Source Code: lib/async_hooks.js
\nThe async_hooks module provides an API to track asynchronous resources. It\ncan be accessed using:
const async_hooks = require('async_hooks');\nAn asynchronous resource represents an object with an associated callback.\nThis callback may be called multiple times, for example, the 'connection'\nevent in net.createServer(), or just a single time like in fs.open().\nA resource can also be closed before the callback is called. AsyncHook does\nnot explicitly distinguish between these different cases but will represent them\nas the abstract concept that is a resource.
If Workers are used, each thread has an independent async_hooks\ninterface, and each thread will use a new set of async IDs.
Following is a simple overview of the public API.
\nconst async_hooks = require('async_hooks');\n\n// Return the ID of the current execution context.\nconst eid = async_hooks.executionAsyncId();\n\n// Return the ID of the handle responsible for triggering the callback of the\n// current execution scope to call.\nconst tid = async_hooks.triggerAsyncId();\n\n// Create a new AsyncHook instance. All of these callbacks are optional.\nconst asyncHook =\n async_hooks.createHook({ init, before, after, destroy, promiseResolve });\n\n// Allow callbacks of this AsyncHook instance to call. This is not an implicit\n// action after running the constructor, and must be explicitly run to begin\n// executing callbacks.\nasyncHook.enable();\n\n// Disable listening for new asynchronous events.\nasyncHook.disable();\n\n//\n// The following are the callbacks that can be passed to createHook().\n//\n\n// init is called during object construction. The resource may not have\n// completed construction when this callback runs, therefore all fields of the\n// resource referenced by \"asyncId\" may not have been populated.\nfunction init(asyncId, type, triggerAsyncId, resource) { }\n\n// Before is called just before the resource's callback is called. It can be\n// called 0-N times for handles (such as TCPWrap), and will be called exactly 1\n// time for requests (such as FSReqCallback).\nfunction before(asyncId) { }\n\n// After is called just after the resource's callback has finished.\nfunction after(asyncId) { }\n\n// Destroy is called when the resource is destroyed.\nfunction destroy(asyncId) { }\n\n// promiseResolve is called only for promise resources, when the\n// `resolve` function passed to the `Promise` constructor is invoked\n// (either directly or through other means of resolving a promise).\nfunction promiseResolve(asyncId) { }\nBy default, promise executions are not assigned asyncIds due to the relatively\nexpensive nature of the promise introspection API provided by\nV8. This means that programs using promises or async/await will not get\ncorrect execution and trigger ids for promise callback contexts by default.
const ah = require('async_hooks');\nPromise.resolve(1729).then(() => {\n console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`);\n});\n// produces:\n// eid 1 tid 0\nObserve that the then() callback claims to have executed in the context of the\nouter scope even though there was an asynchronous hop involved. Also,\nthe triggerAsyncId value is 0, which means that we are missing context about\nthe resource that caused (triggered) the then() callback to be executed.
Installing async hooks via async_hooks.createHook enables promise execution\ntracking:
const ah = require('async_hooks');\nah.createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.\nPromise.resolve(1729).then(() => {\n console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`);\n});\n// produces:\n// eid 7 tid 6\nIn this example, adding any actual hook function enabled the tracking of\npromises. There are two promises in the example above; the promise created by\nPromise.resolve() and the promise returned by the call to then(). In the\nexample above, the first promise got the asyncId 6 and the latter got\nasyncId 7. During the execution of the then() callback, we are executing\nin the context of promise with asyncId 7. This promise was triggered by\nasync resource 6.
Another subtlety with promises is that before and after callbacks are run\nonly on chained promises. That means promises not created by then()/catch()\nwill not have the before and after callbacks fired on them. For more details\nsee the details of the V8 PromiseHooks API.
Library developers that handle their own asynchronous resources performing tasks\nlike I/O, connection pooling, or managing callback queues may use the\nAsyncResource JavaScript API so that all the appropriate callbacks are called.
The class AsyncResource is designed to be extended by the embedder's async\nresources. Using this, users can easily trigger the lifetime events of their\nown resources.
The init hook will trigger when an AsyncResource is instantiated.
The following is an overview of the AsyncResource API.
const { AsyncResource, executionAsyncId } = require('async_hooks');\n\n// AsyncResource() is meant to be extended. Instantiating a\n// new AsyncResource() also triggers init. If triggerAsyncId is omitted then\n// async_hook.executionAsyncId() is used.\nconst asyncResource = new AsyncResource(\n type, { triggerAsyncId: executionAsyncId(), requireManualDestroy: false }\n);\n\n// Run a function in the execution context of the resource. This will\n// * establish the context of the resource\n// * trigger the AsyncHooks before callbacks\n// * call the provided function `fn` with the supplied arguments\n// * trigger the AsyncHooks after callbacks\n// * restore the original execution context\nasyncResource.runInAsyncScope(fn, thisArg, ...args);\n\n// Call AsyncHooks destroy callbacks.\nasyncResource.emitDestroy();\n\n// Return the unique ID assigned to the AsyncResource instance.\nasyncResource.asyncId();\n\n// Return the trigger ID for the AsyncResource instance.\nasyncResource.triggerAsyncId();\nBinds the given function to the current execution context.
\nThe returned function will have an asyncResource property referencing\nthe AsyncResource to which the function is bound.
Binds the given function to execute to this AsyncResource's scope.
The returned function will have an asyncResource property referencing\nthe AsyncResource to which the function is bound.
Call the provided function with the provided arguments in the execution context\nof the async resource. This will establish the context, trigger the AsyncHooks\nbefore callbacks, call the function, trigger the AsyncHooks after callbacks, and\nthen restore the original execution context.
" }, { "textRaw": "`asyncResource.emitDestroy()`", "type": "method", "name": "emitDestroy", "signatures": [ { "return": { "textRaw": "Returns: {AsyncResource} A reference to `asyncResource`.", "name": "return", "type": "AsyncResource", "desc": "A reference to `asyncResource`." }, "params": [] } ], "desc": "Call all destroy hooks. This should only ever be called once. An error will\nbe thrown if it is called more than once. This must be manually called. If\nthe resource is left to be collected by the GC then the destroy hooks will\nnever be called.
Example usage:
\nclass DBQuery extends AsyncResource {\n constructor(db) {\n super('DBQuery');\n this.db = db;\n }\n\n getInfo(query, callback) {\n this.db.get(query, (err, data) => {\n this.runInAsyncScope(callback, null, err, data);\n });\n }\n\n close() {\n this.db = null;\n this.emitDestroy();\n }\n}\nThe following example shows how to use the AsyncResource class to properly\nprovide async tracking for a Worker pool. Other resource pools, such as\ndatabase connection pools, can follow a similar model.
Assuming that the task is adding two numbers, using a file named\ntask_processor.js with the following content:
const { parentPort } = require('worker_threads');\nparentPort.on('message', (task) => {\n parentPort.postMessage(task.a + task.b);\n});\na Worker pool around it could use the following structure:
\nconst { AsyncResource } = require('async_hooks');\nconst { EventEmitter } = require('events');\nconst path = require('path');\nconst { Worker } = require('worker_threads');\n\nconst kTaskInfo = Symbol('kTaskInfo');\nconst kWorkerFreedEvent = Symbol('kWorkerFreedEvent');\n\nclass WorkerPoolTaskInfo extends AsyncResource {\n constructor(callback) {\n super('WorkerPoolTaskInfo');\n this.callback = callback;\n }\n\n done(err, result) {\n this.runInAsyncScope(this.callback, null, err, result);\n this.emitDestroy(); // `TaskInfo`s are used only once.\n }\n}\n\nclass WorkerPool extends EventEmitter {\n constructor(numThreads) {\n super();\n this.numThreads = numThreads;\n this.workers = [];\n this.freeWorkers = [];\n this.tasks = [];\n\n for (let i = 0; i < numThreads; i++)\n this.addNewWorker();\n\n // Any time the kWorkerFreedEvent is emitted, dispatch\n // the next task pending in the queue, if any.\n this.on(kWorkerFreedEvent, () => {\n if (this.tasks.length > 0) {\n const { task, callback } = this.tasks.shift();\n this.runTask(task, callback);\n }\n });\n }\n\n addNewWorker() {\n const worker = new Worker(path.resolve(__dirname, 'task_processor.js'));\n worker.on('message', (result) => {\n // In case of success: Call the callback that was passed to `runTask`,\n // remove the `TaskInfo` associated with the Worker, and mark it as free\n // again.\n worker[kTaskInfo].done(null, result);\n worker[kTaskInfo] = null;\n this.freeWorkers.push(worker);\n this.emit(kWorkerFreedEvent);\n });\n worker.on('error', (err) => {\n // In case of an uncaught exception: Call the callback that was passed to\n // `runTask` with the error.\n if (worker[kTaskInfo])\n worker[kTaskInfo].done(err, null);\n else\n this.emit('error', err);\n // Remove the worker from the list and start a new Worker to replace the\n // current one.\n this.workers.splice(this.workers.indexOf(worker), 1);\n this.addNewWorker();\n });\n this.workers.push(worker);\n this.freeWorkers.push(worker);\n this.emit(kWorkerFreedEvent);\n }\n\n runTask(task, callback) {\n if (this.freeWorkers.length === 0) {\n // No free threads, wait until a worker thread becomes free.\n this.tasks.push({ task, callback });\n return;\n }\n\n const worker = this.freeWorkers.pop();\n worker[kTaskInfo] = new WorkerPoolTaskInfo(callback);\n worker.postMessage(task);\n }\n\n close() {\n for (const worker of this.workers) worker.terminate();\n }\n}\n\nmodule.exports = WorkerPool;\nWithout the explicit tracking added by the WorkerPoolTaskInfo objects,\nit would appear that the callbacks are associated with the individual Worker\nobjects. However, the creation of the Workers is not associated with the\ncreation of the tasks and does not provide information about when tasks\nwere scheduled.
This pool could be used as follows:
\nconst WorkerPool = require('./worker_pool.js');\nconst os = require('os');\n\nconst pool = new WorkerPool(os.cpus().length);\n\nlet finished = 0;\nfor (let i = 0; i < 10; i++) {\n pool.runTask({ a: 42, b: 100 }, (err, result) => {\n console.log(i, err, result);\n if (++finished === 10)\n pool.close();\n });\n}\nEvent listeners triggered by an EventEmitter may be run in a different\nexecution context than the one that was active when eventEmitter.on() was\ncalled.
The following example shows how to use the AsyncResource class to properly\nassociate an event listener with the correct execution context. The same\napproach can be applied to a Stream or a similar event-driven class.
const { createServer } = require('http');\nconst { AsyncResource, executionAsyncId } = require('async_hooks');\n\nconst server = createServer((req, res) => {\n req.on('close', AsyncResource.bind(() => {\n // Execution context is bound to the current outer scope.\n }));\n req.on('close', () => {\n // Execution context is bound to the scope that caused 'close' to emit.\n });\n res.end();\n}).listen(3000);\nRegisters functions to be called for different lifetime events of each async\noperation.
\nThe callbacks init()/before()/after()/destroy() are called for the\nrespective asynchronous event during a resource's lifetime.
All callbacks are optional. For example, if only resource cleanup needs to\nbe tracked, then only the destroy callback needs to be passed. The\nspecifics of all functions that can be passed to callbacks is in the\nHook Callbacks section.
const async_hooks = require('async_hooks');\n\nconst asyncHook = async_hooks.createHook({\n init(asyncId, type, triggerAsyncId, resource) { },\n destroy(asyncId) { }\n});\nThe callbacks will be inherited via the prototype chain:
\nclass MyAsyncCallbacks {\n init(asyncId, type, triggerAsyncId, resource) { }\n destroy(asyncId) {}\n}\n\nclass MyAddedCallbacks extends MyAsyncCallbacks {\n before(asyncId) { }\n after(asyncId) { }\n}\n\nconst asyncHook = async_hooks.createHook(new MyAddedCallbacks());\nBecause promises are asynchronous resources whose lifecycle is tracked\nvia the async hooks mechanism, the init(), before(), after(), and\ndestroy() callbacks must not be async functions that return promises.
If any AsyncHook callbacks throw, the application will print the stack trace\nand exit. The exit path does follow that of an uncaught exception, but\nall 'uncaughtException' listeners are removed, thus forcing the process to\nexit. The 'exit' callbacks will still be called unless the application is run\nwith --abort-on-uncaught-exception, in which case a stack trace will be\nprinted and the application exits, leaving a core file.
The reason for this error handling behavior is that these callbacks are running\nat potentially volatile points in an object's lifetime, for example during\nclass construction and destruction. Because of this, it is deemed necessary to\nbring down the process quickly in order to prevent an unintentional abort in the\nfuture. This is subject to change in the future if a comprehensive analysis is\nperformed to ensure an exception can follow the normal control flow without\nunintentional side effects.
", "type": "module", "displayName": "Error handling" }, { "textRaw": "Printing in AsyncHooks callbacks", "name": "printing_in_asynchooks_callbacks", "desc": "Because printing to the console is an asynchronous operation, console.log()\nwill cause the AsyncHooks callbacks to be called. Using console.log() or\nsimilar asynchronous operations inside an AsyncHooks callback function will thus\ncause an infinite recursion. An easy solution to this when debugging is to use a\nsynchronous logging operation such as fs.writeFileSync(file, msg, flag).\nThis will print to the file and will not invoke AsyncHooks recursively because\nit is synchronous.
const fs = require('fs');\nconst util = require('util');\n\nfunction debug(...args) {\n // Use a function like this one when debugging inside an AsyncHooks callback\n fs.writeFileSync('log.out', `${util.format(...args)}\\n`, { flag: 'a' });\n}\nIf an asynchronous operation is needed for logging, it is possible to keep\ntrack of what caused the asynchronous operation using the information\nprovided by AsyncHooks itself. The logging should then be skipped when\nit was the logging itself that caused AsyncHooks callback to call. By\ndoing this the otherwise infinite recursion is broken.
", "type": "module", "displayName": "Printing in AsyncHooks callbacks" } ] } ], "classes": [ { "textRaw": "Class: `AsyncHook`", "type": "class", "name": "AsyncHook", "desc": "The class AsyncHook exposes an interface for tracking lifetime events\nof asynchronous operations.
Enable the callbacks for a given AsyncHook instance. If no callbacks are\nprovided, enabling is a no-op.
The AsyncHook instance is disabled by default. If the AsyncHook instance\nshould be enabled immediately after creation, the following pattern can be used.
const async_hooks = require('async_hooks');\n\nconst hook = async_hooks.createHook(callbacks).enable();\nDisable the callbacks for a given AsyncHook instance from the global pool of\nAsyncHook callbacks to be executed. Once a hook has been disabled it will not\nbe called again until enabled.
For API consistency disable() also returns the AsyncHook instance.
Resource objects returned by executionAsyncResource() are most often internal\nNode.js handle objects with undocumented APIs. Using any functions or properties\non the object is likely to crash your application and should be avoided.
Using executionAsyncResource() in the top-level execution context will\nreturn an empty object as there is no handle or request object to use,\nbut having an object representing the top-level can be helpful.
const { open } = require('fs');\nconst { executionAsyncId, executionAsyncResource } = require('async_hooks');\n\nconsole.log(executionAsyncId(), executionAsyncResource()); // 1 {}\nopen(__filename, 'r', (err, fd) => {\n console.log(executionAsyncId(), executionAsyncResource()); // 7 FSReqWrap\n});\nThis can be used to implement continuation local storage without the\nuse of a tracking Map to store the metadata:
const { createServer } = require('http');\nconst {\n executionAsyncId,\n executionAsyncResource,\n createHook\n} = require('async_hooks');\nconst sym = Symbol('state'); // Private symbol to avoid pollution\n\ncreateHook({\n init(asyncId, type, triggerAsyncId, resource) {\n const cr = executionAsyncResource();\n if (cr) {\n resource[sym] = cr[sym];\n }\n }\n}).enable();\n\nconst server = createServer((req, res) => {\n executionAsyncResource()[sym] = { state: req.url };\n setTimeout(function() {\n res.end(JSON.stringify(executionAsyncResource()[sym]));\n }, 100);\n}).listen(3000);\nconst async_hooks = require('async_hooks');\n\nconsole.log(async_hooks.executionAsyncId()); // 1 - bootstrap\nfs.open(path, 'r', (err, fd) => {\n console.log(async_hooks.executionAsyncId()); // 6 - open()\n});\nThe ID returned from executionAsyncId() is related to execution timing, not\ncausality (which is covered by triggerAsyncId()):
const server = net.createServer((conn) => {\n // Returns the ID of the server, not of the new connection, because the\n // callback runs in the execution scope of the server's MakeCallback().\n async_hooks.executionAsyncId();\n\n}).listen(port, () => {\n // Returns the ID of a TickObject (process.nextTick()) because all\n // callbacks passed to .listen() are wrapped in a nextTick().\n async_hooks.executionAsyncId();\n});\nPromise contexts may not get precise executionAsyncIds by default.\nSee the section on promise execution tracking.
const server = net.createServer((conn) => {\n // The resource that caused (or triggered) this callback to be called\n // was that of the new connection. Thus the return value of triggerAsyncId()\n // is the asyncId of \"conn\".\n async_hooks.triggerAsyncId();\n\n}).listen(port, () => {\n // Even though all callbacks passed to .listen() are wrapped in a nextTick()\n // the callback itself exists because the call to the server's .listen()\n // was made. So the return value would be the ID of the server.\n async_hooks.triggerAsyncId();\n});\nPromise contexts may not get valid triggerAsyncIds by default. See\nthe section on promise execution tracking.
Key events in the lifetime of asynchronous events have been categorized into\nfour areas: instantiation, before/after the callback is called, and when the\ninstance is destroyed.
", "methods": [ { "textRaw": "`init(asyncId, type, triggerAsyncId, resource)`", "type": "method", "name": "init", "signatures": [ { "params": [ { "textRaw": "`asyncId` {number} A unique ID for the async resource.", "name": "asyncId", "type": "number", "desc": "A unique ID for the async resource." }, { "textRaw": "`type` {string} The type of the async resource.", "name": "type", "type": "string", "desc": "The type of the async resource." }, { "textRaw": "`triggerAsyncId` {number} The unique ID of the async resource in whose execution context this async resource was created.", "name": "triggerAsyncId", "type": "number", "desc": "The unique ID of the async resource in whose execution context this async resource was created." }, { "textRaw": "`resource` {Object} Reference to the resource representing the async operation, needs to be released during _destroy_.", "name": "resource", "type": "Object", "desc": "Reference to the resource representing the async operation, needs to be released during _destroy_." } ] } ], "desc": "Called when a class is constructed that has the possibility to emit an\nasynchronous event. This does not mean the instance must call\nbefore/after before destroy is called, only that the possibility\nexists.
This behavior can be observed by doing something like opening a resource then\nclosing it before the resource can be used. The following snippet demonstrates\nthis.
\nrequire('net').createServer().listen(function() { this.close(); });\n// OR\nclearTimeout(setTimeout(() => {}, 10));\nEvery new resource is assigned an ID that is unique within the scope of the\ncurrent Node.js instance.
", "modules": [ { "textRaw": "`type`", "name": "`type`", "desc": "The type is a string identifying the type of resource that caused\ninit to be called. Generally, it will correspond to the name of the\nresource's constructor.
FSEVENTWRAP, FSREQCALLBACK, GETADDRINFOREQWRAP, GETNAMEINFOREQWRAP, HTTPINCOMINGMESSAGE,\nHTTPCLIENTREQUEST, JSSTREAM, PIPECONNECTWRAP, PIPEWRAP, PROCESSWRAP, QUERYWRAP,\nSHUTDOWNWRAP, SIGNALWRAP, STATWATCHER, TCPCONNECTWRAP, TCPSERVERWRAP, TCPWRAP,\nTTYWRAP, UDPSENDWRAP, UDPWRAP, WRITEWRAP, ZLIB, SSLCONNECTION, PBKDF2REQUEST,\nRANDOMBYTESREQUEST, TLSWRAP, Microtask, Timeout, Immediate, TickObject\nThere is also the PROMISE resource type, which is used to track Promise\ninstances and asynchronous work scheduled by them.
Users are able to define their own type when using the public embedder API.
It is possible to have type name collisions. Embedders are encouraged to use\nunique prefixes, such as the npm package name, to prevent collisions when\nlistening to the hooks.
", "type": "module", "displayName": "`type`" }, { "textRaw": "`triggerAsyncId`", "name": "`triggerasyncid`", "desc": "triggerAsyncId is the asyncId of the resource that caused (or \"triggered\")\nthe new resource to initialize and that caused init to call. This is different\nfrom async_hooks.executionAsyncId() that only shows when a resource was\ncreated, while triggerAsyncId shows why a resource was created.
The following is a simple demonstration of triggerAsyncId:
const { fd } = process.stdout;\n\nasync_hooks.createHook({\n init(asyncId, type, triggerAsyncId) {\n const eid = async_hooks.executionAsyncId();\n fs.writeSync(\n fd,\n `${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\\n`);\n }\n}).enable();\n\nnet.createServer((conn) => {}).listen(8080);\nOutput when hitting the server with nc localhost 8080:
TCPSERVERWRAP(5): trigger: 1 execution: 1\nTCPWRAP(7): trigger: 5 execution: 0\nThe TCPSERVERWRAP is the server which receives the connections.
The TCPWRAP is the new connection from the client. When a new\nconnection is made, the TCPWrap instance is immediately constructed. This\nhappens outside of any JavaScript stack. (An executionAsyncId() of 0 means\nthat it is being executed from C++ with no JavaScript stack above it.) With only\nthat information, it would be impossible to link resources together in\nterms of what caused them to be created, so triggerAsyncId is given the task\nof propagating what resource is responsible for the new resource's existence.
resource is an object that represents the actual async resource that has\nbeen initialized. This can contain useful information that can vary based on\nthe value of type. For instance, for the GETADDRINFOREQWRAP resource type,\nresource provides the host name used when looking up the IP address for the\nhost in net.Server.listen(). The API for accessing this information is\nnot supported, but using the Embedder API, users can provide\nand document their own resource objects. For example, such a resource object\ncould contain the SQL query being executed.
In some cases the resource object is reused for performance reasons, it is\nthus not safe to use it as a key in a WeakMap or add properties to it.
The following is an example with additional information about the calls to\ninit between the before and after calls, specifically what the\ncallback to listen() will look like. The output formatting is slightly more\nelaborate to make calling context easier to see.
const { fd } = process.stdout;\n\nlet indent = 0;\nasync_hooks.createHook({\n init(asyncId, type, triggerAsyncId) {\n const eid = async_hooks.executionAsyncId();\n const indentStr = ' '.repeat(indent);\n fs.writeSync(\n fd,\n `${indentStr}${type}(${asyncId}):` +\n ` trigger: ${triggerAsyncId} execution: ${eid}\\n`);\n },\n before(asyncId) {\n const indentStr = ' '.repeat(indent);\n fs.writeSync(fd, `${indentStr}before: ${asyncId}\\n`);\n indent += 2;\n },\n after(asyncId) {\n indent -= 2;\n const indentStr = ' '.repeat(indent);\n fs.writeSync(fd, `${indentStr}after: ${asyncId}\\n`);\n },\n destroy(asyncId) {\n const indentStr = ' '.repeat(indent);\n fs.writeSync(fd, `${indentStr}destroy: ${asyncId}\\n`);\n },\n}).enable();\n\nnet.createServer(() => {}).listen(8080, () => {\n // Let's wait 10ms before logging the server started.\n setTimeout(() => {\n console.log('>>>', async_hooks.executionAsyncId());\n }, 10);\n});\nOutput from only starting the server:
\nTCPSERVERWRAP(5): trigger: 1 execution: 1\nTickObject(6): trigger: 5 execution: 1\nbefore: 6\n Timeout(7): trigger: 6 execution: 6\nafter: 6\ndestroy: 6\nbefore: 7\n>>> 7\n TickObject(8): trigger: 7 execution: 7\nafter: 7\nbefore: 8\nafter: 8\nAs illustrated in the example, executionAsyncId() and execution each specify\nthe value of the current execution context; which is delineated by calls to\nbefore and after.
Only using execution to graph resource allocation results in the following:
root(1)\n ^\n |\nTickObject(6)\n ^\n |\n Timeout(7)\nThe TCPSERVERWRAP is not part of this graph, even though it was the reason for\nconsole.log() being called. This is because binding to a port without a host\nname is a synchronous operation, but to maintain a completely asynchronous\nAPI the user's callback is placed in a process.nextTick(). Which is why\nTickObject is present in the output and is a 'parent' for .listen()\ncallback.
The graph only shows when a resource was created, not why, so to track\nthe why use triggerAsyncId. Which can be represented with the following\ngraph:
bootstrap(1)\n |\n ˅\nTCPSERVERWRAP(5)\n |\n ˅\n TickObject(6)\n |\n ˅\n Timeout(7)\nWhen an asynchronous operation is initiated (such as a TCP server receiving a\nnew connection) or completes (such as writing data to disk) a callback is\ncalled to notify the user. The before callback is called just before said\ncallback is executed. asyncId is the unique identifier assigned to the\nresource about to execute the callback.
The before callback will be called 0 to N times. The before callback\nwill typically be called 0 times if the asynchronous operation was cancelled\nor, for example, if no connections are received by a TCP server. Persistent\nasynchronous resources like a TCP server will typically call the before\ncallback multiple times, while other operations like fs.open() will call\nit only once.
Called immediately after the callback specified in before is completed.
If an uncaught exception occurs during execution of the callback, then after\nwill run after the 'uncaughtException' event is emitted or a domain's\nhandler runs.
Called after the resource corresponding to asyncId is destroyed. It is also\ncalled asynchronously from the embedder API emitDestroy().
Some resources depend on garbage collection for cleanup, so if a reference is\nmade to the resource object passed to init it is possible that destroy\nwill never be called, causing a memory leak in the application. If the resource\ndoes not depend on garbage collection, then this will not be an issue.
Called when the resolve function passed to the Promise constructor is\ninvoked (either directly or through other means of resolving a promise).
resolve() does not do any observable synchronous work.
The Promise is not necessarily fulfilled or rejected at this point if the\nPromise was resolved by assuming the state of another Promise.
new Promise((resolve) => resolve(true)).then((a) => {});\ncalls the following callbacks:
\ninit for PROMISE with id 5, trigger id: 1\n promise resolve 5 # corresponds to resolve(true)\ninit for PROMISE with id 6, trigger id: 5 # the Promise returned by then()\n before 6 # the then() callback is entered\n promise resolve 6 # the then() callback resolves the promise by returning\n after 6\nThis class is used to create asynchronous state within callbacks and promise\nchains. It allows storing data throughout the lifetime of a web request\nor any other asynchronous duration. It is similar to thread-local storage\nin other languages.
\nWhile you can create your own implementation on top of the async_hooks module,\nAsyncLocalStorage should be preferred as it is a performant and memory safe\nimplementation that involves significant optimizations that are non-obvious to\nimplement.
The following example uses AsyncLocalStorage to build a simple logger\nthat assigns IDs to incoming HTTP requests and includes them in messages\nlogged within each request.
const http = require('http');\nconst { AsyncLocalStorage } = require('async_hooks');\n\nconst asyncLocalStorage = new AsyncLocalStorage();\n\nfunction logWithId(msg) {\n const id = asyncLocalStorage.getStore();\n console.log(`${id !== undefined ? id : '-'}:`, msg);\n}\n\nlet idSeq = 0;\nhttp.createServer((req, res) => {\n asyncLocalStorage.run(idSeq++, () => {\n logWithId('start');\n // Imagine any chain of async operations here\n setImmediate(() => {\n logWithId('finish');\n res.end();\n });\n });\n}).listen(8080);\n\nhttp.get('http://localhost:8080');\nhttp.get('http://localhost:8080');\n// Prints:\n// 0: start\n// 1: start\n// 0: finish\n// 1: finish\nWhen having multiple instances of AsyncLocalStorage, they are independent\nfrom each other. It is safe to instantiate this class multiple times.
Disables the instance of AsyncLocalStorage. All subsequent calls\nto asyncLocalStorage.getStore() will return undefined until\nasyncLocalStorage.run() or asyncLocalStorage.enterWith() is called again.
When calling asyncLocalStorage.disable(), all current contexts linked to the\ninstance will be exited.
Calling asyncLocalStorage.disable() is required before the\nasyncLocalStorage can be garbage collected. This does not apply to stores\nprovided by the asyncLocalStorage, as those objects are garbage collected\nalong with the corresponding async resources.
Use this method when the asyncLocalStorage is not in use anymore\nin the current process.
Returns the current store.\nIf called outside of an asynchronous context initialized by\ncalling asyncLocalStorage.run() or asyncLocalStorage.enterWith(), it\nreturns undefined.
Transitions into the context for the remainder of the current\nsynchronous execution and then persists the store through any following\nasynchronous calls.
\nExample:
\nconst store = { id: 1 };\n// Replaces previous store with the given store object\nasyncLocalStorage.enterWith(store);\nasyncLocalStorage.getStore(); // Returns the store object\nsomeAsyncOperation(() => {\n asyncLocalStorage.getStore(); // Returns the same object\n});\nThis transition will continue for the entire synchronous execution.\nThis means that if, for example, the context is entered within an event\nhandler subsequent event handlers will also run within that context unless\nspecifically bound to another context with an AsyncResource. That is why\nrun() should be preferred over enterWith() unless there are strong reasons\nto use the latter method.
const store = { id: 1 };\n\nemitter.on('my-event', () => {\n asyncLocalStorage.enterWith(store);\n});\nemitter.on('my-event', () => {\n asyncLocalStorage.getStore(); // Returns the same object\n});\n\nasyncLocalStorage.getStore(); // Returns undefined\nemitter.emit('my-event');\nasyncLocalStorage.getStore(); // Returns the same object\nRuns a function synchronously within a context and returns its\nreturn value. The store is not accessible outside of the callback function.\nThe store is accessible to any asynchronous operations created within the\ncallback.
\nThe optional args are passed to the callback function.
If the callback function throws an error, the error is thrown by run() too.\nThe stacktrace is not impacted by this call and the context is exited.
Example:
\nconst store = { id: 2 };\ntry {\n asyncLocalStorage.run(store, () => {\n asyncLocalStorage.getStore(); // Returns the store object\n setTimeout(() => {\n asyncLocalStorage.getStore(); // Returns the store object\n }, 200);\n throw new Error();\n });\n} catch (e) {\n asyncLocalStorage.getStore(); // Returns undefined\n // The error will be caught here\n}\nRuns a function synchronously outside of a context and returns its\nreturn value. The store is not accessible within the callback function or\nthe asynchronous operations created within the callback. Any getStore()\ncall 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.\nThe stacktrace is not impacted by this call and the context is re-entered.
Example:
\n// Within a call to run\ntry {\n asyncLocalStorage.getStore(); // Returns the store object or value\n asyncLocalStorage.exit(() => {\n asyncLocalStorage.getStore(); // Returns undefined\n throw new Error();\n });\n} catch (e) {\n asyncLocalStorage.getStore(); // Returns the same object or value\n // The error will be caught here\n}\nIf, within an async function, only one await call is to run within a context,\nthe following pattern should be used:
async function fn() {\n await asyncLocalStorage.run(new Map(), () => {\n asyncLocalStorage.getStore().set('key', value);\n return foo(); // The return value of foo will be awaited\n });\n}\nIn this example, the store is only available in the callback function and the\nfunctions called by foo. Outside of run, calling getStore will return\nundefined.
In most cases your application or library code should have no issues with\nAsyncLocalStorage. But in rare cases you may face situations when the\ncurrent store is lost in one of asynchronous operations. In those cases,\nconsider the following options.
If your code is callback-based, it is enough to promisify it with\nutil.promisify(), so it starts working with native promises.
If you need to keep using callback-based API, or your code assumes\na custom thenable implementation, use the AsyncResource class\nto associate the asynchronous operation with the correct execution context.
Creates a new instance of AsyncLocalStorage. Store is only provided within a\nrun() call or after an enterWith() call.