| <!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="Making [`Arc`][Arc] itself atomic"><meta name="keywords" content="rust, rustlang, rust-lang, arc_swap"><title>arc_swap - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../normalize.css"><link rel="stylesheet" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../ayu.css" disabled><link rel="stylesheet" href="../dark.css" disabled><link rel="stylesheet" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script defer src="../crates.js"></script><script defer src="../main.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.svg"></head><body class="rustdoc mod crate"><!--[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="sidebar-logo" href="../arc_swap/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../arc_swap/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2 class="location"><a href="#">Crate arc_swap</a></h2><div class="sidebar-elems"><ul class="block"><li class="version">Version 1.6.0</li><li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><li><a href="#reexports">Re-exports</a></li><li><a href="#modules">Modules</a></li><li><a href="#structs">Structs</a></li><li><a href="#traits">Traits</a></li><li><a href="#types">Type Definitions</a></li></ul></section></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Crate <a class="mod" href="#">arc_swap</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../src/arc_swap/lib.rs.html#1-1306">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Making [<code>Arc</code>][Arc] itself atomic</p> |
| <p>The <a href="type.ArcSwap.html" title="ArcSwap"><code>ArcSwap</code></a> type is a container for an <code>Arc</code> that can be changed atomically. Semantically, |
| it is similar to something like <code>Atomic<Arc<T>></code> (if there was such a thing) or |
| <code>RwLock<Arc<T>></code> (but without the need for the locking). It is optimized for read-mostly |
| scenarios, with consistent performance characteristics.</p> |
| <h2 id="motivation"><a href="#motivation">Motivation</a></h2> |
| <p>There are many situations in which one might want to have some data structure that is often |
| read and seldom updated. Some examples might be a configuration of a service, routing tables, |
| snapshot of some data that is renewed every few minutes, etc.</p> |
| <p>In all these cases one needs:</p> |
| <ul> |
| <li>Being able to read the current value of the data structure, fast, often and concurrently from |
| many threads.</li> |
| <li>Using the same version of the data structure over longer period of time ‒ a query should be |
| answered by a consistent version of data, a packet should be routed either by an old or by a |
| new version of the routing table but not by a combination, etc.</li> |
| <li>Perform an update without disrupting the processing.</li> |
| </ul> |
| <p>The first idea would be to use <a href="https://doc.rust-lang.org/std/sync/struct.RwLock.html"><code>RwLock<T></code></a> and keep a read-lock for the whole time of |
| processing. Update would, however, pause all processing until done.</p> |
| <p>Better option would be to have <a href="https://doc.rust-lang.org/std/sync/struct.RwLock.html"><code>RwLock<Arc<T>></code></a>. Then one would lock, clone the [Arc] |
| and unlock. This suffers from CPU-level contention (on the lock and on the reference count of |
| the [Arc]) which makes it relatively slow. Depending on the implementation, an update may be |
| blocked for arbitrary long time by a steady inflow of readers.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">static </span>ROUTING_TABLE: Lazy<RwLock<Arc<RoutingTable>>> = Lazy::new(|| { |
| RwLock::new(Arc::new(RoutingTable)) |
| }); |
| |
| <span class="kw">fn </span>process_packet(packet: Packet) { |
| <span class="kw">let </span>table = Arc::clone(<span class="kw-2">&</span>ROUTING_TABLE.read().unwrap()); |
| table.route(packet); |
| }</code></pre></div> |
| <p>The <a href="type.ArcSwap.html" title="ArcSwap">ArcSwap</a> can be used instead, which solves the above problems and has better performance |
| characteristics than the <a href="https://doc.rust-lang.org/std/sync/struct.RwLock.html">RwLock</a>, both in contended and non-contended scenarios.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">static </span>ROUTING_TABLE: Lazy<ArcSwap<RoutingTable>> = Lazy::new(|| { |
| ArcSwap::from_pointee(RoutingTable) |
| }); |
| |
| <span class="kw">fn </span>process_packet(packet: Packet) { |
| <span class="kw">let </span>table = ROUTING_TABLE.load(); |
| table.route(packet); |
| }</code></pre></div> |
| <h2 id="crate-contents"><a href="#crate-contents">Crate contents</a></h2> |
| <p>At the heart of the crate there are <a href="type.ArcSwap.html" title="ArcSwap"><code>ArcSwap</code></a> and <a href="type.ArcSwapOption.html" title="ArcSwapOption"><code>ArcSwapOption</code></a> types, containers for an |
| [<code>Arc</code>] and [<code>Option<Arc></code>][Option].</p> |
| <p>Technically, these are type aliases for partial instantiations of the <a href="struct.ArcSwapAny.html" title="ArcSwapAny"><code>ArcSwapAny</code></a> type. The |
| <a href="struct.ArcSwapAny.html" title="ArcSwapAny"><code>ArcSwapAny</code></a> is more flexible and allows tweaking of many things (can store other things than |
| [<code>Arc</code>]s, can configure the locking <a href="strategy/trait.Strategy.html" title="Strategy"><code>Strategy</code></a>). For details about the tweaking, see the |
| documentation of the <a href="strategy/index.html" title="strategy"><code>strategy</code></a> module and the <a href="trait.RefCnt.html" title="RefCnt"><code>RefCnt</code></a> trait.</p> |
| <p>The <a href="cache/index.html" title="cache"><code>cache</code></a> module provides means for speeding up read access of the contained data at the |
| cost of delayed reclamation.</p> |
| <p>The <a href="access/index.html" title="access"><code>access</code></a> module can be used to do projections into the contained data to separate parts |
| of application from each other (eg. giving a component access to only its own part of |
| configuration while still having it reloaded as a whole).</p> |
| <h2 id="before-using"><a href="#before-using">Before using</a></h2> |
| <p>The data structure is a bit niche. Before using, please check the |
| <a href="docs/limitations/index.html" title="docs::limitations">limitations and common pitfalls</a> and the <a href="docs/performance/index.html" title="docs::performance">performance |
| characteristics</a>, including choosing the right <a href="docs/performance/index.html#read-operations" title="docs::performance">read |
| operation</a>.</p> |
| <p>You can also get an inspiration about what’s possible in the <a href="docs/patterns/index.html" title="docs::patterns">common patterns</a> |
| section.</p> |
| <h2 id="examples"><a href="#examples">Examples</a></h2> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::sync::Arc; |
| |
| <span class="kw">use </span>arc_swap::ArcSwap; |
| <span class="kw">use </span>crossbeam_utils::thread; |
| |
| <span class="kw">fn </span>main() { |
| <span class="kw">let </span>config = ArcSwap::from(Arc::new(String::default())); |
| thread::scope(|scope| { |
| scope.spawn(|<span class="kw">_</span>| { |
| <span class="kw">let </span>new_conf = Arc::new(<span class="string">"New configuration"</span>.to_owned()); |
| config.store(new_conf); |
| }); |
| <span class="kw">for _ in </span><span class="number">0</span>..<span class="number">10 </span>{ |
| scope.spawn(|<span class="kw">_</span>| { |
| <span class="kw">loop </span>{ |
| <span class="kw">let </span>cfg = config.load(); |
| <span class="kw">if </span>!cfg.is_empty() { |
| <span class="macro">assert_eq!</span>(<span class="kw-2">**</span>cfg, <span class="string">"New configuration"</span>); |
| <span class="kw">return</span>; |
| } |
| } |
| }); |
| } |
| }).unwrap(); |
| }</code></pre></div> |
| </div></details><h2 id="reexports" class="small-section-header"><a href="#reexports">Re-exports</a></h2><div class="item-table"><div class="item-row"><div class="item-left import-item" id="reexport.Cache"><code>pub use crate::cache::<a class="struct" href="cache/struct.Cache.html" title="struct arc_swap::cache::Cache">Cache</a>;</code></div></div><div class="item-row"><div class="item-left import-item" id="reexport.DefaultStrategy"><code>pub use crate::strategy::<a class="type" href="strategy/type.DefaultStrategy.html" title="type arc_swap::strategy::DefaultStrategy">DefaultStrategy</a>;</code></div></div><div class="item-row"><div class="item-left import-item" id="reexport.IndependentStrategy"><code>pub use crate::strategy::IndependentStrategy;</code></div></div></div><h2 id="modules" class="small-section-header"><a href="#modules">Modules</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="mod" href="access/index.html" title="arc_swap::access mod">access</a></div><div class="item-right docblock-short">Abstracting over accessing parts of stored value.</div></div><div class="item-row"><div class="item-left module-item"><a class="mod" href="cache/index.html" title="arc_swap::cache mod">cache</a></div><div class="item-right docblock-short">Caching handle into the <a href="struct.ArcSwapAny.html" title="ArcSwapAny">ArcSwapAny</a>.</div></div><div class="item-row"><div class="item-left module-item"><a class="mod" href="docs/index.html" title="arc_swap::docs mod">docs</a></div><div class="item-right docblock-short">Additional documentation.</div></div><div class="item-row"><div class="item-left module-item"><a class="mod" href="strategy/index.html" title="arc_swap::strategy mod">strategy</a></div><div class="item-right docblock-short">Strategies for protecting the reference counts.</div></div></div><h2 id="structs" class="small-section-header"><a href="#structs">Structs</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.ArcSwapAny.html" title="arc_swap::ArcSwapAny struct">ArcSwapAny</a></div><div class="item-right docblock-short">An atomic storage for a reference counted smart pointer like <a href="https://doc.rust-lang.org/std/sync/struct.Arc.html"><code>Arc</code></a> or <code>Option<Arc></code>.</div></div><div class="item-row"><div class="item-left module-item"><a class="struct" href="struct.Guard.html" title="arc_swap::Guard struct">Guard</a></div><div class="item-right docblock-short">A temporary storage of the pointer.</div></div></div><h2 id="traits" class="small-section-header"><a href="#traits">Traits</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="trait" href="trait.AsRaw.html" title="arc_swap::AsRaw trait">AsRaw</a></div><div class="item-right docblock-short">A trait describing things that can be turned into a raw pointer.</div></div><div class="item-row"><div class="item-left module-item"><a class="trait" href="trait.RefCnt.html" title="arc_swap::RefCnt trait">RefCnt</a></div><div class="item-right docblock-short">A trait describing smart reference counted pointers.</div></div></div><h2 id="types" class="small-section-header"><a href="#types">Type Definitions</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="type" href="type.ArcSwap.html" title="arc_swap::ArcSwap type">ArcSwap</a></div><div class="item-right docblock-short">An atomic storage for <code>Arc</code>.</div></div><div class="item-row"><div class="item-left module-item"><a class="type" href="type.ArcSwapOption.html" title="arc_swap::ArcSwapOption type">ArcSwapOption</a></div><div class="item-right docblock-short">An atomic storage for <code>Option<Arc></code>.</div></div></div></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="arc_swap" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
