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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Dispatches trace events to `Subscriber`s."><title>tracing::dispatcher - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="tracing" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="../sidebar-items.js"></script><script defer src="../../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc mod"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button><a class="logo-container" href="../../tracing/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt=""></a></nav><nav class="sidebar"><div class="sidebar-crate"><a class="logo-container" href="../../tracing/index.html"><img src="https://raw.githubusercontent.com/tokio-rs/tracing/master/assets/logo-type.png" alt="logo"></a><h2><a href="../../tracing/index.html">tracing</a><span class="version">0.1.40</span></h2></div><h2 class="location"><a href="#">Module dispatcher</a></h2><div class="sidebar-elems"><section><ul class="block"><li><a href="#structs">Structs</a></li><li><a href="#functions">Functions</a></li></ul></section><h2><a href="../index.html">In crate tracing</a></h2></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../../tracing/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Module <a href="../index.html">tracing</a>::<wbr><a class="mod" href="#">dispatcher</a><button id="copy-path" title="Copy item path to clipboard"><img src="../../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../../src/tracing/dispatcher.rs.html#1-145">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Dispatches trace events to <a href="../trait.Subscriber.html" title="trait tracing::Subscriber"><code>Subscriber</code></a>s.</p>
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<p>The <em>dispatcher</em> is the component of the tracing system which is responsible
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for forwarding trace data from the instrumentation points that generate it
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to the subscriber that collects it.</p>
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<h2 id="using-the-trace-dispatcher"><a href="#using-the-trace-dispatcher">Using the Trace Dispatcher</a></h2>
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<p>Every thread in a program using <code>tracing</code> has a <em>default subscriber</em>. When
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events occur, or spans are created, they are dispatched to the thread’s
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current subscriber.</p>
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<h3 id="setting-the-default-subscriber"><a href="#setting-the-default-subscriber">Setting the Default Subscriber</a></h3>
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<p>By default, the current subscriber is an empty implementation that does
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nothing. To use a subscriber implementation, it must be set as the default.
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There are two methods for doing so: <a href="fn.with_default.html" title="fn tracing::dispatcher::with_default"><code>with_default</code></a> and
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<a href="fn.set_global_default.html" title="fn tracing::dispatcher::set_global_default"><code>set_global_default</code></a>. <code>with_default</code> sets the default subscriber for the
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duration of a scope, while <code>set_global_default</code> sets a default subscriber
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for the entire process.</p>
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<p>To use either of these functions, we must first wrap our subscriber in a
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<a href="../struct.Dispatch.html" title="struct tracing::Dispatch"><code>Dispatch</code></a>, a cloneable, type-erased reference to a subscriber. For
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example:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>dispatcher::Dispatch;
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<span class="kw">let </span>my_subscriber = FooSubscriber::new();
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<span class="kw">let </span>my_dispatch = Dispatch::new(my_subscriber);</code></pre></div>
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<p>Then, we can use <a href="fn.with_default.html" title="fn tracing::dispatcher::with_default"><code>with_default</code></a> to set our <code>Dispatch</code> as the default for
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the duration of a block:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// no default subscriber
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</span>dispatcher::with_default(<span class="kw-2">&</span>my_dispatch, || {
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<span class="comment">// my_subscriber is the default
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</span>});
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<span class="comment">// no default subscriber again</span></code></pre></div>
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<p>It’s important to note that <code>with_default</code> will not propagate the current
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thread’s default subscriber to any threads spawned within the <code>with_default</code>
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block. To propagate the default subscriber to new threads, either use
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<code>with_default</code> from the new thread, or use <code>set_global_default</code>.</p>
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<p>As an alternative to <code>with_default</code>, we can use <a href="fn.set_global_default.html" title="fn tracing::dispatcher::set_global_default"><code>set_global_default</code></a> to
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set a <code>Dispatch</code> as the default for all threads, for the lifetime of the
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program. For example:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// no default subscriber
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</span>dispatcher::set_global_default(my_dispatch)
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<span class="comment">// `set_global_default` will return an error if the global default
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// subscriber has already been set.
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</span>.expect(<span class="string">"global default was already set!"</span>);
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<span class="comment">// `my_subscriber` is now the default</span></code></pre></div>
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<pre class="ignore" style="white-space:normal;font:inherit;">
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<strong>Note</strong>: The thread-local scoped dispatcher (<code>with_default</code>)
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requires the Rust standard library. <code>no_std</code> users should
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use <a href="fn.set_global_default.html"><code>set_global_default</code></a>
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instead.
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</pre>
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<h3 id="accessing-the-default-subscriber"><a href="#accessing-the-default-subscriber">Accessing the Default Subscriber</a></h3>
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<p>A thread’s current default subscriber can be accessed using the
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<a href="fn.get_default.html" title="fn tracing::dispatcher::get_default"><code>get_default</code></a> function, which executes a closure with a reference to the
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currently default <code>Dispatch</code>. This is used primarily by <code>tracing</code>
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instrumentation.</p>
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</div></details><h2 id="structs" class="section-header"><a href="#structs">Structs</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.DefaultGuard.html" title="struct tracing::dispatcher::DefaultGuard">DefaultGuard</a></div><div class="desc docblock-short">A guard that resets the current default dispatcher to the prior
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default dispatcher when dropped.</div></li><li><div class="item-name"><a class="struct" href="struct.Dispatch.html" title="struct tracing::dispatcher::Dispatch">Dispatch</a></div><div class="desc docblock-short"><code>Dispatch</code> trace data to a <a href="../trait.Subscriber.html" title="trait tracing::Subscriber"><code>Subscriber</code></a>.</div></li><li><div class="item-name"><a class="struct" href="struct.SetGlobalDefaultError.html" title="struct tracing::dispatcher::SetGlobalDefaultError">SetGlobalDefaultError</a></div><div class="desc docblock-short">Returned if setting the global dispatcher fails.</div></li><li><div class="item-name"><a class="struct" href="struct.WeakDispatch.html" title="struct tracing::dispatcher::WeakDispatch">WeakDispatch</a></div><div class="desc docblock-short"><code>WeakDispatch</code> is a version of <a href="../struct.Dispatch.html" title="struct tracing::Dispatch"><code>Dispatch</code></a> that holds a non-owning reference
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to a <a href="../trait.Subscriber.html" title="trait tracing::Subscriber"><code>Subscriber</code></a>.</div></li></ul><h2 id="functions" class="section-header"><a href="#functions">Functions</a></h2><ul class="item-table"><li><div class="item-name"><a class="fn" href="fn.get_default.html" title="fn tracing::dispatcher::get_default">get_default</a></div><div class="desc docblock-short">Executes a closure with a reference to this thread’s current <a href="../struct.Dispatch.html" title="struct tracing::Dispatch">dispatcher</a>.</div></li><li><div class="item-name"><a class="fn" href="fn.set_default.html" title="fn tracing::dispatcher::set_default">set_default</a></div><div class="desc docblock-short">Sets the dispatch as the default dispatch for the duration of the lifetime
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of the returned DefaultGuard</div></li><li><div class="item-name"><a class="fn" href="fn.set_global_default.html" title="fn tracing::dispatcher::set_global_default">set_global_default</a></div><div class="desc docblock-short">Sets this dispatch as the global default for the duration of the entire program.
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Will be used as a fallback if no thread-local dispatch has been set in a thread
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(using <code>with_default</code>.)</div></li><li><div class="item-name"><a class="fn" href="fn.with_default.html" title="fn tracing::dispatcher::with_default">with_default</a></div><div class="desc docblock-short">Sets this dispatch as the default for the duration of a closure.</div></li></ul></section></div></main></body></html>
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