Node.js v8.1.2 Documentation

Async Hooks#

Stability: 1 - Experimental

The async_hooks module provides an API to register callbacks tracking the lifetime of asynchronous resources created inside a Node.js application. It can be accessed using:

const async_hooks = require('async_hooks');


An asynchronous resource represents an object with an associated callback. This callback may be called multiple times, for example, the connection event in net.createServer, or just a single time like in A resource can also be closed before the callback is called. AsyncHook does not explicitly distinguish between these different cases but will represent them as the abstract concept that is a resource.

Public API#


Following is a simple overview of the public API.

const async_hooks = require('async_hooks');

// Return the ID of the current execution context.
const cid = async_hooks.currentId();

// Return the ID of the handle responsible for triggering the callback of the
// current execution scope to call.
const tid = async_hooks.triggerId();

// Create a new AsyncHook instance. All of these callbacks are optional.
const asyncHook = async_hooks.createHook({ init, before, after, destroy });

// Allow callbacks of this AsyncHook instance to call. This is not an implicit
// action after running the constructor, and must be explicitly run to begin
// executing callbacks.

// Disable listening for new asynchronous events.

// The following are the callbacks that can be passed to createHook().

// init is called during object construction. The resource may not have
// completed construction when this callback runs, therefore all fields of the
// resource referenced by "asyncId" may not have been populated.
function init(asyncId, type, triggerId, resource) { }

// before is called just before the resource's callback is called. It can be
// called 0-N times for handles (e.g. TCPWrap), and will be called exactly 1
// time for requests (e.g. FSReqWrap).
function before(asyncId) { }

// after is called just after the resource's callback has finished.
function after(asyncId) { }

// destroy is called when an AsyncWrap instance is destroyed.
function destroy(asyncId) { }


  • callbacks <Object> the callbacks to register
  • Returns: {AsyncHook} instance used for disabling and enabling hooks

Registers functions to be called for different lifetime events of each async operation.

The callbacks init()/before()/after()/destroy() are called for the respective asynchronous event during a resource's lifetime.

All callbacks are optional. So, for example, if only resource cleanup needs to be tracked then only the destroy callback needs to be passed. The specifics of all functions that can be passed to callbacks is in the section Hook Callbacks.

Error Handling#

If any AsyncHook callbacks throw, the application will print the stack trace and exit. The exit path does follow that of an uncaught exception but all uncaughtException listeners are removed, thus forcing the process to exit. The 'exit' callbacks will still be called unless the application is run with --abort-on-uncaught-exception, in which case a stack trace will be printed and the application exits, leaving a core file.

The reason for this error handling behavior is that these callbacks are running at potentially volatile points in an object's lifetime, for example during class construction and destruction. Because of this, it is deemed necessary to bring down the process quickly in order to prevent an unintentional abort in the future. This is subject to change in the future if a comprehensive analysis is performed to ensure an exception can follow the normal control flow without unintentional side effects.

Printing in AsyncHooks callbacks#

Because printing to the console is an asynchronous operation, console.log() will cause the AsyncHooks callbacks to be called. Using console.log() or similar asynchronous operations inside an AsyncHooks callback function will thus cause an infinite recursion. An easily solution to this when debugging is to use a synchronous logging operation such as fs.writeSync(1, msg). This will print to stdout because 1 is the file descriptor for stdout and will not invoke AsyncHooks recursively because it is synchronous.

const fs = require('fs');
const util = require('util');

function debug(...args) {
  // use a function like this one when debugging inside an AsyncHooks callback
  fs.writeSync(1, `${util.format(...args)}\n`);

If an asynchronous operation is needed for logging, it is possible to keep track of what caused the asynchronous operation using the information provided by AsyncHooks itself. The logging should then be skipped when it was the logging itself that caused AsyncHooks callback to call. By doing this the otherwise infinite recursion is broken.


  • Returns <AsyncHook> A reference to asyncHook.

Enable the callbacks for a given AsyncHook instance. If no callbacks are provided enabling is a noop.

The AsyncHook instance is by default disabled. If the AsyncHook instance should be enabled immediately after creation, the following pattern can be used.

const async_hooks = require('async_hooks');

const hook = async_hooks.createHook(callbacks).enable();


  • Returns <AsyncHook> A reference to asyncHook.

Disable the callbacks for a given AsyncHook instance from the global pool of AsyncHook callbacks to be executed. Once a hook has been disabled it will not be called again until enabled.

For API consistency disable() also returns the AsyncHook instance.

Hook Callbacks#

Key events in the lifetime of asynchronous events have been categorized into four areas: instantiation, before/after the callback is called, and when the instance is destructed.

init(asyncId, type, triggerId, resource)#
  • asyncId <number> a unique ID for the async resource
  • type <string> the type of the async resource
  • triggerId <number> the unique ID of the async resource in whose execution context this async resource was created
  • resource <Object> reference to the resource representing the async operation, needs to be released during destroy

Called when a class is constructed that has the possibility to emit an asynchronous event. This does not mean the instance must call before/after before destroy is called, only that the possibility exists.

This behavior can be observed by doing something like opening a resource then closing it before the resource can be used. The following snippet demonstrates this.

require('net').createServer().listen(function() { this.close(); });
// OR
clearTimeout(setTimeout(() => {}, 10));

Every new resource is assigned a unique ID.


The type is a string that represents the type of resource that caused init to call. Generally it will be the name of the resource's constructor. The resource types provided by the built-in Node.js modules are:


Users are be able to define their own type when using the public embedder API.

Note: It is possible to have type name collisions. Embedders are encouraged to use a unique prefixes, such as the npm package name, to prevent collisions when listening to the hooks.


triggerId is the asyncId of the resource that caused (or "triggered") the new resource to initialize and that caused init to call. This is different from async_hooks.currentId() that only shows when a resource was created, while triggerId shows why a resource was created.

The following is a simple demonstration of triggerId:

  init(asyncId, type, triggerId) {
    const cId = async_hooks.currentId();
      1, `${type}(${asyncId}): trigger: ${triggerId} scope: ${cId}\n`);

require('net').createServer((conn) => {}).listen(8080);

Output when hitting the server with nc localhost 8080:

TCPWRAP(2): trigger: 1 scope: 1
TCPWRAP(4): trigger: 2 scope: 0

The first TCPWRAP is the server which receives the connections.

The second TCPWRAP is the new connection from the client. When a new connection is made the TCPWrap instance is immediately constructed. This happens outside of any JavaScript stack (side note: a currentId() of 0 means it's being executed from C++, with no JavaScript stack above it). With only that information it would be impossible to link resources together in terms of what caused them to be created, so triggerId is given the task of propagating what resource is responsible for the new resource's existence.


resource is an object that represents the actual resource. This can contain useful information such as the hostname for the GETADDRINFOREQWRAP resource type, which will be used when looking up the ip for the hostname in net.Server.listen. The API for getting this information is currently not considered public, but using the Embedder API users can provide and document their own resource objects. Such as resource object could for example contain the SQL query being executed.

Note: In some cases the resource object is reused for performance reasons, it is thus not safe to use it as a key in a WeakMap or add properties to it.

asynchronous context example#

Below is another example with additional information about the calls to init between the before and after calls, specifically what the callback to listen() will look like. The output formatting is slightly more elaborate to make calling context easier to see.

let indent = 0;
  init(asyncId, type, triggerId) {
    const cId = async_hooks.currentId();
    const indentStr = ' '.repeat(indent);
      `${indentStr}${type}(${asyncId}): trigger: ${triggerId} scope: ${cId}\n`);
  before(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeSync(1, `${indentStr}before:  ${asyncId}\n`);
    indent += 2;
  after(asyncId) {
    indent -= 2;
    const indentStr = ' '.repeat(indent);
    fs.writeSync(1, `${indentStr}after:   ${asyncId}\n`);
  destroy(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeSync(1, `${indentStr}destroy: ${asyncId}\n`);

require('net').createServer(() => {}).listen(8080, () => {
  // Let's wait 10ms before logging the server started.
  setTimeout(() => {
    console.log('>>>', async_hooks.currentId());
  }, 10);

Output from only starting the server:

TCPWRAP(2): trigger: 1 scope: 1
TickObject(3): trigger: 2 scope: 1
before:  3
  Timeout(4): trigger: 3 scope: 3
  TIMERWRAP(5): trigger: 3 scope: 3
after:   3
destroy: 3
before:  5
  before:  4
    TTYWRAP(6): trigger: 4 scope: 4
    SIGNALWRAP(7): trigger: 4 scope: 4
    TTYWRAP(8): trigger: 4 scope: 4
>>> 4
    TickObject(9): trigger: 4 scope: 4
  after:   4
after:   5
before:  9
after:   9
destroy: 4
destroy: 9
destroy: 5

Note: As illustrated in the example, currentId() and scope each specify the value of the current execution context; which is delineated by calls to before and after.

Only using scope to graph resource allocation results in the following:

TTYWRAP(6) -> Timeout(4) -> TIMERWRAP(5) -> TickObject(3) -> root(1)

The TCPWRAP isn't part of this graph; even though it was the reason for console.log() being called. This is because binding to a port without a hostname is actually synchronous, but to maintain a completely asynchronous API the user's callback is placed in a process.nextTick().

The graph only shows when a resource was created, not why, so to track the why use triggerId.


When an asynchronous operation is initiated (such as a TCP server receiving a new connection) or completes (such as writing data to disk) a callback is called to notify the user. The before callback is called just before said callback is executed. asyncId is the unique identifier assigned to the resource about to execute the callback.

The before callback will be called 0 to N times. The before callback will typically be called 0 times if the asynchronous operation was cancelled or for example if no connections are received by a TCP server. Asynchronous like the TCP server will typically call the before callback multiple times, while other operations like will only call it once.


Called immediately after the callback specified in before is completed.

Note: If an uncaught exception occurs during execution of the callback then after will run after the 'uncaughtException' event is emitted or a domain's handler runs.


Called after the resource corresponding to asyncId is destroyed. It is also called asynchronously from the embedder API emitDestroy().

Note: Some resources depend on GC for cleanup, so if a reference is made to the resource object passed to init it's possible that destroy is never called, causing a memory leak in the application. Of course if the resource doesn't depend on GC then this isn't an issue.


  • Returns <number> the asyncId of the current execution context. Useful to track when something calls.

For example:

console.log(async_hooks.currentId());  // 1 - bootstrap, 'r', (err, fd) => {
  console.log(async_hooks.currentId());  // 6 - open()

It is important to note that the ID returned fom currentId() is related to execution timing, not causality (which is covered by triggerId()). For example:

const server = net.createServer(function onConnection(conn) {
  // Returns the ID of the server, not of the new connection, because the
  // onConnection callback runs in the execution scope of the server's
  // MakeCallback().

}).listen(port, function onListening() {
  // Returns the ID of a TickObject (i.e. process.nextTick()) because all
  // callbacks passed to .listen() are wrapped in a nextTick().


  • Returns <number> the ID of the resource responsible for calling the callback that is currently being executed.

For example:

const server = net.createServer((conn) => {
  // The resource that caused (or triggered) this callback to be called
  // was that of the new connection. Thus the return value of triggerId()
  // is the asyncId of "conn".

}).listen(port, () => {
  // Even though all callbacks passed to .listen() are wrapped in a nextTick()
  // the callback itself exists because the call to the server's .listen()
  // was made. So the return value would be the ID of the server.

JavaScript Embedder API#

Library developers that handle their own I/O, a connection pool, or callback queues will need to hook into the AsyncWrap API so that all the appropriate callbacks are called. To accommodate this a JavaScript API is provided.

class AsyncResource()#

The class AsyncResource was 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.

It is important that before/after calls are unwound in the same order they are called. Otherwise an unrecoverable exception will occur and node will abort.

The following is an overview of the AsyncResource API.

const { AsyncResource } = require('async_hooks');

// AsyncResource() is meant to be extended. Instantiating a
// new AsyncResource() also triggers init. If triggerId is omitted then
// async_hook.currentId() is used.
const asyncResource = new AsyncResource(type, triggerId);

// Call AsyncHooks before callbacks.

// Call AsyncHooks after callbacks.

// Call AsyncHooks destroy callbacks.

// Return the unique ID assigned to the AsyncResource instance.

// Return the trigger ID for the AsyncResource instance.

AsyncResource(type[, triggerId])#

  • arguments
    • type <string> the type of ascyc event
    • triggerId <number> the ID of the execution context that created this async event

Example usage:

class DBQuery extends AsyncResource {
  constructor(db) {
    this.db = db;

  getInfo(query, callback) {
    this.db.get(query, (err, data) => {
      callback(err, data);

  close() {
    this.db = null;


Call all before callbacks and let them know a new asynchronous execution context is being entered. If nested calls to emitBefore() are made, the stack of asyncIds will be tracked and properly unwound.


Call all after callbacks. If nested calls to emitBefore() were made, then make sure the stack is unwound properly. Otherwise an error will be thrown.

If the user's callback throws an exception then emitAfter() will automatically be called for all asyncIds on the stack if the error is handled by a domain or 'uncaughtException' handler.


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.


  • Returns <number> the unique asyncId assigned to the resource.


  • Returns <number> the same triggerId that is passed to the AsyncResource constructor.