Nodejs Async Hooks 101
Tags: node, async, hooks, asynchronousoperation
Published: 2023-05-19 07:54:06

Node.js is a highly scalable and performance-oriented platform that enables developers to create applications that are fast, efficient, and lightweight. One of Node.js' key features is its ability to handle multiple asynchronous operations at the same time using a single-threaded event loop.

Async Hooks is a new Node.js feature that allows developers to access the event loop and track the lifecycle of asynchronous operations in their applications. This feature provides information about how asynchronous operations are carried out and can aid in the identification of potential performance issues and bottlenecks.

Developers can use Async Hooks to get information about the current state of an asynchronous operation and receive notifications when it begins, ends, or encounters an error. This data can be used to determine how long an asynchronous operation takes, what resources it consumes, and how it affects the application's overall performance.

Async Hooks are implemented as Node.js callbacks registered with the **async_hooks** module. Callbacks are informed about the current asynchronous operation and can be used to record data, track resources, and manage performance.

Async Hooks can be used to provide custom logic for monitoring and managing asynchronous actions in addition to offering performance insights. Async Hooks can be used by developers, for instance, to provide unique resource management, error handling, and logging solutions.

Async Hooks, a potent new feature of Node.js that gives developers more insight into the inner workings of asynchronous actions, is summarised above. With this knowledge, programmers may create apps that are quicker and more effective, and they can also rapidly locate and fix performance problems.

Example of **aync_hook** implementation:

In this example, we define a set of hooks for tracking asynchronous operations. The hooks include `init` , `before` , `after` , `destroy` , and `promiseResolve` .

We then create an Async Hook using `async_hooks.createHook` and pass in the hooks. The created hook is then enabled using `asyncHook.enable()` .

We also have a `setTimeout` function that will execute after 1000 milliseconds, which will trigger the hooks to be called.

Finally, when the Node.js process is about to exit, the hook is disabled using a`syncHook.disable()` .

When you run this code, you will see output from the hooks that log information about the asynchronous operation triggered by the `setTimeout` function.

```
const async_hooks = require('async_hooks');
const hooks = {
init: (asyncId, type, triggerAsyncId, resource) => {
console.log(`Init Hook: asyncId: ${asyncId}, type: ${type}`);
},
before: (asyncId) => {
console.log(`Before Hook: asyncId: ${asyncId}`);
},
after: (asyncId) => {
console.log(`After Hook: asyncId: ${asyncId}`);
},
destroy: (asyncId) => {
console.log(`Destroy Hook: asyncId: ${asyncId}`);
},
promiseResolve: (asyncId) => {
console.log(`Promise Resolve Hook: asyncId: ${asyncId}`);
},
};

const asyncHook = async_hooks.createHook(hooks);
asyncHook.enable();
setTimeout(() => {
console.log('Timeout finished');
}, 1000);
process.on('exit', () => {
asyncHook.disable();
});

```
**Async Hooks should be used in the following scenarios**:

1. <u>Performance Monitoring</u>: Async Hooks offers information on the manner in which asynchronous operations are carried out, how long they take to complete, and what resources they use. This data can be used to track your application's performance and spot any potential performance bottlenecks.

2. <u>Resource management</u>: Resources connected to asynchronous operations can be managed and tracked using async hooks. Async Hooks can be used, for instance, to track and release file descriptors, network connections, and other resources that
asynchronous actions acquire and use.

3. <u>Error Handling</u>: Custom error handling for asynchronous activities can be implemented using async hooks. Instead of handling errors in each individual asynchronous action, you may utilize Async Hooks to find errors and handle them centrally.

4. <u>Logging</u>: Async Hooks can be used to log information about asynchronous operations and their lifecycle. This information can be used for debugging and troubleshooting, as well as for performance analysis and optimization.

Async Hook **Resource Management** Example:

```
const async_hooks = require('async_hooks');
const fs = require('fs');
const activeResources = new Map();
const hooks = {
init: (asyncId, type, triggerAsyncId, resource) => {
if (type === 'FSREQWRAP') {
activeResources.set(asyncId, resource);
}
},
before: (asyncId) => {},
after: (asyncId) => {},
destroy: (asyncId) => {
const resource = activeResources.get(asyncId);
if (resource) {
resource.close();
activeResources.delete(asyncId);
}
},
promiseResolve: (asyncId) => {},
};

const asyncHook = async_hooks.createHook(hooks);
asyncHook.enable();
fs.open('test.txt', 'r', (err, fd) => {
if (err) {
console.error(err);
return;
}
console.log(`Opened file descriptor: ${fd}`);
});
process.on('exit', () => {
asyncHook.disable();
});
```
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