Asynchronous context tracking | Node.js v16.20.2 Documentation Node.js * About this documentation * Usage and example * Assertion testing * Asynchronous context tracking * Async hooks * Buffer * C++ addons * C/C++ addons with Node-API * C++ embedder API * Child processes * Cluster * Command-line options * Console * Corepack * Crypto * Debugger * Deprecated APIs * Diagnostics Channel * DNS * Domain * Errors * Events * File system * Globals * HTTP * HTTP/2 * HTTPS * Inspector * Internationalization * Modules: CommonJS modules * Modules: ECMAScript modules * Modules: node:module API * Modules: Packages * Net * OS * Path * Performance hooks * Permissions * Process * Punycode * Query strings * Readline * REPL * Report * Stream * String decoder * Test runner * Timers * TLS/SSL * Trace events * TTY * UDP/datagram * URL * Utilities * V8 * VM * WASI * Web Crypto API * Web Streams API * Worker threads * Zlib * Code repository and issue tracker Node.js v16.20.2 documentation * Node.js v16.20.2 * ► ▼ Table of contents * Asynchronous context tracking * Introduction * Class: AsyncLocalStorage * new AsyncLocalStorage() * asyncLocalStorage.disable() * asyncLocalStorage.getStore() * asyncLocalStorage.enterWith(store) * asyncLocalStorage.run(store, callback[, ...args]) * asyncLocalStorage.exit(callback[, ...args]) * Usage with async/await * Troubleshooting: Context loss * Class: AsyncResource * new AsyncResource(type[, options]) * Static method: AsyncResource.bind(fn[, type[, thisArg]]) * asyncResource.bind(fn[, thisArg]) * asyncResource.runInAsyncScope(fn[, thisArg, ...args]) * asyncResource.emitDestroy() * asyncResource.asyncId() * asyncResource.triggerAsyncId() * Using AsyncResource for a Worker thread pool * Integrating AsyncResource with EventEmitter * ► ▼ Index * About this documentation * Usage and example * Index * Assertion testing * Asynchronous context tracking * Async hooks * Buffer * C++ addons * C/C++ addons with Node-API * C++ embedder API * Child processes * Cluster * Command-line options * Console * Corepack * Crypto * Debugger * Deprecated APIs * Diagnostics Channel * DNS * Domain * Errors * Events * File system * Globals * HTTP * HTTP/2 * HTTPS * Inspector * Internationalization * Modules: CommonJS modules * Modules: ECMAScript modules * Modules: node:module API * Modules: Packages * Net * OS * Path * Performance hooks * Permissions * Process * Punycode * Query strings * Readline * REPL * Report * Stream * String decoder * Test runner * Timers * TLS/SSL * Trace events * TTY * UDP/datagram * URL * Utilities * V8 * VM * WASI * Web Crypto API * Web Streams API * Worker threads * Zlib * Code repository and issue tracker * ► ▼ Other versions * 20.x * 19.x * 18.x LTS * 17.x * 16.x LTS * ► ▼ Options * View on single page * View as JSON * Edit on GitHub Table of contents * Asynchronous context tracking * Introduction * Class: AsyncLocalStorage * new AsyncLocalStorage() * asyncLocalStorage.disable() * asyncLocalStorage.getStore() * asyncLocalStorage.enterWith(store) * asyncLocalStorage.run(store, callback[, ...args]) * asyncLocalStorage.exit(callback[, ...args]) * Usage with async/await * Troubleshooting: Context loss * Class: AsyncResource * new AsyncResource(type[, options]) * Static method: AsyncResource.bind(fn[, type[, thisArg]]) * asyncResource.bind(fn[, thisArg]) * asyncResource.runInAsyncScope(fn[, thisArg, ...args]) * asyncResource.emitDestroy() * asyncResource.asyncId() * asyncResource.triggerAsyncId() * Using AsyncResource for a Worker thread pool * Integrating AsyncResource with EventEmitter Asynchronous context tracking # Stability: 2 - Stable Source Code: lib/async_hooks.js Introduction # These classes are used to associate state and propagate it throughout callbacks and promise chains. They allow storing data throughout the lifetime of a web request or any other asynchronous duration. It is similar to thread-local storage in other languages. The AsyncLocalStorage and AsyncResource classes are part of the node:async_hooks module: import { AsyncLocalStorage , AsyncResource } from 'node:async_hooks' ; const { AsyncLocalStorage , AsyncResource } = require ( 'node:async_hooks' ); Class: AsyncLocalStorage # History Version Changes v16.4.0 AsyncLocalStorage is now Stable. Previously, it had been Experimental. v13.10.0, v12.17.0 Added in: v13.10.0, v12.17.0 This class creates stores that stay coherent through asynchronous operations. While you can create your own implementation on top of the node:async_hooks module, AsyncLocalStorage should be preferred as it is a performant and memory safe implementation that involves significant optimizations that are non-obvious to implement. The following example uses AsyncLocalStorage to build a simple logger that assigns IDs to incoming HTTP requests and includes them in messages logged within each request. import http from 'node:http' ; import { AsyncLocalStorage } from 'node:async_hooks' ; const asyncLocalStorage = new AsyncLocalStorage (); function logWithId ( msg ) { const id = asyncLocalStorage. getStore (); console . log ( ` ${id !== undefined ? id : '-' } :` , msg); } let idSeq = 0 ; http. createServer ( ( req, res ) => { asyncLocalStorage. run (idSeq++, () => { logWithId ( 'start' ); // Imagine any chain of async operations here setImmediate ( () => { logWithId ( 'finish' ); res. end (); }); }); }). listen ( 8080 ); http. get ( 'http://localhost:8080' ); http. get ( 'http://localhost:8080' ); // Prints: // 0: start // 1: start // 0: finish // 1: finish const http = require ( 'node:http' ); const { AsyncLocalStorage } = require ( 'node:async_hooks' ); const asyncLocalStorage = new AsyncLocalStorage (); function logWithId ( msg ) { const id = asyncLocalStorage. getStore (); console . log ( ` ${id !== undefined ? id : '-' } :` , msg); } let idSeq = 0 ; http. createServer ( ( req, res ) => { asyncLocalStorage. run (idSeq++, () => { logWithId ( 'start' ); // Imagine any chain of async operations here setImmediate ( () => { logWithId ( 'finish' ); res. end (); }); }); }). listen ( 8080 ); http. get ( 'http://localhost:8080' ); http. get ( 'http://localhost:8080' ); // Prints: // 0: start // 1: start // 0: finish // 1: finish Each instance of AsyncLocalStorage maintains an independent storage context. Multiple instances can safely exist simultaneously without risk of interfering with each other's data. new AsyncLocalStorage() # Added in: v13.10.0, v12.17.0 Creates a new instance of AsyncLocalStorage . Store is only provided within a run() call or after an enterWith() call. asyncLocalStorage.disable() # Added in: v13.10.0, v12.17.0 Stability: 1 - Experimental Disables the instance of AsyncLocalStorage . All subsequent calls to asyncLocalStorage.getStore() will return undefined until asyncLocalStorage.run() or asyncLocalStorage.enterWith() is called again. When calling asyncLocalStorage.disable() , all current contexts linked to the instance will be exited. Calling asyncLocalStorage.disable() is required before the asyncLocalStorage can be garbage collected. This does not apply to stores provided by the asyncLocalStorage , as those objects are garbage collected along with the corresponding async resources. Use this method when the asyncLocalStorage is not in use anymore in the current process. asyncLocalStorage.getStore() # Added in: v13.10.0, v12.17.0 * Returns: <any> Returns the current store. If called outside of an asynchronous context initialized by calling asyncLocalStorage.run() or asyncLocalStorage.enterWith() , it returns undefined . asyncLocalStorage.enterWith(store) # Added in: v13.11.0, v12.17.0 Stability: 1 - Experimental * store <any> Transitions into the context for the remainder of the current synchronous execution and then persists the store through any following asynchronous calls. Example: const store = { id : 1 }; // Replaces previous store with the given store object asyncLocalStorage. enterWith (store); asyncLocalStorage. getStore (); // Returns the store object someAsyncOperation ( () => { asyncLocalStorage. getStore (); // Returns the same object }); This transition will continue for the entire synchronous execution. This means that if, for example, the context is entered within an event handler subsequent event handlers will also run within that context unless specifically bound to another context with an AsyncResource . That is why run() should be preferred over enterWith() unless there are strong reasons to use the latter method. const store = { id : 1 }; emitter. on ( 'my-event' , () => { asyncLocalStorage. enterWith (store); }); emitter. on ( 'my-event' , () => { asyncLocalStorage. getStore (); // Returns the same object }); asyncLocalStorage. getStore (); // Returns undefined emitter. emit ( 'my-event' ); asyncLocalStorage. getStore (); // Returns the same object asyncLocalStorage.run(store, callback[, ...args]) # Added in: v13.10.0, v12.17.0 * store <any> * callback <Function> * ...args <any> Runs a function synchronously within a context and returns its return value. The store is not accessible outside of the callback function. The store is accessible to any asynchronous operations created within the callback. The optional args are passed to the callback function. If the callback function throws an error, the error is thrown by run() too. The stacktrace is not impacted by this call and the context is exited. Example: const store = { id : 2 }; try { asyncLocalStorage. run (store, () => { asyncLocalStorage. getStore (); // Returns the store object setTimeout ( () => { asyncLocalStorage. getStore (); // Returns the store object }, 200 ); throw new Error (); }); } catch (e) { asyncLocalStorage. getStore (); // Returns undefined // The error will be caught here } asyncLocalStorage.exit(callback[, ...args]) # Added in: v13.10.0, v12.17.0 Stability: 1 - Experimental * callback <Function> * ...args <any> Runs a function synchronously outside of a context and returns its return value. The store is not accessible within the callback function or the asynchronous operations created within the callback. Any getStore() call done within the callback function will always return undefined . The optional args are passed to the callback function. If the callback function throws an error, the error is thrown by exit() too. The stacktrace is not impacted by this call and the context is re-entered. Example: // Within a call to run try { asyncLocalStorage. getStore (); // Returns the store object or value asyncLocalStorage. exit ( () => { asyncLocalStorage. getStore (); // Returns undefined throw new Error (); }); } catch (e) { asyncLocalStorage. getStore (); // Returns the same object or value // The error will be caught here } Usage with async/await # If, within an async function, only one await call is to run within a context, the following pattern should be used: async function fn ( ) { await asyncLocalStorage. run ( new Map (), () => { asyncLocalStorage. getStore (). set ( 'key' , value); return foo (); // The return value of foo will be awaited }); } In this example, the store is only available in the callback function and the functions called by foo . Outside of run , calling getStore will return undefined . Troubleshooting: Context loss # In most cases, AsyncLocalStorage works without issues. In rare situations, the current store is lost in one of the asynchronous operations. If your code is callback-based, it is enough to promisify it with util.promisify() so it starts working with native promises. If you need to use a callback-based API or your code assumes a custom thenable implementation, use the AsyncResource class to associate the asynchronous operation with the correct execution context. Find the function call responsible for the context loss by logging the content of asyncLocalStorage.getStore() after the calls you suspect are responsible for the loss. When the code logs undefined , the last callback called is probably responsible for the context loss. Class: AsyncResource # History Version Changes v16.4.0 AsyncResource is now Stable. Previously, it had been Experimental. The class AsyncResource is designed to be extended by the embedder's async resources. Using this, users can easily trigger the lifetime events of their own resources. The init hook will trigger when an AsyncResource is instantiated. The following is an overview of the AsyncResource API. import { AsyncResource , executionAsyncId } from 'node:async_hooks' ; // AsyncResource() is meant to be extended. Instantiating a // new AsyncResource() also triggers init. If triggerAsyncId is omitted then // async_hook.executionAsyncId() is used. const asyncResource = new AsyncResource ( type, { triggerAsyncId : executionAsyncId (), requireManualDestroy : false } ); // Run a function in the execution context of the resource. This will // * establish the context of the resource // * trigger the AsyncHooks before callbacks // * call the provided function `fn` with the supplied arguments // * trigger the AsyncHooks after callbacks // * restore the original execution context asyncResource. runInAsyncScope (fn, thisArg, ...args); // Call AsyncHooks destroy callbacks. asyncResource. emitDestroy (); // Return the unique ID assigned to the AsyncResource instance. asyncResource. asyncId (); // Return the trigger ID for the AsyncResource instance. asyncResource. triggerAsyncId (); const { AsyncResource , executionAsyncId } = require ( 'node:async_hooks' ); // AsyncResource() is meant to be extended. Instantiating a // new AsyncResource() also triggers init. If triggerAsyncId is omitted then // async_hook.executionAsyncId() is used. const asyncResource = new AsyncResource ( type, { triggerAsyncId : executionAsyncId (), requireManualDestroy : false } ); // Run a function in the execution context of the resource. This will // * establish the context of the resource // * trigger the AsyncHooks before callbacks // * call the provided function `fn` with the supplied arguments // * trigger the AsyncHooks after callbacks // * restore the original execution context asyncResource. runInAsyncScope (fn, thisArg, ...args); // Call AsyncHooks destroy callbacks. asyncResource. emitDestroy (); // Return the unique ID assigned to the AsyncResource instance. asyncResource. asyncId (); // Return the trigger ID for the AsyncResource instance. asyncResource. triggerAsyncId (); new AsyncResource(type[, options]) # * type <string> The type of async event. * options <Object> * triggerAsyncId <number> The ID of the execution context that created this async event. Default: executionAsyncId() . * requireManualDestroy <boolean> If set to true , disables emitDestroy when the object is garbage collected. This usually does not need to be set (even if emitDestroy is called manually), unless the resource's asyncId is retrieved and the sensitive API's emitDestroy is called with it. When set to false , the emitDestroy call on garbage collection will only take place if there is at least one active destroy hook. Default: false . Example usage: class DBQuery extends AsyncResource { constructor ( db ) { super ( 'DBQuery' ); this . db = db; } getInfo ( query, callback ) { this . db . get (query, ( err, data ) => { this . runInAsyncScope (callback, null , err, data); }); } close ( ) { this . db = null ; this . emitDestroy (); } } Static method: AsyncResource.bind(fn[, type[, thisArg]]) # History Version Changes v16.15.0 Changed the default when thisArg is undefined to use this from the caller. v16.0.0 Added optional thisArg. v14.8.0, v12.19.0 Added in: v14.8.0, v12.19.0 * fn <Function> The function to bind to the current execution context. * type <string> An optional name to associate with the underlying AsyncResource . * thisArg <any> Binds the given function to the current execution context. The returned function will have an asyncResource property referencing the AsyncResource to which the function is bound. asyncResource.bind(fn[, thisArg]) # History Version Changes v16.15.0 Changed the default when thisArg is undefined to use this from the caller. v16.0.0 Added optional thisArg. v14.8.0, v12.19.0 Added in: v14.8.0, v12.19.0 * fn <Function> The function to bind to the current AsyncResource . * thisArg <any> Binds the given function to execute to this AsyncResource 's scope. The returned function will have an asyncResource property referencing the AsyncResource to which the function is bound. asyncResource.runInAsyncScope(fn[, thisArg, ...args]) # Added in: v9.6.0 * fn <Function> The function to call in the execution context of this async resource. * thisArg <any> The receiver to be used for the function call. * ...args <any> Optional arguments to pass to the function. Call the provided function with the provided arguments in the execution context of the async resource. This will establish the context, trigger the AsyncHooks before callbacks, call the function, trigger the AsyncHooks after callbacks, and then restore the original execution context. asyncResource.emitDestroy() # * Returns: <AsyncResource> A reference to asyncResource . Call all destroy hooks. This should only ever be called once. An error will be thrown if it is called more than once. This must be manually called. If the resource is left to be collected by the GC then the destroy hooks will never be called. asyncResource.asyncId() # * Returns: <number> The unique asyncId assigned to the resource. asyncResource.triggerAsyncId() # * Returns: <number> The same triggerAsyncId that is passed to the AsyncResource constructor. Using AsyncResource for a Worker thread pool # The following example shows how to use the AsyncResource class to properly provide async tracking for a Worker pool. Other resource po