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<div class="picker"><div class="toc"><ul>
<li><span class="stability_2"><a href="#diagnostics-channel">Diagnostics Channel</a></span>
<ul>
<li><a href="#public-api">Public API</a>
<ul>
<li><a href="#overview">Overview</a>
<ul>
<li><a href="#diagnostics_channelhassubscribersname"><code>diagnostics_channel.hasSubscribers(name)</code></a></li>
<li><a href="#diagnostics_channelchannelname"><code>diagnostics_channel.channel(name)</code></a></li>
<li><a href="#diagnostics_channelsubscribename-onmessage"><code>diagnostics_channel.subscribe(name, onMessage)</code></a></li>
<li><a href="#diagnostics_channelunsubscribename-onmessage"><code>diagnostics_channel.unsubscribe(name, onMessage)</code></a></li>
<li><span class="stability_1"><a href="#diagnostics_channeltracingchannelnameorchannels"><code>diagnostics_channel.tracingChannel(nameOrChannels)</code></a></span></li>
</ul>
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<li><a href="#class-channel">Class: <code>Channel</code></a>
<ul>
<li><a href="#channelhassubscribers"><code>channel.hasSubscribers</code></a></li>
<li><a href="#channelpublishmessage"><code>channel.publish(message)</code></a></li>
<li><span class="stability_0"><a href="#channelsubscribeonmessage"><code>channel.subscribe(onMessage)</code></a></span></li>
<li><span class="stability_0"><a href="#channelunsubscribeonmessage"><code>channel.unsubscribe(onMessage)</code></a></span></li>
<li><span class="stability_1"><a href="#channelbindstorestore-transform"><code>channel.bindStore(store[, transform])</code></a></span></li>
<li><span class="stability_1"><a href="#channelunbindstorestore"><code>channel.unbindStore(store)</code></a></span></li>
<li><span class="stability_1"><a href="#channelrunstorescontext-fn-thisarg-args"><code>channel.runStores(context, fn[, thisArg[, ...args]])</code></a></span></li>
</ul>
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<li><span class="stability_1"><a href="#class-tracingchannel">Class: <code>TracingChannel</code></a></span>
<ul>
<li><span class="stability_1"><a href="#tracingchannelsubscribesubscribers"><code>tracingChannel.subscribe(subscribers)</code></a></span></li>
<li><span class="stability_1"><a href="#tracingchannelunsubscribesubscribers"><code>tracingChannel.unsubscribe(subscribers)</code></a></span></li>
<li><span class="stability_1"><a href="#tracingchanneltracesyncfn-context-thisarg-args"><code>tracingChannel.traceSync(fn[, context[, thisArg[, ...args]]])</code></a></span></li>
<li><span class="stability_1"><a href="#tracingchanneltracepromisefn-context-thisarg-args"><code>tracingChannel.tracePromise(fn[, context[, thisArg[, ...args]]])</code></a></span></li>
<li><span class="stability_1"><a href="#tracingchanneltracecallbackfn-position-context-thisarg-args"><code>tracingChannel.traceCallback(fn, position, context, thisArg, ...args)</code></a></span></li>
</ul>
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<li><a href="#tracingchannel-channels">TracingChannel Channels</a>
<ul>
<li><a href="#startevent"><code>start(event)</code></a></li>
<li><a href="#endevent"><code>end(event)</code></a></li>
<li><a href="#asyncstartevent"><code>asyncStart(event)</code></a></li>
<li><a href="#asyncendevent"><code>asyncEnd(event)</code></a></li>
<li><a href="#errorevent"><code>error(event)</code></a></li>
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<li><a href="https://nodejs.org/docs/latest-v18.x/api/diagnostics_channel.html">18.x <b>LTS</b></a></li>
<li><a href="https://nodejs.org/docs/latest-v17.x/api/diagnostics_channel.html">17.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v16.x/api/diagnostics_channel.html">16.x</a></li>
<li><a href="https://nodejs.org/docs/latest-v15.x/api/diagnostics_channel.html">15.x</a></li></ol></div>
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<details role="navigation" id="toc" open><summary>Table of contents</summary><ul>
<li><span class="stability_2"><a href="#diagnostics-channel">Diagnostics Channel</a></span>
<ul>
<li><a href="#public-api">Public API</a>
<ul>
<li><a href="#overview">Overview</a>
<ul>
<li><a href="#diagnostics_channelhassubscribersname"><code>diagnostics_channel.hasSubscribers(name)</code></a></li>
<li><a href="#diagnostics_channelchannelname"><code>diagnostics_channel.channel(name)</code></a></li>
<li><a href="#diagnostics_channelsubscribename-onmessage"><code>diagnostics_channel.subscribe(name, onMessage)</code></a></li>
<li><a href="#diagnostics_channelunsubscribename-onmessage"><code>diagnostics_channel.unsubscribe(name, onMessage)</code></a></li>
<li><span class="stability_1"><a href="#diagnostics_channeltracingchannelnameorchannels"><code>diagnostics_channel.tracingChannel(nameOrChannels)</code></a></span></li>
</ul>
</li>
<li><a href="#class-channel">Class: <code>Channel</code></a>
<ul>
<li><a href="#channelhassubscribers"><code>channel.hasSubscribers</code></a></li>
<li><a href="#channelpublishmessage"><code>channel.publish(message)</code></a></li>
<li><span class="stability_0"><a href="#channelsubscribeonmessage"><code>channel.subscribe(onMessage)</code></a></span></li>
<li><span class="stability_0"><a href="#channelunsubscribeonmessage"><code>channel.unsubscribe(onMessage)</code></a></span></li>
<li><span class="stability_1"><a href="#channelbindstorestore-transform"><code>channel.bindStore(store[, transform])</code></a></span></li>
<li><span class="stability_1"><a href="#channelunbindstorestore"><code>channel.unbindStore(store)</code></a></span></li>
<li><span class="stability_1"><a href="#channelrunstorescontext-fn-thisarg-args"><code>channel.runStores(context, fn[, thisArg[, ...args]])</code></a></span></li>
</ul>
</li>
<li><span class="stability_1"><a href="#class-tracingchannel">Class: <code>TracingChannel</code></a></span>
<ul>
<li><span class="stability_1"><a href="#tracingchannelsubscribesubscribers"><code>tracingChannel.subscribe(subscribers)</code></a></span></li>
<li><span class="stability_1"><a href="#tracingchannelunsubscribesubscribers"><code>tracingChannel.unsubscribe(subscribers)</code></a></span></li>
<li><span class="stability_1"><a href="#tracingchanneltracesyncfn-context-thisarg-args"><code>tracingChannel.traceSync(fn[, context[, thisArg[, ...args]]])</code></a></span></li>
<li><span class="stability_1"><a href="#tracingchanneltracepromisefn-context-thisarg-args"><code>tracingChannel.tracePromise(fn[, context[, thisArg[, ...args]]])</code></a></span></li>
<li><span class="stability_1"><a href="#tracingchanneltracecallbackfn-position-context-thisarg-args"><code>tracingChannel.traceCallback(fn, position, context, thisArg, ...args)</code></a></span></li>
</ul>
</li>
<li><a href="#tracingchannel-channels">TracingChannel Channels</a>
<ul>
<li><a href="#startevent"><code>start(event)</code></a></li>
<li><a href="#endevent"><code>end(event)</code></a></li>
<li><a href="#asyncstartevent"><code>asyncStart(event)</code></a></li>
<li><a href="#asyncendevent"><code>asyncEnd(event)</code></a></li>
<li><a href="#errorevent"><code>error(event)</code></a></li>
</ul>
</li>
<li><span class="stability_1"><a href="#built-in-channels">Built-in Channels</a></span>
<ul>
<li><a href="#http">HTTP</a></li>
<li><a href="#net">NET</a></li>
<li><a href="#udp">UDP</a></li>
<li><a href="#process">Process</a></li>
<li><a href="#worker-thread">Worker Thread</a></li>
</ul>
</li>
</ul>
</li>
</ul>
</li>
</ul></details>
<div role="main" id="apicontent">
<h2>Diagnostics Channel<span><a class="mark" href="#diagnostics-channel" id="diagnostics-channel">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_diagnostics_channel"></a></h2>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v19.2.0, v18.13.0</td>
<td><p>diagnostics_channel is now Stable.</p></td></tr>
<tr><td>v15.1.0, v14.17.0</td>
<td><p><span>Added in: v15.1.0, v14.17.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<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/v20.15.1/lib/diagnostics_channel.js">lib/diagnostics_channel.js</a></p>
<p>The <code>node:diagnostics_channel</code> module provides an API to create named channels
to report arbitrary message data for diagnostics purposes.</p>
<p>It can be accessed using:</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);</code><button class="copy-button">copy</button></pre>
<p>It is intended that a module writer wanting to report diagnostics messages
will create one or many top-level channels to report messages through.
Channels may also be acquired at runtime but it is not encouraged
due to the additional overhead of doing so. Channels may be exported for
convenience, but as long as the name is known it can be acquired anywhere.</p>
<p>If you intend for your module to produce diagnostics data for others to
consume it is recommended that you include documentation of what named
channels are used along with the shape of the message data. Channel names
should generally include the module name to avoid collisions with data from
other modules.</p>
<section><h3>Public API<span><a class="mark" href="#public-api" id="public-api">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_public_api"></a></h3>
<h4>Overview<span><a class="mark" href="#overview" id="overview">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_overview"></a></h4>
<p>Following is a simple overview of the public API.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-comment">// Get a reusable channel object</span>
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
<span class="hljs-keyword">function</span> <span class="hljs-title function_">onMessage</span>(<span class="hljs-params">message, name</span>) {
<span class="hljs-comment">// Received data</span>
}
<span class="hljs-comment">// Subscribe to the channel</span>
diagnostics_channel.<span class="hljs-title function_">subscribe</span>(<span class="hljs-string">'my-channel'</span>, onMessage);
<span class="hljs-comment">// Check if the channel has an active subscriber</span>
<span class="hljs-keyword">if</span> (channel.<span class="hljs-property">hasSubscribers</span>) {
<span class="hljs-comment">// Publish data to the channel</span>
channel.<span class="hljs-title function_">publish</span>({
<span class="hljs-attr">some</span>: <span class="hljs-string">'data'</span>,
});
}
<span class="hljs-comment">// Unsubscribe from the channel</span>
diagnostics_channel.<span class="hljs-title function_">unsubscribe</span>(<span class="hljs-string">'my-channel'</span>, onMessage);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-comment">// Get a reusable channel object</span>
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
<span class="hljs-keyword">function</span> <span class="hljs-title function_">onMessage</span>(<span class="hljs-params">message, name</span>) {
<span class="hljs-comment">// Received data</span>
}
<span class="hljs-comment">// Subscribe to the channel</span>
diagnostics_channel.<span class="hljs-title function_">subscribe</span>(<span class="hljs-string">'my-channel'</span>, onMessage);
<span class="hljs-comment">// Check if the channel has an active subscriber</span>
<span class="hljs-keyword">if</span> (channel.<span class="hljs-property">hasSubscribers</span>) {
<span class="hljs-comment">// Publish data to the channel</span>
channel.<span class="hljs-title function_">publish</span>({
<span class="hljs-attr">some</span>: <span class="hljs-string">'data'</span>,
});
}
<span class="hljs-comment">// Unsubscribe from the channel</span>
diagnostics_channel.<span class="hljs-title function_">unsubscribe</span>(<span class="hljs-string">'my-channel'</span>, onMessage);</code><button class="copy-button">copy</button></pre>
<h5><code>diagnostics_channel.hasSubscribers(name)</code><span><a class="mark" href="#diagnostics_channelhassubscribersname" id="diagnostics_channelhassubscribersname">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_diagnostics_channel_hassubscribers_name"></a></h5>
<div class="api_metadata">
<span>Added in: v15.1.0, v14.17.0</span>
</div>
<ul>
<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The channel name</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If there are active subscribers</li>
</ul>
<p>Check if there are active subscribers to the named channel. This is helpful if
the message you want to send might be expensive to prepare.</p>
<p>This API is optional but helpful when trying to publish messages from very
performance-sensitive code.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">if</span> (diagnostics_channel.<span class="hljs-title function_">hasSubscribers</span>(<span class="hljs-string">'my-channel'</span>)) {
<span class="hljs-comment">// There are subscribers, prepare and publish message</span>
}</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">if</span> (diagnostics_channel.<span class="hljs-title function_">hasSubscribers</span>(<span class="hljs-string">'my-channel'</span>)) {
<span class="hljs-comment">// There are subscribers, prepare and publish message</span>
}</code><button class="copy-button">copy</button></pre>
<h5><code>diagnostics_channel.channel(name)</code><span><a class="mark" href="#diagnostics_channelchannelname" id="diagnostics_channelchannelname">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_diagnostics_channel_channel_name"></a></h5>
<div class="api_metadata">
<span>Added in: v15.1.0, v14.17.0</span>
</div>
<ul>
<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The channel name</li>
<li>Returns: <a href="diagnostics_channel.html#class-channel" class="type"><Channel></a> The named channel object</li>
</ul>
<p>This is the primary entry-point for anyone wanting to publish to a named
channel. It produces a channel object which is optimized to reduce overhead at
publish time as much as possible.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);</code><button class="copy-button">copy</button></pre>
<h5><code>diagnostics_channel.subscribe(name, onMessage)</code><span><a class="mark" href="#diagnostics_channelsubscribename-onmessage" id="diagnostics_channelsubscribename-onmessage">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_diagnostics_channel_subscribe_name_onmessage"></a></h5>
<div class="api_metadata">
<span>Added in: v18.7.0, v16.17.0</span>
</div>
<ul>
<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The channel name</li>
<li><code>onMessage</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The handler to receive channel messages
<ul>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The message data</li>
<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The name of the channel</li>
</ul>
</li>
</ul>
<p>Register a message handler to subscribe to this channel. This message handler
will be run synchronously whenever a message is published to the channel. Any
errors thrown in the message handler will trigger an <a href="process.html#event-uncaughtexception"><code>'uncaughtException'</code></a>.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
diagnostics_channel.<span class="hljs-title function_">subscribe</span>(<span class="hljs-string">'my-channel'</span>, <span class="hljs-function">(<span class="hljs-params">message, name</span>) =></span> {
<span class="hljs-comment">// Received data</span>
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
diagnostics_channel.<span class="hljs-title function_">subscribe</span>(<span class="hljs-string">'my-channel'</span>, <span class="hljs-function">(<span class="hljs-params">message, name</span>) =></span> {
<span class="hljs-comment">// Received data</span>
});</code><button class="copy-button">copy</button></pre>
<h5><code>diagnostics_channel.unsubscribe(name, onMessage)</code><span><a class="mark" href="#diagnostics_channelunsubscribename-onmessage" id="diagnostics_channelunsubscribename-onmessage">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_diagnostics_channel_unsubscribe_name_onmessage"></a></h5>
<div class="api_metadata">
<span>Added in: v18.7.0, v16.17.0</span>
</div>
<ul>
<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The channel name</li>
<li><code>onMessage</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The previous subscribed handler to remove</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> <code>true</code> if the handler was found, <code>false</code> otherwise.</li>
</ul>
<p>Remove a message handler previously registered to this channel with
<a href="#diagnostics_channelsubscribename-onmessage"><code>diagnostics_channel.subscribe(name, onMessage)</code></a>.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">function</span> <span class="hljs-title function_">onMessage</span>(<span class="hljs-params">message, name</span>) {
<span class="hljs-comment">// Received data</span>
}
diagnostics_channel.<span class="hljs-title function_">subscribe</span>(<span class="hljs-string">'my-channel'</span>, onMessage);
diagnostics_channel.<span class="hljs-title function_">unsubscribe</span>(<span class="hljs-string">'my-channel'</span>, onMessage);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">function</span> <span class="hljs-title function_">onMessage</span>(<span class="hljs-params">message, name</span>) {
<span class="hljs-comment">// Received data</span>
}
diagnostics_channel.<span class="hljs-title function_">subscribe</span>(<span class="hljs-string">'my-channel'</span>, onMessage);
diagnostics_channel.<span class="hljs-title function_">unsubscribe</span>(<span class="hljs-string">'my-channel'</span>, onMessage);</code><button class="copy-button">copy</button></pre>
<h5><code>diagnostics_channel.tracingChannel(nameOrChannels)</code><span><a class="mark" href="#diagnostics_channeltracingchannelnameorchannels" id="diagnostics_channeltracingchannelnameorchannels">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_diagnostics_channel_tracingchannel_nameorchannels"></a></h5>
<div class="api_metadata">
<span>Added in: v19.9.0</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>nameOrChannels</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="diagnostics_channel.html#class-tracingchannel" class="type"><TracingChannel></a> Channel name or
object containing all the <a href="#tracingchannel-channels">TracingChannel Channels</a></li>
<li>Returns: <a href="diagnostics_channel.html#class-tracingchannel" class="type"><TracingChannel></a> Collection of channels to trace with</li>
</ul>
<p>Creates a <a href="#class-tracingchannel"><code>TracingChannel</code></a> wrapper for the given
<a href="#tracingchannel-channels">TracingChannel Channels</a>. If a name is given, the corresponding tracing
channels will be created in the form of <code>tracing:${name}:${eventType}</code> where
<code>eventType</code> corresponds to the types of <a href="#tracingchannel-channels">TracingChannel Channels</a>.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">const</span> channelsByName = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
<span class="hljs-comment">// or...</span>
<span class="hljs-keyword">const</span> channelsByCollection = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>({
<span class="hljs-attr">start</span>: diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'tracing:my-channel:start'</span>),
<span class="hljs-attr">end</span>: diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'tracing:my-channel:end'</span>),
<span class="hljs-attr">asyncStart</span>: diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'tracing:my-channel:asyncStart'</span>),
<span class="hljs-attr">asyncEnd</span>: diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'tracing:my-channel:asyncEnd'</span>),
<span class="hljs-attr">error</span>: diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'tracing:my-channel:error'</span>),
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">const</span> channelsByName = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
<span class="hljs-comment">// or...</span>
<span class="hljs-keyword">const</span> channelsByCollection = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>({
<span class="hljs-attr">start</span>: diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'tracing:my-channel:start'</span>),
<span class="hljs-attr">end</span>: diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'tracing:my-channel:end'</span>),
<span class="hljs-attr">asyncStart</span>: diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'tracing:my-channel:asyncStart'</span>),
<span class="hljs-attr">asyncEnd</span>: diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'tracing:my-channel:asyncEnd'</span>),
<span class="hljs-attr">error</span>: diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'tracing:my-channel:error'</span>),
});</code><button class="copy-button">copy</button></pre>
<h4>Class: <code>Channel</code><span><a class="mark" href="#class-channel" id="class-channel">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_class_channel"></a></h4>
<div class="api_metadata">
<span>Added in: v15.1.0, v14.17.0</span>
</div>
<p>The class <code>Channel</code> represents an individual named channel within the data
pipeline. It is used to track subscribers and to publish messages when there
are subscribers present. It exists as a separate object to avoid channel
lookups at publish time, enabling very fast publish speeds and allowing
for heavy use while incurring very minimal cost. Channels are created with
<a href="#diagnostics_channelchannelname"><code>diagnostics_channel.channel(name)</code></a>, constructing a channel directly
with <code>new Channel(name)</code> is not supported.</p>
<h5><code>channel.hasSubscribers</code><span><a class="mark" href="#channelhassubscribers" id="channelhassubscribers">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_channel_hassubscribers"></a></h5>
<div class="api_metadata">
<span>Added in: v15.1.0, v14.17.0</span>
</div>
<ul>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> If there are active subscribers</li>
</ul>
<p>Check if there are active subscribers to this channel. This is helpful if
the message you want to send might be expensive to prepare.</p>
<p>This API is optional but helpful when trying to publish messages from very
performance-sensitive code.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
<span class="hljs-keyword">if</span> (channel.<span class="hljs-property">hasSubscribers</span>) {
<span class="hljs-comment">// There are subscribers, prepare and publish message</span>
}</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
<span class="hljs-keyword">if</span> (channel.<span class="hljs-property">hasSubscribers</span>) {
<span class="hljs-comment">// There are subscribers, prepare and publish message</span>
}</code><button class="copy-button">copy</button></pre>
<h5><code>channel.publish(message)</code><span><a class="mark" href="#channelpublishmessage" id="channelpublishmessage">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_channel_publish_message"></a></h5>
<div class="api_metadata">
<span>Added in: v15.1.0, v14.17.0</span>
</div>
<ul>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The message to send to the channel subscribers</li>
</ul>
<p>Publish a message to any subscribers to the channel. This will trigger
message handlers synchronously so they will execute within the same context.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
channel.<span class="hljs-title function_">publish</span>({
<span class="hljs-attr">some</span>: <span class="hljs-string">'message'</span>,
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
channel.<span class="hljs-title function_">publish</span>({
<span class="hljs-attr">some</span>: <span class="hljs-string">'message'</span>,
});</code><button class="copy-button">copy</button></pre>
<h5><code>channel.subscribe(onMessage)</code><span><a class="mark" href="#channelsubscribeonmessage" id="channelsubscribeonmessage">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_channel_subscribe_onmessage"></a></h5>
<div class="api_metadata">
<span>Added in: v15.1.0, v14.17.0</span><span>Deprecated since: v18.7.0, v16.17.0</span>
</div>
<p></p><div class="api_stability api_stability_0"><a href="documentation.html#stability-index">Stability: 0</a> - Deprecated: Use <a href="#diagnostics_channelsubscribename-onmessage"><code>diagnostics_channel.subscribe(name, onMessage)</code></a></div><p></p>
<ul>
<li><code>onMessage</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The handler to receive channel messages
<ul>
<li><code>message</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The message data</li>
<li><code>name</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#String_type" class="type"><string></a> | <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Symbol_type" class="type"><symbol></a> The name of the channel</li>
</ul>
</li>
</ul>
<p>Register a message handler to subscribe to this channel. This message handler
will be run synchronously whenever a message is published to the channel. Any
errors thrown in the message handler will trigger an <a href="process.html#event-uncaughtexception"><code>'uncaughtException'</code></a>.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
channel.<span class="hljs-title function_">subscribe</span>(<span class="hljs-function">(<span class="hljs-params">message, name</span>) =></span> {
<span class="hljs-comment">// Received data</span>
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
channel.<span class="hljs-title function_">subscribe</span>(<span class="hljs-function">(<span class="hljs-params">message, name</span>) =></span> {
<span class="hljs-comment">// Received data</span>
});</code><button class="copy-button">copy</button></pre>
<h5><code>channel.unsubscribe(onMessage)</code><span><a class="mark" href="#channelunsubscribeonmessage" id="channelunsubscribeonmessage">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_channel_unsubscribe_onmessage"></a></h5>
<div class="api_metadata">
<details class="changelog"><summary>History</summary>
<table>
<tbody><tr><th>Version</th><th>Changes</th></tr>
<tr><td>v18.7.0, v16.17.0</td>
<td><p><span>Deprecated since: v18.7.0, v16.17.0</span></p></td></tr>
<tr><td>v17.1.0, v16.14.0, v14.19.0</td>
<td><p>Added return value. Added to channels without subscribers.</p></td></tr>
<tr><td>v15.1.0, v14.17.0</td>
<td><p><span>Added in: v15.1.0, v14.17.0</span></p></td></tr>
</tbody></table>
</details>
</div>
<p></p><div class="api_stability api_stability_0"><a href="documentation.html#stability-index">Stability: 0</a> - Deprecated: Use <a href="#diagnostics_channelunsubscribename-onmessage"><code>diagnostics_channel.unsubscribe(name, onMessage)</code></a></div><p></p>
<ul>
<li><code>onMessage</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The previous subscribed handler to remove</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> <code>true</code> if the handler was found, <code>false</code> otherwise.</li>
</ul>
<p>Remove a message handler previously registered to this channel with
<a href="#channelsubscribeonmessage"><code>channel.subscribe(onMessage)</code></a>.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
<span class="hljs-keyword">function</span> <span class="hljs-title function_">onMessage</span>(<span class="hljs-params">message, name</span>) {
<span class="hljs-comment">// Received data</span>
}
channel.<span class="hljs-title function_">subscribe</span>(onMessage);
channel.<span class="hljs-title function_">unsubscribe</span>(onMessage);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
<span class="hljs-keyword">function</span> <span class="hljs-title function_">onMessage</span>(<span class="hljs-params">message, name</span>) {
<span class="hljs-comment">// Received data</span>
}
channel.<span class="hljs-title function_">subscribe</span>(onMessage);
channel.<span class="hljs-title function_">unsubscribe</span>(onMessage);</code><button class="copy-button">copy</button></pre>
<h5><code>channel.bindStore(store[, transform])</code><span><a class="mark" href="#channelbindstorestore-transform" id="channelbindstorestore-transform">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_channel_bindstore_store_transform"></a></h5>
<div class="api_metadata">
<span>Added in: v19.9.0</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="async_context.html#class-asynclocalstorage" class="type"><AsyncLocalStorage></a> The store to which to bind the context data</li>
<li><code>transform</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> Transform context data before setting the store context</li>
</ul>
<p>When <a href="#channelrunstorescontext-fn-thisarg-args"><code>channel.runStores(context, ...)</code></a> is called, the given context data
will be applied to any store bound to the channel. If the store has already been
bound the previous <code>transform</code> function will be replaced with the new one.
The <code>transform</code> function may be omitted to set the given context data as the
context directly.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</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> store = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>();
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
channel.<span class="hljs-title function_">bindStore</span>(store, <span class="hljs-function">(<span class="hljs-params">data</span>) =></span> {
<span class="hljs-keyword">return</span> { data };
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</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> store = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>();
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
channel.<span class="hljs-title function_">bindStore</span>(store, <span class="hljs-function">(<span class="hljs-params">data</span>) =></span> {
<span class="hljs-keyword">return</span> { data };
});</code><button class="copy-button">copy</button></pre>
<h5><code>channel.unbindStore(store)</code><span><a class="mark" href="#channelunbindstorestore" id="channelunbindstorestore">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_channel_unbindstore_store"></a></h5>
<div class="api_metadata">
<span>Added in: v19.9.0</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="async_context.html#class-asynclocalstorage" class="type"><AsyncLocalStorage></a> The store to unbind from the channel.</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> <code>true</code> if the store was found, <code>false</code> otherwise.</li>
</ul>
<p>Remove a message handler previously registered to this channel with
<a href="#channelbindstorestore-transform"><code>channel.bindStore(store)</code></a>.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</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> store = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>();
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
channel.<span class="hljs-title function_">bindStore</span>(store);
channel.<span class="hljs-title function_">unbindStore</span>(store);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</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> store = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>();
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
channel.<span class="hljs-title function_">bindStore</span>(store);
channel.<span class="hljs-title function_">unbindStore</span>(store);</code><button class="copy-button">copy</button></pre>
<h5><code>channel.runStores(context, fn[, thisArg[, ...args]])</code><span><a class="mark" href="#channelrunstorescontext-fn-thisarg-args" id="channelrunstorescontext-fn-thisarg-args">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_channel_runstores_context_fn_thisarg_args"></a></h5>
<div class="api_metadata">
<span>Added in: v19.9.0</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>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> Message to send to subscribers and bind to stores</li>
<li><code>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> Handler to run within the entered storage context</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>Applies the given data to any AsyncLocalStorage instances bound to the channel
for the duration of the given function, then publishes to the channel within
the scope of that data is applied to the stores.</p>
<p>If a transform function was given to <a href="#channelbindstorestore-transform"><code>channel.bindStore(store)</code></a> it will be
applied to transform the message data before it becomes the context value for
the store. The prior storage context is accessible from within the transform
function in cases where context linking is required.</p>
<p>The context applied to the store should be accessible in any async code which
continues from execution which began during the given function, however
there are some situations in which <a href="async_context.html#troubleshooting-context-loss">context loss</a> may occur.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</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> store = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>();
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
channel.<span class="hljs-title function_">bindStore</span>(store, <span class="hljs-function">(<span class="hljs-params">message</span>) =></span> {
<span class="hljs-keyword">const</span> parent = store.<span class="hljs-title function_">getStore</span>();
<span class="hljs-keyword">return</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Span</span>(message, parent);
});
channel.<span class="hljs-title function_">runStores</span>({ <span class="hljs-attr">some</span>: <span class="hljs-string">'message'</span> }, <span class="hljs-function">() =></span> {
store.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Span({ some: 'message' })</span>
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</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> store = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>();
<span class="hljs-keyword">const</span> channel = diagnostics_channel.<span class="hljs-title function_">channel</span>(<span class="hljs-string">'my-channel'</span>);
channel.<span class="hljs-title function_">bindStore</span>(store, <span class="hljs-function">(<span class="hljs-params">message</span>) =></span> {
<span class="hljs-keyword">const</span> parent = store.<span class="hljs-title function_">getStore</span>();
<span class="hljs-keyword">return</span> <span class="hljs-keyword">new</span> <span class="hljs-title class_">Span</span>(message, parent);
});
channel.<span class="hljs-title function_">runStores</span>({ <span class="hljs-attr">some</span>: <span class="hljs-string">'message'</span> }, <span class="hljs-function">() =></span> {
store.<span class="hljs-title function_">getStore</span>(); <span class="hljs-comment">// Span({ some: 'message' })</span>
});</code><button class="copy-button">copy</button></pre>
<h4>Class: <code>TracingChannel</code><span><a class="mark" href="#class-tracingchannel" id="class-tracingchannel">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_class_tracingchannel"></a></h4>
<div class="api_metadata">
<span>Added in: v19.9.0</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>The class <code>TracingChannel</code> is a collection of <a href="#tracingchannel-channels">TracingChannel Channels</a> which
together express a single traceable action. It is used to formalize and
simplify the process of producing events for tracing application flow.
<a href="#diagnostics_channeltracingchannelnameorchannels"><code>diagnostics_channel.tracingChannel()</code></a> is used to construct a
<code>TracingChannel</code>. As with <code>Channel</code> it is recommended to create and reuse a
single <code>TracingChannel</code> at the top-level of the file rather than creating them
dynamically.</p>
<h5><code>tracingChannel.subscribe(subscribers)</code><span><a class="mark" href="#tracingchannelsubscribesubscribers" id="tracingchannelsubscribesubscribers">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_tracingchannel_subscribe_subscribers"></a></h5>
<div class="api_metadata">
<span>Added in: v19.9.0</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>subscribers</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> Set of <a href="#tracingchannel-channels">TracingChannel Channels</a> subscribers
<ul>
<li><code>start</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#startevent"><code>start</code> event</a> subscriber</li>
<li><code>end</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#endevent"><code>end</code> event</a> subscriber</li>
<li><code>asyncStart</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#asyncstartevent"><code>asyncStart</code> event</a> subscriber</li>
<li><code>asyncEnd</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#asyncendevent"><code>asyncEnd</code> event</a> subscriber</li>
<li><code>error</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#errorevent"><code>error</code> event</a> subscriber</li>
</ul>
</li>
</ul>
<p>Helper to subscribe a collection of functions to the corresponding channels.
This is the same as calling <a href="#channelsubscribeonmessage"><code>channel.subscribe(onMessage)</code></a> on each channel
individually.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">const</span> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
channels.<span class="hljs-title function_">subscribe</span>({
<span class="hljs-title function_">start</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle start message</span>
},
<span class="hljs-title function_">end</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle end message</span>
},
<span class="hljs-title function_">asyncStart</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle asyncStart message</span>
},
<span class="hljs-title function_">asyncEnd</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle asyncEnd message</span>
},
<span class="hljs-title function_">error</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle error message</span>
},
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">const</span> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
channels.<span class="hljs-title function_">subscribe</span>({
<span class="hljs-title function_">start</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle start message</span>
},
<span class="hljs-title function_">end</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle end message</span>
},
<span class="hljs-title function_">asyncStart</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle asyncStart message</span>
},
<span class="hljs-title function_">asyncEnd</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle asyncEnd message</span>
},
<span class="hljs-title function_">error</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle error message</span>
},
});</code><button class="copy-button">copy</button></pre>
<h5><code>tracingChannel.unsubscribe(subscribers)</code><span><a class="mark" href="#tracingchannelunsubscribesubscribers" id="tracingchannelunsubscribesubscribers">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_tracingchannel_unsubscribe_subscribers"></a></h5>
<div class="api_metadata">
<span>Added in: v19.9.0</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>subscribers</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> Set of <a href="#tracingchannel-channels">TracingChannel Channels</a> subscribers
<ul>
<li><code>start</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#startevent"><code>start</code> event</a> subscriber</li>
<li><code>end</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#endevent"><code>end</code> event</a> subscriber</li>
<li><code>asyncStart</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#asyncstartevent"><code>asyncStart</code> event</a> subscriber</li>
<li><code>asyncEnd</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#asyncendevent"><code>asyncEnd</code> event</a> subscriber</li>
<li><code>error</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> The <a href="#errorevent"><code>error</code> event</a> subscriber</li>
</ul>
</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> <code>true</code> if all handlers were successfully unsubscribed,
and <code>false</code> otherwise.</li>
</ul>
<p>Helper to unsubscribe a collection of functions from the corresponding channels.
This is the same as calling <a href="#channelunsubscribeonmessage"><code>channel.unsubscribe(onMessage)</code></a> on each channel
individually.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">const</span> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
channels.<span class="hljs-title function_">unsubscribe</span>({
<span class="hljs-title function_">start</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle start message</span>
},
<span class="hljs-title function_">end</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle end message</span>
},
<span class="hljs-title function_">asyncStart</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle asyncStart message</span>
},
<span class="hljs-title function_">asyncEnd</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle asyncEnd message</span>
},
<span class="hljs-title function_">error</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle error message</span>
},
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">const</span> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
channels.<span class="hljs-title function_">unsubscribe</span>({
<span class="hljs-title function_">start</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle start message</span>
},
<span class="hljs-title function_">end</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle end message</span>
},
<span class="hljs-title function_">asyncStart</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle asyncStart message</span>
},
<span class="hljs-title function_">asyncEnd</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle asyncEnd message</span>
},
<span class="hljs-title function_">error</span>(<span class="hljs-params">message</span>) {
<span class="hljs-comment">// Handle error message</span>
},
});</code><button class="copy-button">copy</button></pre>
<h5><code>tracingChannel.traceSync(fn[, context[, thisArg[, ...args]]])</code><span><a class="mark" href="#tracingchanneltracesyncfn-context-thisarg-args" id="tracingchanneltracesyncfn-context-thisarg-args">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_tracingchannel_tracesync_fn_context_thisarg_args"></a></h5>
<div class="api_metadata">
<span>Added in: v19.9.0</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>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> Function to wrap a trace around</li>
<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> Shared object to correlate events through</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>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The return value of the given function</li>
</ul>
<p>Trace a synchronous function call. This will always produce a <a href="#startevent"><code>start</code> event</a>
and <a href="#endevent"><code>end</code> event</a> around the execution and may produce an <a href="#errorevent"><code>error</code> event</a>
if the given function throws an error. This will run the given function using
<a href="#channelrunstorescontext-fn-thisarg-args"><code>channel.runStores(context, ...)</code></a> on the <code>start</code> channel which ensures all
events should have any bound stores set to match this trace context.</p>
<p>To ensure only correct trace graphs are formed, events will only be published
if subscribers are present prior to starting the trace. Subscriptions which are
added after the trace begins will not receive future events from that trace,
only future traces will be seen.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">const</span> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
channels.<span class="hljs-title function_">traceSync</span>(<span class="hljs-function">() =></span> {
<span class="hljs-comment">// Do something</span>
}, {
<span class="hljs-attr">some</span>: <span class="hljs-string">'thing'</span>,
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">const</span> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
channels.<span class="hljs-title function_">traceSync</span>(<span class="hljs-function">() =></span> {
<span class="hljs-comment">// Do something</span>
}, {
<span class="hljs-attr">some</span>: <span class="hljs-string">'thing'</span>,
});</code><button class="copy-button">copy</button></pre>
<h5><code>tracingChannel.tracePromise(fn[, context[, thisArg[, ...args]]])</code><span><a class="mark" href="#tracingchanneltracepromisefn-context-thisarg-args" id="tracingchanneltracepromisefn-context-thisarg-args">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_tracingchannel_tracepromise_fn_context_thisarg_args"></a></h5>
<div class="api_metadata">
<span>Added in: v19.9.0</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>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> Promise-returning function to wrap a trace around</li>
<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> Shared object to correlate trace events through</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>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise" class="type"><Promise></a> Chained from promise returned by the given function</li>
</ul>
<p>Trace a promise-returning function call. This will always produce a
<a href="#startevent"><code>start</code> event</a> and <a href="#endevent"><code>end</code> event</a> around the synchronous portion of the
function execution, and will produce an <a href="#asyncstartevent"><code>asyncStart</code> event</a> and
<a href="#asyncendevent"><code>asyncEnd</code> event</a> when a promise continuation is reached. It may also
produce an <a href="#errorevent"><code>error</code> event</a> if the given function throws an error or the
returned promise rejects. This will run the given function using
<a href="#channelrunstorescontext-fn-thisarg-args"><code>channel.runStores(context, ...)</code></a> on the <code>start</code> channel which ensures all
events should have any bound stores set to match this trace context.</p>
<p>To ensure only correct trace graphs are formed, events will only be published
if subscribers are present prior to starting the trace. Subscriptions which are
added after the trace begins will not receive future events from that trace,
only future traces will be seen.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">const</span> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
channels.<span class="hljs-title function_">tracePromise</span>(<span class="hljs-keyword">async</span> () => {
<span class="hljs-comment">// Do something</span>
}, {
<span class="hljs-attr">some</span>: <span class="hljs-string">'thing'</span>,
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">const</span> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
channels.<span class="hljs-title function_">tracePromise</span>(<span class="hljs-keyword">async</span> () => {
<span class="hljs-comment">// Do something</span>
}, {
<span class="hljs-attr">some</span>: <span class="hljs-string">'thing'</span>,
});</code><button class="copy-button">copy</button></pre>
<h5><code>tracingChannel.traceCallback(fn, position, context, thisArg, ...args)</code><span><a class="mark" href="#tracingchanneltracecallbackfn-position-context-thisarg-args" id="tracingchanneltracecallbackfn-position-context-thisarg-args">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_tracingchannel_tracecallback_fn_position_context_thisarg_args"></a></h5>
<div class="api_metadata">
<span>Added in: v19.9.0</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>fn</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function" class="type"><Function></a> callback using function to wrap a trace around</li>
<li><code>position</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><number></a> Zero-indexed argument position of expected callback
(defaults to last argument if <code>undefined</code> is passed)</li>
<li><code>context</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> Shared object to correlate trace events through (defaults
to <code>{}</code> if <code>undefined</code> is passed)</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> arguments to pass to the function (must include the callback)</li>
<li>Returns: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Data_types" class="type"><any></a> The return value of the given function</li>
</ul>
<p>Trace a callback-receiving function call. The callback is expected to follow
the error as first arg convention typically used. This will always produce a
<a href="#startevent"><code>start</code> event</a> and <a href="#endevent"><code>end</code> event</a> around the synchronous portion of the
function execution, and will produce a <a href="#asyncstartevent"><code>asyncStart</code> event</a> and
<a href="#asyncendevent"><code>asyncEnd</code> event</a> around the callback execution. It may also produce an
<a href="#errorevent"><code>error</code> event</a> if the given function throws or the first argument passed to
the callback is set. This will run the given function using
<a href="#channelrunstorescontext-fn-thisarg-args"><code>channel.runStores(context, ...)</code></a> on the <code>start</code> channel which ensures all
events should have any bound stores set to match this trace context.</p>
<p>To ensure only correct trace graphs are formed, events will only be published
if subscribers are present prior to starting the trace. Subscriptions which are
added after the trace begins will not receive future events from that trace,
only future traces will be seen.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</span>;
<span class="hljs-keyword">const</span> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
channels.<span class="hljs-title function_">traceCallback</span>(<span class="hljs-function">(<span class="hljs-params">arg1, callback</span>) =></span> {
<span class="hljs-comment">// Do something</span>
<span class="hljs-title function_">callback</span>(<span class="hljs-literal">null</span>, <span class="hljs-string">'result'</span>);
}, <span class="hljs-number">1</span>, {
<span class="hljs-attr">some</span>: <span class="hljs-string">'thing'</span>,
}, thisArg, arg1, callback);</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</span>);
<span class="hljs-keyword">const</span> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
channels.<span class="hljs-title function_">traceCallback</span>(<span class="hljs-function">(<span class="hljs-params">arg1, callback</span>) =></span> {
<span class="hljs-comment">// Do something</span>
<span class="hljs-title function_">callback</span>(<span class="hljs-literal">null</span>, <span class="hljs-string">'result'</span>);
}, {
<span class="hljs-attr">some</span>: <span class="hljs-string">'thing'</span>,
}, thisArg, arg1, callback);</code><button class="copy-button">copy</button></pre>
<p>The callback will also be run with <a href="#channelrunstorescontext-fn-thisarg-args"><code>channel.runStores(context, ...)</code></a> which
enables context loss recovery in some cases.</p>
<pre class="with-64-chars"><input class="js-flavor-toggle" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> diagnostics_channel <span class="hljs-keyword">from</span> <span class="hljs-string">'node:diagnostics_channel'</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> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
<span class="hljs-keyword">const</span> myStore = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>();
<span class="hljs-comment">// The start channel sets the initial store data to something</span>
<span class="hljs-comment">// and stores that store data value on the trace context object</span>
channels.<span class="hljs-property">start</span>.<span class="hljs-title function_">bindStore</span>(myStore, <span class="hljs-function">(<span class="hljs-params">data</span>) =></span> {
<span class="hljs-keyword">const</span> span = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Span</span>(data);
data.<span class="hljs-property">span</span> = span;
<span class="hljs-keyword">return</span> span;
});
<span class="hljs-comment">// Then asyncStart can restore from that data it stored previously</span>
channels.<span class="hljs-property">asyncStart</span>.<span class="hljs-title function_">bindStore</span>(myStore, <span class="hljs-function">(<span class="hljs-params">data</span>) =></span> {
<span class="hljs-keyword">return</span> data.<span class="hljs-property">span</span>;
});</code><code class="language-js cjs"><span class="hljs-keyword">const</span> diagnostics_channel = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:diagnostics_channel'</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> channels = diagnostics_channel.<span class="hljs-title function_">tracingChannel</span>(<span class="hljs-string">'my-channel'</span>);
<span class="hljs-keyword">const</span> myStore = <span class="hljs-keyword">new</span> <span class="hljs-title class_">AsyncLocalStorage</span>();
<span class="hljs-comment">// The start channel sets the initial store data to something</span>
<span class="hljs-comment">// and stores that store data value on the trace context object</span>
channels.<span class="hljs-property">start</span>.<span class="hljs-title function_">bindStore</span>(myStore, <span class="hljs-function">(<span class="hljs-params">data</span>) =></span> {
<span class="hljs-keyword">const</span> span = <span class="hljs-keyword">new</span> <span class="hljs-title class_">Span</span>(data);
data.<span class="hljs-property">span</span> = span;
<span class="hljs-keyword">return</span> span;
});
<span class="hljs-comment">// Then asyncStart can restore from that data it stored previously</span>
channels.<span class="hljs-property">asyncStart</span>.<span class="hljs-title function_">bindStore</span>(myStore, <span class="hljs-function">(<span class="hljs-params">data</span>) =></span> {
<span class="hljs-keyword">return</span> data.<span class="hljs-property">span</span>;
});</code><button class="copy-button">copy</button></pre>
<h4>TracingChannel Channels<span><a class="mark" href="#tracingchannel-channels" id="tracingchannel-channels">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_tracingchannel_channels"></a></h4>
<p>A TracingChannel is a collection of several diagnostics_channels representing
specific points in the execution lifecycle of a single traceable action. The
behavior is split into five diagnostics_channels consisting of <code>start</code>,
<code>end</code>, <code>asyncStart</code>, <code>asyncEnd</code>, and <code>error</code>. A single traceable action will
share the same event object between all events, this can be helpful for
managing correlation through a weakmap.</p>
<p>These event objects will be extended with <code>result</code> or <code>error</code> values when
the task "completes". In the case of a synchronous task the <code>result</code> will be
the return value and the <code>error</code> will be anything thrown from the function.
With callback-based async functions the <code>result</code> will be the second argument
of the callback while the <code>error</code> will either be a thrown error visible in the
<code>end</code> event or the first callback argument in either of the <code>asyncStart</code> or
<code>asyncEnd</code> events.</p>
<p>To ensure only correct trace graphs are formed, events should only be published
if subscribers are present prior to starting the trace. Subscriptions which are
added after the trace begins should not receive future events from that trace,
only future traces will be seen.</p>
<p>Tracing channels should follow a naming pattern of:</p>
<ul>
<li><code>tracing:module.class.method:start</code> or <code>tracing:module.function:start</code></li>
<li><code>tracing:module.class.method:end</code> or <code>tracing:module.function:end</code></li>
<li><code>tracing:module.class.method:asyncStart</code> or <code>tracing:module.function:asyncStart</code></li>
<li><code>tracing:module.class.method:asyncEnd</code> or <code>tracing:module.function:asyncEnd</code></li>
<li><code>tracing:module.class.method:error</code> or <code>tracing:module.function:error</code></li>
</ul>
<h5><code>start(event)</code><span><a class="mark" href="#startevent" id="startevent">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_start_event"></a></h5>
<ul>
<li>Name: <code>tracing:${name}:start</code></li>
</ul>
<p>The <code>start</code> event represents the point at which a function is called. At this
point the event data may contain function arguments or anything else available
at the very start of the execution of the function.</p>
<h5><code>end(event)</code><span><a class="mark" href="#endevent" id="endevent">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_end_event"></a></h5>
<ul>
<li>Name: <code>tracing:${name}:end</code></li>
</ul>
<p>The <code>end</code> event represents the point at which a function call returns a value.
In the case of an async function this is when the promise returned not when the
function itself makes a return statement internally. At this point, if the
traced function was synchronous the <code>result</code> field will be set to the return
value of the function. Alternatively, the <code>error</code> field may be present to
represent any thrown errors.</p>
<p>It is recommended to listen specifically to the <code>error</code> event to track errors
as it may be possible for a traceable action to produce multiple errors. For
example, an async task which fails may be started internally before the sync
part of the task then throws an error.</p>
<h5><code>asyncStart(event)</code><span><a class="mark" href="#asyncstartevent" id="asyncstartevent">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_asyncstart_event"></a></h5>
<ul>
<li>Name: <code>tracing:${name}:asyncStart</code></li>
</ul>
<p>The <code>asyncStart</code> event represents the callback or continuation of a traceable
function being reached. At this point things like callback arguments may be
available, or anything else expressing the "result" of the action.</p>
<p>For callbacks-based functions, the first argument of the callback will be
assigned to the <code>error</code> field, if not <code>undefined</code> or <code>null</code>, and the second
argument will be assigned to the <code>result</code> field.</p>
<p>For promises, the argument to the <code>resolve</code> path will be assigned to <code>result</code>
or the argument to the <code>reject</code> path will be assign to <code>error</code>.</p>
<p>It is recommended to listen specifically to the <code>error</code> event to track errors
as it may be possible for a traceable action to produce multiple errors. For
example, an async task which fails may be started internally before the sync
part of the task then throws an error.</p>
<h5><code>asyncEnd(event)</code><span><a class="mark" href="#asyncendevent" id="asyncendevent">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_asyncend_event"></a></h5>
<ul>
<li>Name: <code>tracing:${name}:asyncEnd</code></li>
</ul>
<p>The <code>asyncEnd</code> event represents the callback of an asynchronous function
returning. It's not likely event data will change after the <code>asyncStart</code> event,
however it may be useful to see the point where the callback completes.</p>
<h5><code>error(event)</code><span><a class="mark" href="#errorevent" id="errorevent">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_error_event"></a></h5>
<ul>
<li>Name: <code>tracing:${name}:error</code></li>
</ul>
<p>The <code>error</code> event represents any error produced by the traceable function
either synchronously or asynchronously. If an error is thrown in the
synchronous portion of the traced function the error will be assigned to the
<code>error</code> field of the event and the <code>error</code> event will be triggered. If an error
is received asynchronously through a callback or promise rejection it will also
be assigned to the <code>error</code> field of the event and trigger the <code>error</code> event.</p>
<p>It is possible for a single traceable function call to produce errors multiple
times so this should be considered when consuming this event. For example, if
another async task is triggered internally which fails and then the sync part
of the function then throws and error two <code>error</code> events will be emitted, one
for the sync error and one for the async error.</p>
<h4>Built-in Channels<span><a class="mark" href="#built-in-channels" id="built-in-channels">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_built_in_channels"></a></h4>
<p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p>
<p>While the diagnostics_channel API is now considered stable, the built-in
channels currently available are not. Each channel must be declared stable
independently.</p>
<h5>HTTP<span><a class="mark" href="#http" id="http">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_http"></a></h5>
<p><code>http.client.request.start</code></p>
<ul>
<li><code>request</code> <a href="http.html#class-httpclientrequest" class="type"><http.ClientRequest></a></li>
</ul>
<p>Emitted when client starts a request.</p>
<p><code>http.client.response.finish</code></p>
<ul>
<li><code>request</code> <a href="http.html#class-httpclientrequest" class="type"><http.ClientRequest></a></li>
<li><code>response</code> <a href="http.html#class-httpincomingmessage" class="type"><http.IncomingMessage></a></li>
</ul>
<p>Emitted when client receives a response.</p>
<p><code>http.server.request.start</code></p>
<ul>
<li><code>request</code> <a href="http.html#class-httpincomingmessage" class="type"><http.IncomingMessage></a></li>
<li><code>response</code> <a href="http.html#class-httpserverresponse" class="type"><http.ServerResponse></a></li>
<li><code>socket</code> <a href="net.html#class-netsocket" class="type"><net.Socket></a></li>
<li><code>server</code> <a href="http.html#class-httpserver" class="type"><http.Server></a></li>
</ul>
<p>Emitted when server receives a request.</p>
<p><code>http.server.response.finish</code></p>
<ul>
<li><code>request</code> <a href="http.html#class-httpincomingmessage" class="type"><http.IncomingMessage></a></li>
<li><code>response</code> <a href="http.html#class-httpserverresponse" class="type"><http.ServerResponse></a></li>
<li><code>socket</code> <a href="net.html#class-netsocket" class="type"><net.Socket></a></li>
<li><code>server</code> <a href="http.html#class-httpserver" class="type"><http.Server></a></li>
</ul>
<p>Emitted when server sends a response.</p>
<h5>NET<span><a class="mark" href="#net" id="net">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_net"></a></h5>
<p><code>net.client.socket</code></p>
<ul>
<li><code>socket</code> <a href="net.html#class-netsocket" class="type"><net.Socket></a></li>
</ul>
<p>Emitted when a new TCP or pipe client socket is created.</p>
<p><code>net.server.socket</code></p>
<ul>
<li><code>socket</code> <a href="net.html#class-netsocket" class="type"><net.Socket></a></li>
</ul>
<p>Emitted when a new TCP or pipe connection is received.</p>
<h5>UDP<span><a class="mark" href="#udp" id="udp">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_udp"></a></h5>
<p><code>udp.socket</code></p>
<ul>
<li><code>socket</code> <a href="dgram.html#class-dgramsocket" class="type"><dgram.Socket></a></li>
</ul>
<p>Emitted when a new UDP socket is created.</p>
<h5>Process<span><a class="mark" href="#process" id="process">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_process"></a></h5>
<div class="api_metadata">
<span>Added in: v16.18.0</span>
</div>
<p><code>child_process</code></p>
<ul>
<li><code>process</code> <a href="child_process.html#class-childprocess" class="type"><ChildProcess></a></li>
</ul>
<p>Emitted when a new process is created.</p>
<h5>Worker Thread<span><a class="mark" href="#worker-thread" id="worker-thread">#</a></span><a aria-hidden="true" class="legacy" id="diagnostics_channel_worker_thread"></a></h5>
<div class="api_metadata">
<span>Added in: v16.18.0</span>
</div>
<p><code>worker_threads</code></p>
<ul>
<li><code>worker</code> <a href="worker_threads.html#class-worker"><code>Worker</code></a></li>
</ul>
<p>Emitted when a new thread is created.</p></section>
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