</span> (e) { asyncLocalStorage.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Returns the same object or value</span> <span class="hljs-comment">// The error will be caught here</span> }</code></pre> <h4>Usage with <code>async/await</code><span><a class="mark" href="#usage-with-asyncawait" id="usage-with-asyncawait">#</a></span><a aria-hidden="true" class="legacy" id="async_context_usage_with_async_await"></a></h4> <p>If, within an async function, only one <code>await</code> call is to run within a context, the following pattern should be used:</p> <pre><code class="language-js"><span class="hljs-keyword">async</span> <span class="hljs-keyword">function</span> <span class="hljs-title function_">fn</span>(<span class="hljs-params"></span>) { <span class="hljs-keyword">await</span> asyncLocalStorage.<span class="hljs-title function_">run</span>(<span class="hljs-keyword">new</span> <span class="hljs-title class_">Map</span>(), <span class="hljs-function">() =></span> { asyncLocalStorage.<span class="hljs-title function_">getStore</span>().<span class="hljs-title function_">set</span>(<span class="hljs-string">'key'</span>, value); <span class="hljs-keyword">return</span> <span class="hljs-title function_">foo</span>(); <span class="hljs-comment">// The return value of foo will be awaited</span> }); }</code></pre> <p>In this example, the store is only available in the callback function and the functions called by <code>foo</code>. Outside of <code>run</code>, calling <code>getStore</code> will return <code>undefined</code>.</p> <h4>Troubleshooting: Context loss<span><a class="mark" href="#troubleshooting-context-loss" id="troubleshooting-context-loss">#</a></span><a aria-hidden="true" class="legacy" id="async_context_troubleshooting_context_loss"></a></h4> <p>In most cases, <code>AsyncLocalStorage</code> works without issues. In rare situations, the current store is lost in one of the asynchronous operations.</p> <p>If your code is callback-based, it is enough to promisify it with <a href="util.html#utilpromisifyoriginal"><code>util.promisify()</code></a> so it starts working with native promises.</p> <p>If you need to use a callback-based API or your code assumes a custom thenable implementation, use the <a href="#class-asyncresource"><code>AsyncResource</code></a> class to associate the asynchronous operation with the correct execution context. Find the function call responsible for the context loss by logging the content of <code>asyncLocalStorage.getStore()</code> after the calls you suspect are responsible for the loss. When the code logs <code>undefined</code>, the last callback called is probably responsible for the context loss.</p> </section><section><h3>Class: <code>AsyncResource</code><span><a class="mark" href="#class-asyncresource" id="class-asyncresource">#</a></span><a aria-hidden="true" class="legacy" id="async_context_class_asyncresource"></a></h3> <div class="api_metadata"> <details class="changelog"><summary>History</summary> <table> <tbody><tr><th>Version</th><th>Changes</th></tr> <tr><td>v16.4.0</td> <td><p>AsyncResource is now Stable. Previously, it had been Experimental.</p></td></tr> </tbody></table> </details> </div> <p>The class <code>AsyncResource</code> is designed to be extended by the embedder's async resources. Using this, users can easily trigger the lifetime events of their own resources.</p> <p>The <code>init</code> hook will trigger when an <code>AsyncResource</code> is instantiated.</p> <p>The following is an overview of the <code>AsyncResource</code> API.</p> <pre class="with-72-chars"><input class="js-flavor-selector" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { <span class="hljs-title class_">AsyncResource</span>, executionAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; <span class="hljs-comment">// AsyncResource() is meant to be extended. Instantiating a</span> <span class="hljs-comment">// new AsyncResource() also triggers init. If triggerAsyncId is omitted then</span> <span class="hljs-comment">// async_hook.executionAsyncId() is used.</span> <span class="hljs-keyword">const</span> asyncResource = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncResource</span>( type, { <span class="hljs-attr">triggerAsyncId</span>: <span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-attr">requireManualDestroy</span>: <span class="hljs-literal">false</span> } ); <span class="hljs-comment">// Run a function in the execution context of the resource. This will</span> <span class="hljs-comment">// * establish the context of the resource</span> <span class="hljs-comment">// * trigger the AsyncHooks before callbacks</span> <span class="hljs-comment">// * call the provided function `fn` with the supplied arguments</span> <span class="hljs-comment">// * trigger the AsyncHooks after callbacks</span> <span class="hljs-comment">// * restore the original execution context</span> asyncResource.<span class="hljs-title function_">runInAsyncScope</span>(fn, thisArg, ...args); <span class="hljs-comment">// Call AsyncHooks destroy callbacks.</span> asyncResource.<span class="hljs-title function_">emitDestroy</span>(); <span class="hljs-comment">// Return the unique ID assigned to the AsyncResource instance.</span> asyncResource.<span class="hljs-title function_">asyncId</span>(); <span class="hljs-comment">// Return the trigger ID for the AsyncResource instance.</span> asyncResource.<span class="hljs-title function_">triggerAsyncId</span>();</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncResource</span>, executionAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); <span class="hljs-comment">// AsyncResource() is meant to be extended. Instantiating a</span> <span class="hljs-comment">// new AsyncResource() also triggers init. If triggerAsyncId is omitted then</span> <span class="hljs-comment">// async_hook.executionAsyncId() is used.</span> <span class="hljs-keyword">const</span> asyncResource = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncResource</span>( type, { <span class="hljs-attr">triggerAsyncId</span>: <span class="hljs-title function_">executionAsyncId</span>(), <span class="hljs-attr">requireManualDestroy</span>: <span class="hljs-literal">false</span> } ); <span class="hljs-comment">// Run a function in the execution context of the resource. This will</span> <span class="hljs-comment">// * establish the context of the resource</span> <span class="hljs-comment">// * trigger the AsyncHooks before callbacks</span> <span class="hljs-comment">// * call the provided function `fn` with the supplied arguments</span> <span class="hljs-comment">// * trigger the AsyncHooks after callbacks</span> <span class="hljs-comment">// * restore the original execution context</span> asyncResource.<span class="hljs-title function_">runInAsyncScope</span>(fn, thisArg, ...args); <span class="hljs-comment">// Call AsyncHooks destroy callbacks.</span> asyncResource.<span class="hljs-title function_">emitDestroy</span>(); <span class="hljs-comment">// Return the unique ID assigned to the AsyncResource instance.</span> asyncResource.<span class="hljs-title function_">asyncId</span>(); <span class="hljs-comment">// Return the trigger ID for the AsyncResource instance.</span> asyncResource.<span class="hljs-title function_">triggerAsyncId</span>();</code></pre> <h4><code>new AsyncResource(type[, options])</code><span><a class="mark" href="#new-asyncresourcetype-options" id="new-asyncresourcetype-options">#</a></span><a aria-hidden="true" class="legacy" id="async_context_new_asyncresource_type_options"></a></h4> <ul> <li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> The type of async event.</li> <li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> <ul> <li><code>triggerAsyncId</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The ID of the execution context that created this async event. <strong>Default:</strong> <code>executionAsyncId()</code>.</li> <li><code>requireManualDestroy</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If set to <code>true</code>, disables <code>emitDestroy</code> when the object is garbage collected. This usually does not need to be set (even if <code>emitDestroy</code> is called manually), unless the resource's <code>asyncId</code> is retrieved and the sensitive API's <code>emitDestroy</code> is called with it. When set to <code>false</code>, the <code>emitDestroy</code> call on garbage collection will only take place if there is at least one active <code>destroy</code> hook. <strong>Default:</strong> <code>false</code>.</li> </ul> </li> </ul> <p>Example usage:</p> <pre><code class="language-js"><span class="hljs-keyword">class</span> <span class="hljs-title class_">DBQuery</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">AsyncResource</span> { <span class="hljs-title function_">constructor</span>(<span class="hljs-params">db</span>) { <span class="hljs-variable language_">super</span>(<span class="hljs-string">'DBQuery'</span>); <span class="hljs-variable language_">this</span>.<span class="hljs-property">db</span> = db; } <span class="hljs-title function_">getInfo</span>(<span class="hljs-params">query, callback</span>) { <span class="hljs-variable language_">this</span>.<span class="hljs-property">db</span>.<span class="hljs-title function_">get</span>(query, <span class="hljs-function">(<span class="hljs-params">err, data</span>) =></span> { <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runInAsyncScope</span>(callback, <span class="hljs-literal">null</span>, err, data); }); } <span class="hljs-title function_">close</span>(<span class="hljs-params"></span>) { <span class="hljs-variable language_">this</span>.<span class="hljs-property">db</span> = <span class="hljs-literal">null</span>; <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emitDestroy</span>(); } }</code></pre> <h4>Static method: <code>AsyncResource.bind(fn[, type[, thisArg]])</code><span><a class="mark" href="#static-method-asyncresourcebindfn-type-thisarg" id="static-method-asyncresourcebindfn-type-thisarg">#</a></span><a aria-hidden="true" class="legacy" id="async_context_static_method_asyncresource_bind_fn_type_thisarg"></a></h4> <div class="api_metadata"> <details class="changelog"><summary>History</summary> <table> <tbody><tr><th>Version</th><th>Changes</th></tr> <tr><td>v16.15.0</td> <td><p>Changed the default when <code>thisArg</code> is undefined to use <code>this</code> from the caller.</p></td></tr> <tr><td>v16.0.0</td> <td><p>Added optional thisArg.</p></td></tr> <tr><td>v14.8.0, v12.19.0</td> <td><p><span>Added in: v14.8.0, v12.19.0</span></p></td></tr> </tbody></table> </details> </div> <ul> <li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The function to bind to the current execution context.</li> <li><code>type</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> An optional name to associate with the underlying <code>AsyncResource</code>.</li> <li><code>thisArg</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> </ul> <p>Binds the given function to the current execution context.</p> <p>The returned function will have an <code>asyncResource</code> property referencing the <code>AsyncResource</code> to which the function is bound.</p> <h4><code>asyncResource.bind(fn[, thisArg])</code><span><a class="mark" href="#asyncresourcebindfn-thisarg" id="asyncresourcebindfn-thisarg">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asyncresource_bind_fn_thisarg"></a></h4> <div class="api_metadata"> <details class="changelog"><summary>History</summary> <table> <tbody><tr><th>Version</th><th>Changes</th></tr> <tr><td>v16.15.0</td> <td><p>Changed the default when <code>thisArg</code> is undefined to use <code>this</code> from the caller.</p></td></tr> <tr><td>v16.0.0</td> <td><p>Added optional thisArg.</p></td></tr> <tr><td>v14.8.0, v12.19.0</td> <td><p><span>Added in: v14.8.0, v12.19.0</span></p></td></tr> </tbody></table> </details> </div> <ul> <li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The function to bind to the current <code>AsyncResource</code>.</li> <li><code>thisArg</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a></li> </ul> <p>Binds the given function to execute to this <code>AsyncResource</code>'s scope.</p> <p>The returned function will have an <code>asyncResource</code> property referencing the <code>AsyncResource</code> to which the function is bound.</p> <h4><code>asyncResource.runInAsyncScope(fn[, thisArg, ...args])</code><span><a class="mark" href="#asyncresourceruninasyncscopefn-thisarg-args" id="asyncresourceruninasyncscopefn-thisarg-args">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asyncresource_runinasyncscope_fn_thisarg_args"></a></h4> <div class="api_metadata"> <span>Added in: v9.6.0</span> </div> <ul> <li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The function to call in the execution context of this async resource.</li> <li><code>thisArg</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The receiver to be used for the function call.</li> <li><code>...args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> Optional arguments to pass to the function.</li> </ul> <p>Call the provided function with the provided arguments in the execution context of the async resource. This will establish the context, trigger the AsyncHooks before callbacks, call the function, trigger the AsyncHooks after callbacks, and then restore the original execution context.</p> <h4><code>asyncResource.emitDestroy()</code><span><a class="mark" href="#asyncresourceemitdestroy" id="asyncresourceemitdestroy">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asyncresource_emitdestroy"></a></h4> <ul> <li>Returns: <a href="async_hooks.html#class-asyncresource" class="type"><AsyncResource></a> A reference to <code>asyncResource</code>.</li> </ul> <p>Call all <code>destroy</code> hooks. This should only ever be called once. An error will be thrown if it is called more than once. This <strong>must</strong> be manually called. If the resource is left to be collected by the GC then the <code>destroy</code> hooks will never be called.</p> <h4><code>asyncResource.asyncId()</code><span><a class="mark" href="#asyncresourceasyncid" id="asyncresourceasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asyncresource_asyncid"></a></h4> <ul> <li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The unique <code>asyncId</code> assigned to the resource.</li> </ul> <h4><code>asyncResource.triggerAsyncId()</code><span><a class="mark" href="#asyncresourcetriggerasyncid" id="asyncresourcetriggerasyncid">#</a></span><a aria-hidden="true" class="legacy" id="async_context_asyncresource_triggerasyncid"></a></h4> <ul> <li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> The same <code>triggerAsyncId</code> that is passed to the <code>AsyncResource</code> constructor.</li> </ul> <p><a id="async-resource-worker-pool"></a></p> <h4>Using <code>AsyncResource</code> for a <code>Worker</code> thread pool<span><a class="mark" href="#using-asyncresource-for-a-worker-thread-pool" id="using-asyncresource-for-a-worker-thread-pool">#</a></span><a aria-hidden="true" class="legacy" id="async_context_using_asyncresource_for_a_worker_thread_pool"></a></h4> <p>The following example shows how to use the <code>AsyncResource</code> class to properly provide async tracking for a <a href="worker_threads.html#class-worker"><code>Worker</code></a> pool. Other resource pools, such as database connection pools, can follow a similar model.</p> <p>Assuming that the task is adding two numbers, using a file named <code>task_processor.js</code> with the following content:</p> <pre class="with-54-chars"><input class="js-flavor-selector" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { parentPort } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:worker_threads'</span>; parentPort.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">task</span>) =></span> { parentPort.<span class="hljs-title function_">postMessage</span>(task.<span class="hljs-property">a</span> + task.<span class="hljs-property">b</span>); });</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { parentPort } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:worker_threads'</span>); parentPort.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">task</span>) =></span> { parentPort.<span class="hljs-title function_">postMessage</span>(task.<span class="hljs-property">a</span> + task.<span class="hljs-property">b</span>); });</code></pre> <p>a Worker pool around it could use the following structure:</p> <pre class="with-54-chars"><input class="js-flavor-selector" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { <span class="hljs-title class_">AsyncResource</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; <span class="hljs-keyword">import</span> { <span class="hljs-title class_">EventEmitter</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:events'</span>; <span class="hljs-keyword">import</span> path <span class="hljs-keyword">from</span> <span class="hljs-string">'node:path'</span>; <span class="hljs-keyword">import</span> { <span class="hljs-title class_">Worker</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:worker_threads'</span>; <span class="hljs-keyword">const</span> kTaskInfo = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kTaskInfo'</span>); <span class="hljs-keyword">const</span> kWorkerFreedEvent = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kWorkerFreedEvent'</span>); <span class="hljs-keyword">class</span> <span class="hljs-title class_">WorkerPoolTaskInfo</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">AsyncResource</span> { <span class="hljs-title function_">constructor</span>(<span class="hljs-params">callback</span>) { <span class="hljs-variable language_">super</span>(<span class="hljs-string">'WorkerPoolTaskInfo'</span>); <span class="hljs-variable language_">this</span>.<span class="hljs-property">callback</span> = callback; } <span class="hljs-title function_">done</span>(<span class="hljs-params">err, result</span>) { <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runInAsyncScope</span>(<span class="hljs-variable language_">this</span>.<span class="hljs-property">callback</span>, <span class="hljs-literal">null</span>, err, result); <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emitDestroy</span>(); <span class="hljs-comment">// `TaskInfo`s are used only once.</span> } } <span class="hljs-keyword">export</span> <span class="hljs-keyword">default</span> <span class="hljs-keyword">class</span> <span class="hljs-title class_">WorkerPool</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">EventEmitter</span> { <span class="hljs-title function_">constructor</span>(<span class="hljs-params">numThreads</span>) { <span class="hljs-variable language_">super</span>(); <span class="hljs-variable language_">this</span>.<span class="hljs-property">numThreads</span> = numThreads; <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span> = []; <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span> = []; <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span> = []; <span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i < numThreads; i++) <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>(); <span class="hljs-comment">// Any time the kWorkerFreedEvent is emitted, dispatch</span> <span class="hljs-comment">// the next task pending in the queue, if any.</span> <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">on</span>(kWorkerFreedEvent, <span class="hljs-function">() =></span> { <span class="hljs-keyword">if</span> (<span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-property">length</span> > <span class="hljs-number">0</span>) { <span class="hljs-keyword">const</span> { task, callback } = <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-title function_">shift</span>(); <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runTask</span>(task, callback); } }); } <span class="hljs-title function_">addNewWorker</span>(<span class="hljs-params"></span>) { <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(<span class="hljs-string">'task_processer.js'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>)); worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">result</span>) =></span> { <span class="hljs-comment">// In case of success: Call the callback that was passed to `runTask`,</span> <span class="hljs-comment">// remove the `TaskInfo` associated with the Worker, and mark it as free</span> <span class="hljs-comment">// again.</span> worker[kTaskInfo].<span class="hljs-title function_">done</span>(<span class="hljs-literal">null</span>, result); worker[kTaskInfo] = <span class="hljs-literal">null</span>; <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker); <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent); }); worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">err</span>) =></span> { <span class="hljs-comment">// In case of an uncaught exception: Call the callback that was passed to</span> <span class="hljs-comment">// `runTask` with the error.</span> <span class="hljs-keyword">if</span> (worker[kTaskInfo]) worker[kTaskInfo].<span class="hljs-title function_">done</span>(err, <span class="hljs-literal">null</span>); <span class="hljs-keyword">else</span> <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'error'</span>, err); <span class="hljs-comment">// Remove the worker from the list and start a new Worker to replace the</span> <span class="hljs-comment">// current one.</span> <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">splice</span>(<span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">indexOf</span>(worker), <span class="hljs-number">1</span>); <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>(); }); <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">push</span>(worker); <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker); <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent); } <span class="hljs-title function_">runTask</span>(<span class="hljs-params">task, callback</span>) { <span class="hljs-keyword">if</span> (<span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-property">length</span> === <span class="hljs-number">0</span>) { <span class="hljs-comment">// No free threads, wait until a worker thread becomes free.</span> <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-title function_">push</span>({ task, callback }); <span class="hljs-keyword">return</span>; } <span class="hljs-keyword">const</span> worker = <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">pop</span>(); worker[kTaskInfo] = <span class="hljs-keyword">new</span> <span class="hljs-title class_">WorkerPoolTaskInfo</span>(callback); worker.<span class="hljs-title function_">postMessage</span>(task); } <span class="hljs-title function_">close</span>(<span class="hljs-params"></span>) { <span class="hljs-keyword">for</span> (<span class="hljs-keyword">const</span> worker <span class="hljs-keyword">of</span> <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>) worker.<span class="hljs-title function_">terminate</span>(); } }</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncResource</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); <span class="hljs-keyword">const</span> { <span class="hljs-title class_">EventEmitter</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:events'</span>); <span class="hljs-keyword">const</span> path = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:path'</span>); <span class="hljs-keyword">const</span> { <span class="hljs-title class_">Worker</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:worker_threads'</span>); <span class="hljs-keyword">const</span> kTaskInfo = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kTaskInfo'</span>); <span class="hljs-keyword">const</span> kWorkerFreedEvent = <span class="hljs-title class_">Symbol</span>(<span class="hljs-string">'kWorkerFreedEvent'</span>); <span class="hljs-keyword">class</span> <span class="hljs-title class_">WorkerPoolTaskInfo</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">AsyncResource</span> { <span class="hljs-title function_">constructor</span>(<span class="hljs-params">callback</span>) { <span class="hljs-variable language_">super</span>(<span class="hljs-string">'WorkerPoolTaskInfo'</span>); <span class="hljs-variable language_">this</span>.<span class="hljs-property">callback</span> = callback; } <span class="hljs-title function_">done</span>(<span class="hljs-params">err, result</span>) { <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runInAsyncScope</span>(<span class="hljs-variable language_">this</span>.<span class="hljs-property">callback</span>, <span class="hljs-literal">null</span>, err, result); <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emitDestroy</span>(); <span class="hljs-comment">// `TaskInfo`s are used only once.</span> } } <span class="hljs-keyword">class</span> <span class="hljs-title class_">WorkerPool</span> <span class="hljs-keyword">extends</span> <span class="hljs-title class_ inherited__">EventEmitter</span> { <span class="hljs-title function_">constructor</span>(<span class="hljs-params">numThreads</span>) { <span class="hljs-variable language_">super</span>(); <span class="hljs-variable language_">this</span>.<span class="hljs-property">numThreads</span> = numThreads; <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span> = []; <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span> = []; <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span> = []; <span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i < numThreads; i++) <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>(); <span class="hljs-comment">// Any time the kWorkerFreedEvent is emitted, dispatch</span> <span class="hljs-comment">// the next task pending in the queue, if any.</span> <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">on</span>(kWorkerFreedEvent, <span class="hljs-function">() =></span> { <span class="hljs-keyword">if</span> (<span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-property">length</span> > <span class="hljs-number">0</span>) { <span class="hljs-keyword">const</span> { task, callback } = <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-title function_">shift</span>(); <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">runTask</span>(task, callback); } }); } <span class="hljs-title function_">addNewWorker</span>(<span class="hljs-params"></span>) { <span class="hljs-keyword">const</span> worker = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Worker</span>(path.<span class="hljs-title function_">resolve</span>(__dirname, <span class="hljs-string">'task_processor.js'</span>)); worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'message'</span>, <span class="hljs-function">(<span class="hljs-params">result</span>) =></span> { <span class="hljs-comment">// In case of success: Call the callback that was passed to `runTask`,</span> <span class="hljs-comment">// remove the `TaskInfo` associated with the Worker, and mark it as free</span> <span class="hljs-comment">// again.</span> worker[kTaskInfo].<span class="hljs-title function_">done</span>(<span class="hljs-literal">null</span>, result); worker[kTaskInfo] = <span class="hljs-literal">null</span>; <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker); <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent); }); worker.<span class="hljs-title function_">on</span>(<span class="hljs-string">'error'</span>, <span class="hljs-function">(<span class="hljs-params">err</span>) =></span> { <span class="hljs-comment">// In case of an uncaught exception: Call the callback that was passed to</span> <span class="hljs-comment">// `runTask` with the error.</span> <span class="hljs-keyword">if</span> (worker[kTaskInfo]) worker[kTaskInfo].<span class="hljs-title function_">done</span>(err, <span class="hljs-literal">null</span>); <span class="hljs-keyword">else</span> <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(<span class="hljs-string">'error'</span>, err); <span class="hljs-comment">// Remove the worker from the list and start a new Worker to replace the</span> <span class="hljs-comment">// current one.</span> <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">splice</span>(<span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">indexOf</span>(worker), <span class="hljs-number">1</span>); <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">addNewWorker</span>(); }); <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>.<span class="hljs-title function_">push</span>(worker); <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">push</span>(worker); <span class="hljs-variable language_">this</span>.<span class="hljs-title function_">emit</span>(kWorkerFreedEvent); } <span class="hljs-title function_">runTask</span>(<span class="hljs-params">task, callback</span>) { <span class="hljs-keyword">if</span> (<span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-property">length</span> === <span class="hljs-number">0</span>) { <span class="hljs-comment">// No free threads, wait until a worker thread becomes free.</span> <span class="hljs-variable language_">this</span>.<span class="hljs-property">tasks</span>.<span class="hljs-title function_">push</span>({ task, callback }); <span class="hljs-keyword">return</span>; } <span class="hljs-keyword">const</span> worker = <span class="hljs-variable language_">this</span>.<span class="hljs-property">freeWorkers</span>.<span class="hljs-title function_">pop</span>(); worker[kTaskInfo] = <span class="hljs-keyword">new</span> <span class="hljs-title class_">WorkerPoolTaskInfo</span>(callback); worker.<span class="hljs-title function_">postMessage</span>(task); } <span class="hljs-title function_">close</span>(<span class="hljs-params"></span>) { <span class="hljs-keyword">for</span> (<span class="hljs-keyword">const</span> worker <span class="hljs-keyword">of</span> <span class="hljs-variable language_">this</span>.<span class="hljs-property">workers</span>) worker.<span class="hljs-title function_">terminate</span>(); } } <span class="hljs-variable language_">module</span>.<span class="hljs-property">exports</span> = <span class="hljs-title class_">WorkerPool</span>;</code></pre> <p>Without the explicit tracking added by the <code>WorkerPoolTaskInfo</code> objects, it would appear that the callbacks are associated with the individual <code>Worker</code> objects. However, the creation of the <code>Worker</code>s is not associated with the creation of the tasks and does not provide information about when tasks were scheduled.</p> <p>This pool could be used as follows:</p> <pre class="with-47-chars"><input class="js-flavor-selector" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> <span class="hljs-title class_">WorkerPool</span> <span class="hljs-keyword">from</span> <span class="hljs-string">'./worker_pool.js'</span>; <span class="hljs-keyword">import</span> os <span class="hljs-keyword">from</span> <span class="hljs-string">'node:os'</span>; <span class="hljs-keyword">const</span> pool = <span class="hljs-keyword">new</span> <span class="hljs-title class_">WorkerPool</span>(os.<span class="hljs-title function_">cpus</span>().<span class="hljs-property">length</span>); <span class="hljs-keyword">let</span> finished = <span class="hljs-number">0</span>; <span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i < <span class="hljs-number">10</span>; i++) { pool.<span class="hljs-title function_">runTask</span>({ <span class="hljs-attr">a</span>: <span class="hljs-number">42</span>, <span class="hljs-attr">b</span>: <span class="hljs-number">100</span> }, <span class="hljs-function">(<span class="hljs-params">err, result</span>) =></span> { <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(i, err, result); <span class="hljs-keyword">if</span> (++finished === <span class="hljs-number">10</span>) pool.<span class="hljs-title function_">close</span>(); }); }</code><code class="language-js cjs"><span class="hljs-keyword">const</span> <span class="hljs-title class_">WorkerPool</span> = <span class="hljs-built_in">require</span>(<span class="hljs-string">'./worker_pool.js'</span>); <span class="hljs-keyword">const</span> os = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:os'</span>); <span class="hljs-keyword">const</span> pool = <span class="hljs-keyword">new</span> <span class="hljs-title class_">WorkerPool</span>(os.<span class="hljs-title function_">cpus</span>().<span class="hljs-property">length</span>); <span class="hljs-keyword">let</span> finished = <span class="hljs-number">0</span>; <span class="hljs-keyword">for</span> (<span class="hljs-keyword">let</span> i = <span class="hljs-number">0</span>; i < <span class="hljs-number">10</span>; i++) { pool.<span class="hljs-title function_">runTask</span>({ <span class="hljs-attr">a</span>: <span class="hljs-number">42</span>, <span class="hljs-attr">b</span>: <span class="hljs-number">100</span> }, <span class="hljs-function">(<span class="hljs-params">err, result</span>) =></span> { <span class="hljs-variable language_">console</span>.<span class="hljs-title function_">log</span>(i, err, result); <span class="hljs-keyword">if</span> (++finished === <span class="hljs-number">10</span>) pool.<span class="hljs-title function_">close</span>(); }); }</code></pre> <h4>Integrating <code>AsyncResource</code> with <code>EventEmitter</code><span><a class="mark" href="#integrating-asyncresource-with-eventemitter" id="integrating-asyncresource-with-eventemitter">#</a></span><a aria-hidden="true" class="legacy" id="async_context_integrating_asyncresource_with_eventemitter"></a></h4> <p>Event listeners triggered by an <a href="events.html#class-eventemitter"><code>EventEmitter</code></a> may be run in a different execution context than the one that was active when <code>eventEmitter.on()</code> was called.</p> <p>The following example shows how to use the <code>AsyncResource</code> class to properly associate an event listener with the correct execution context. The same approach can be applied to a <a href="stream.html#stream"><code>Stream</code></a> or a similar event-driven class.</p> <pre class="with-72-chars"><input class="js-flavor-selector" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { createServer } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:http'</span>; <span class="hljs-keyword">import</span> { <span class="hljs-title class_">AsyncResource</span>, executionAsyncId } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:async_hooks'</span>; <span class="hljs-keyword">const</span> server = <span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { req.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-title class_">AsyncResource</span>.<span class="hljs-title function_">bind</span>(<span class="hljs-function">() =></span> { <span class="hljs-comment">// Execution context is bound to the current outer scope.</span> })); req.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-function">() =></span> { <span class="hljs-comment">// Execution context is bound to the scope that caused 'close' to emit.</span> }); res.<span class="hljs-title function_">end</span>(); }).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> { createServer } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:http'</span>); <span class="hljs-keyword">const</span> { <span class="hljs-title class_">AsyncResource</span>, executionAsyncId } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:async_hooks'</span>); <span class="hljs-keyword">const</span> server = <span class="hljs-title function_">createServer</span>(<span class="hljs-function">(<span class="hljs-params">req, res</span>) =></span> { req.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-title class_">AsyncResource</span>.<span class="hljs-title function_">bind</span>(<span class="hljs-function">() =></span> { <span class="hljs-comment">// Execution context is bound to the current outer scope.</span> })); req.<span class="hljs-title function_">on</span>(<span class="hljs-string">'close'</span>, <span class="hljs-function">() =></span> { <span class="hljs-comment">// Execution context is bound to the scope that caused 'close' to emit.</span> }); res.<span class="hljs-title function_">end</span>(); }).<span class="hljs-title function_">listen</span>(<span class="hljs-number">3000</span>);</code></pre></section> <!-- API END --> </div> </div> </div> </body> </html>