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Node.js v10.15.0-rc.2 Documentation
Table of Contents
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.
If Worker
s are used, each thread has an independent async_hooks
interface, and each thread will use a new set of async IDs.
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)[src]#
-
callbacks
<Object> The Hook Callbacks to registerinit
<Function> Theinit
callback.before
<Function> Thebefore
callback.after
<Function> Theafter
callback.destroy
<Function> Thedestroy
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 easy solution to this when debugging is to use a
synchronous logging operation such as fs.writeSync(process.stdout.fd, msg)
.
This will print to 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(process.stdout.fd, `${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()#
- 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 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()#
- 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 destroyed.
init(asyncId, type, triggerAsyncId, resource)#
asyncId
<number> A unique ID for the async resource.type
<string> The type of the async resource.triggerAsyncId
<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 an ID that is unique within the scope of the current Node.js instance.
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.
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.
triggerAsyncId
#
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. (An executionAsyncId()
of 0
means
that it is 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 an
isChainedPromise
property, set to true
if the promise has a parent promise,
and false
otherwise. For example, in the case of b = a.then(handler)
, a
is
considered a parent Promise
of b
. Here, b
is considered a chained promise.
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: ${eid}\n`);
},
before(asyncId) {
const indentStr = ' '.repeat(indent);
fs.writeSync(process.stdout.fd, `${indentStr}before: ${asyncId}\n`);
indent += 2;
},
after(asyncId) {
indent -= 2;
const indentStr = ' '.repeat(indent);
fs.writeSync(process.stdout.fd, `${indentStr}after: ${asyncId}\n`);
},
destroy(asyncId) {
const indentStr = ' '.repeat(indent);
fs.writeSync(process.stdout.fd, `${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
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)#
asyncId
<number>
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)#
asyncId
<number>
Called immediately after the callback specified in before
is completed.
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)#
asyncId
<number>
Called after the resource corresponding to asyncId
is destroyed. It is also
called asynchronously from the embedder API emitDestroy()
.
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)#
asyncId
<number>
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.
The Promise
is not necessarily fulfilled or rejected at this point if the
Promise
was resolved by assuming the state of another Promise
.
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.
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()
):
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()#
- Returns: <number> The ID of the resource responsible for calling the callback that is currently being executed.
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 triggerAsyncId
s by default. See
the section on promise execution tracking.
Promise execution tracking#
By default, promise executions are not assigned asyncId
s 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.
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:
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[src]#
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.
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 }
);
// 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])[src]#
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> Disables automaticemitDestroy
when the object is garbage collected. This usually does not need to be set (even ifemitDestroy
is called manually), unless the resource'sasyncId
is retrieved and the sensitive API'semitDestroy
is called with it. 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();
}
}
asyncResource.runInAsyncScope(fn[, thisArg, ...args])[src]#
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.emitBefore()[src]#
asyncResource.runInAsyncScope()
instead.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
asyncId
s will be tracked and properly unwound.
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. For this reason, the emitBefore
and emitAfter
APIs are
considered deprecated. Please use runInAsyncScope
, as it provides a much safer
alternative.
asyncResource.emitAfter()[src]#
asyncResource.runInAsyncScope()
instead.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 asyncId
s on the stack if the error is handled by a domain or
'uncaughtException'
handler.
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. For this reason, the emitBefore
and emitAfter
APIs are
considered deprecated. Please use runInAsyncScope
, as it provides a much safer
alternative.
asyncResource.emitDestroy()[src]#
- 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()[src]#
- Returns: <number> The unique
asyncId
assigned to the resource.
asyncResource.triggerAsyncId()[src]#
- Returns: <number> The same
triggerAsyncId
that is passed to theAsyncResource
constructor.