Table of Contents

• Async Hooks
  • Terminology
  • Public API
    • Overview
      • async_hooks.createHook(callbacks)
        • Error Handling
        • Printing in AsyncHooks callbacks
      • asyncHook.enable()
      • asyncHook.disable()
      • Hook Callbacks
        • init(asyncId, type, triggerAsyncId, resource)
          • type
          • triggerId
          • resource
          • Asynchronous context example
        • before(asyncId)
        • after(asyncId)
        • destroy(asyncId)
        • promiseResolve(asyncId)
      • async_hooks.executionAsyncId()
      • async_hooks.triggerAsyncId()
  • Promise execution tracking
  • JavaScript Embedder API
    • class AsyncResource()
      • AsyncResource(type[, options])
      • asyncResource.emitBefore()
      • asyncResource.emitAfter()
      • asyncResource.emitDestroy()
      • asyncResource.asyncId()
      • asyncResource.triggerAsyncId()

Async Hooks#

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');

Terminology#

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 fs.open. 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#

Overview#

Following is a simple overview of the public API.

const async_hooks = require('async_hooks');

// Return the ID of the current execution context.
const eid = async_hooks.executionAsyncId();

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

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

// 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.
asyncHook.enable();

// Disable listening for new asynchronous events.
asyncHook.disable();

//
// 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, triggerAsyncId, 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) { }

// promiseResolve is called only for promise resources, when the
// `resolve` function passed to the `Promise` constructor is invoked
// (either directly or through other means of resolving a promise).
function promiseResolve(asyncId) { }

async_hooks.createHook(callbacks)#

• callbacks <Object> The Hook Callbacks to register
  • init <Function> The init callback.
  • before <Function> The before callback.
  • after <Function> The after callback.
  • destroy <Function> The destroy callback.
• 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. 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 Hook Callbacks section.

const async_hooks = require('async_hooks');

const asyncHook = async_hooks.createHook({
  init(asyncId, type, triggerAsyncId, resource) { },
  destroy(asyncId) { }
});

Note that the callbacks will be inherited via the prototype chain:

class MyAsyncCallbacks {
  init(asyncId, type, triggerAsyncId, resource) { }
  destroy(asyncId) {}
}

class MyAddedCallbacks extends MyAsyncCallbacks {
  before(asyncId) { }
  after(asyncId) { }
}

const asyncHook = async_hooks.createHook(new MyAddedCallbacks());

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.

asyncHook.enable()#

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

The AsyncHook instance is disabled by default. 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();

asyncHook.disable()#

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 destroyed.

init(asyncId, type, triggerAsyncId, resource)#

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 an ID that is unique within the scope of the current process.

type#

The type is a string identifying the type of resource that caused init to be called. Generally, it will correspond to the name of the resource's constructor.

FSEVENTWRAP, FSREQWRAP, GETADDRINFOREQWRAP, GETNAMEINFOREQWRAP, HTTPPARSER,
JSSTREAM, PIPECONNECTWRAP, PIPEWRAP, PROCESSWRAP, QUERYWRAP, SHUTDOWNWRAP,
SIGNALWRAP, STATWATCHER, TCPCONNECTWRAP, TCPSERVER, TCPWRAP, TIMERWRAP, TTYWRAP,
UDPSENDWRAP, UDPWRAP, WRITEWRAP, ZLIB, SSLCONNECTION, PBKDF2REQUEST,
RANDOMBYTESREQUEST, TLSWRAP, Timeout, Immediate, TickObject

There is also the PROMISE resource type, which is used to track Promise instances and asynchronous work scheduled by them.

Users are 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 unique prefixes, such as the npm package name, to prevent collisions when listening to the hooks.

triggerId#

triggerAsyncId 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.executionAsyncId() that only shows when a resource was created, while triggerAsyncId shows why a resource was created.

The following is a simple demonstration of triggerAsyncId:

async_hooks.createHook({
  init(asyncId, type, triggerAsyncId) {
    const eid = async_hooks.executionAsyncId();
    fs.writeSync(
      1, `${type}(${asyncId}): trigger: ${triggerAsyncId} execution: ${eid}\n`);
  }
}).enable();

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

Output when hitting the server with nc localhost 8080:

TCPSERVERWRAP(2): trigger: 1 execution: 1
TCPWRAP(4): trigger: 2 execution: 0

The TCPSERVERWRAP is the server which receives the connections.

The 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 executionAsyncId() 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 triggerAsyncId is given the task of propagating what resource is responsible for the new resource's existence.

resource#

resource is an object that represents the actual async resource that has been initialized. This can contain useful information that can vary based on the value of type. For instance, for the GETADDRINFOREQWRAP resource type, resource provides the hostname used when looking up the IP address for the hostname in net.Server.listen(). The API for accessing this information is currently not considered public, but using the Embedder API, users can provide and document their own resource objects. For example, such a resource object could contain the SQL query being executed.

In the case of Promises, the resource object will have promise property that refers to the Promise that is being initialized, and a parentId property set to the asyncId of a parent Promise, if there is one, and undefined otherwise. For example, in the case of b = a.then(handler), a is considered a parent Promise of b.

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#

The following is an 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;
async_hooks.createHook({
  init(asyncId, type, triggerAsyncId) {
    const eid = async_hooks.executionAsyncId();
    const indentStr = ' '.repeat(indent);
    fs.writeSync(
      1,
      `${indentStr}${type}(${asyncId}):` +
      ` trigger: ${triggerAsyncId} execution: ${eid}\n`);
  },
  before(asyncId) {
    const indentStr = ' '.repeat(indent);
    fs.writeSync(1, `${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`);
  },
}).enable();

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

Output from only starting the server:

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

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

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

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

The TCPSERVERWRAP is not 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 a synchronous operation, 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 triggerAsyncId.

before(asyncId)#

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. Persistent asynchronous resources like a TCP server will typically call the before callback multiple times, while other operations like fs.open() will call it only once.

after(asyncId)#

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.

destroy(asyncId)#

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

Note: Some resources depend on garbage collection for cleanup, so if a reference is made to the resource object passed to init it is possible that destroy will never be called, causing a memory leak in the application. If the resource does not depend on garbage collection, then this will not be an issue.

promiseResolve(asyncId)#

Called when the resolve function passed to the Promise constructor is invoked (either directly or through other means of resolving a promise).

Note that resolve() does not do any observable synchronous work.

Note: This does not necessarily mean that the Promise is fulfilled or rejected at this point, if the Promise was resolved by assuming the state of another Promise.

For example:

new Promise((resolve) => resolve(true)).then((a) => {});

calls the following callbacks:

init for PROMISE with id 5, trigger id: 1
  promise resolve 5      # corresponds to resolve(true)
init for PROMISE with id 6, trigger id: 5  # the Promise returned by then()
  before 6               # the then() callback is entered
  promise resolve 6      # the then() callback resolves the promise by returning
  after 6

async_hooks.executionAsyncId()#

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

For example:

const async_hooks = require('async_hooks');

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

The ID returned from executionAsyncId() is related to execution timing, not causality (which is covered by triggerAsyncId()). 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().
  async_hooks.executionAsyncId();

}).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().
  async_hooks.executionAsyncId();
});

Note that promise contexts may not get precise executionAsyncIds by default. See the section on promise execution tracking.

async_hooks.triggerAsyncId()#

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 triggerAsyncId()
  // is the asyncId of "conn".
  async_hooks.triggerAsyncId();

}).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.
  async_hooks.triggerAsyncId();
});

Note that promise contexts may not get valid triggerAsyncIds by default. See the section on promise execution tracking.

Promise execution tracking#

By default, promise executions are not assigned asyncIds due to the relatively expensive nature of the promise introspection API provided by V8. This means that programs using promises or async/await will not get correct execution and trigger ids for promise callback contexts by default.

Here's an example:

const ah = require('async_hooks');
Promise.resolve(1729).then(() => {
  console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`);
});
// produces:
// eid 1 tid 0

Observe that the then callback claims to have executed in the context of the outer scope even though there was an asynchronous hop involved. Also note that the triggerAsyncId value is 0, which means that we are missing context about the resource that caused (triggered) the then callback to be executed.

Installing async hooks via async_hooks.createHook enables promise execution tracking. Example:

const ah = require('async_hooks');
ah.createHook({ init() {} }).enable(); // forces PromiseHooks to be enabled.
Promise.resolve(1729).then(() => {
  console.log(`eid ${ah.executionAsyncId()} tid ${ah.triggerAsyncId()}`);
});
// produces:
// eid 7 tid 6

In this example, adding any actual hook function enabled the tracking of promises. There are two promises in the example above; the promise created by Promise.resolve() and the promise returned by the call to then. In the example above, the first promise got the asyncId 6 and the latter got asyncId 7. During the execution of the then callback, we are executing in the context of promise with asyncId 7. This promise was triggered by async resource 6.

Another subtlety with promises is that before and after callbacks are run only on chained promises. That means promises not created by then/catch will not have the before and after callbacks fired on them. For more details see the details of the V8 PromiseHooks API.

JavaScript Embedder API#

Library developers that handle their own asynchronous resources performing tasks like I/O, connection pooling, or managing callback queues may use the AsyncWrap JavaScript API so that all the appropriate callbacks are called.

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.

Note: before and after calls must be unwound in the same order that they are called. Otherwise, an unrecoverable exception will occur and the process will abort.

The following is an overview of the AsyncResource API.

const { AsyncResource, executionAsyncId } = require('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 }
);

// Call AsyncHooks before callbacks.
asyncResource.emitBefore();

// Call AsyncHooks after callbacks.
asyncResource.emitAfter();

// 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();

AsyncResource(type[, options])#

Example usage:

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

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

  close() {
    this.db = null;
    this.emitDestroy();
  }
}

asyncResource.emitBefore()#

Call all before callbacks to notify that 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.

asyncResource.emitAfter()#

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, emitAfter() will automatically be called for all asyncIds on the stack if the error is handled by a domain or 'uncaughtException' handler.

asyncResource.emitDestroy()#

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()#

asyncResource.triggerAsyncId()#