async hooks

Async Hooks

What are async hooks?

Async hooks are a way to track asynchronous operations in Node.js. Asynchronous operations are those that happen outside of the main thread of execution, such as I/O operations or timers.

Why use async hooks?

You can use async hooks to:

  • Debug asynchronous code

  • Measure the performance of asynchronous operations

  • Track the flow of data through your application

Async Hook API

The async hooks API consists of the following functions:

  • createHook(callbacks): Creates an async hook. Callbacks are functions that will be called when certain asynchronous events occur.

  • AsyncHook.disable(): Disables the async hook.

  • AsyncHook.enable(): Enables the async hook.

  • executionAsyncResource(): Returns the async resource that is currently executing.

  • emitBefore(): Emits an event before the given async resource executes.

  • emitAfter(): Emits an event after the given async resource executes.

Real-world example

Here is an example of how to use async hooks to track the time spent in asynchronous operations:

const async_hooks = require("node:async_hooks");

const hook = async_hooks.createHook({
  init(asyncId, type, triggerAsyncId, resource) {
    // Initialize the async resource
  },
  destroy(asyncId) {
    // Destroy the async resource
  },
  before(asyncId) {
    // Start the timer
  },
  after(asyncId) {
    // Stop the timer and record the time spent
  },
});

hook.enable();

// Do some asynchronous work
setTimeout(() => {
  console.log("Hello world!");
}, 1000);

// Disable the hook when you are finished
hook.disable();

Applications in the real world

Async hooks can be used in a variety of real-world applications, including:

  • Debugging asynchronous code

  • Measuring the performance of asynchronous operations

  • Tracking the flow of data through your application

  • Fraud detection

  • Load balancing

Simplified Terminology

Asynchronous Resource:

Imagine a resource like a door that you can open or close. When you open the door, a special event (called a callback) happens. This event can happen again and again, like when someone keeps opening and closing the door.

AsyncHook:

This is like a secret listener that watches all the doors (resources) and knows when they are opened or closed. It assigns each door a unique number called an "async ID."

Worker Threads:

Think of worker threads as if you had multiple doors in different rooms. Each room has its own listener that tracks the doors in that room only.

Real-World Example:

Let's say you have a server that listens for incoming connections. When a connection is made, a callback event is triggered. Using AsyncHook, you can track all the active connections and see when they are closed. This information can help you debug errors and improve the performance of your server.

Complete Code Example:

const async_hooks = require('async_hooks');

const hook = async_hooks.createHook({
  init(asyncId, type, triggerAsyncId, resource) {
    console.log(`New resource created: ${asyncId}`);
  },
  destroy(asyncId) {
    console.log(`Resource closed or destroyed: ${asyncId}`);
  },
});

hook.enable();

console.log('Starting server...');

// Create a server and listen for connections
const net = require('net');
net.createServer().on('connection', (socket) => {
  console.log('New connection established!');
});

Potential Applications:

  • Debugging and profiling applications

  • Tracking performance and resource usage

  • Identifying and managing resource leaks

Overview

Async hooks allow you to track asynchronous events in Node.js. They can be useful for debugging, performance monitoring, and error handling.

AsyncHook Class

The AsyncHook class is the main interface for working with async hooks. You can create an instance of this class by passing it a callback function. The callback function will be called whenever an asynchronous event occurs.

Callbacks

The AsyncHook class has five callback functions:

  • init: This function is called when a new asynchronous event is created.

  • before: This function is called just before the callback function of an asynchronous event is called.

  • after: This function is called just after the callback function of an asynchronous event has finished.

  • destroy: This function is called when an asynchronous event is destroyed.

  • promiseResolve: This function 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).

Example Usage

The following code shows how to use the AsyncHook class to track asynchronous events:

const asyncHook = async_hooks.createHook({
  init: (asyncId, type, triggerAsyncId, resource) => {
    // This function is called when a new asynchronous event is created.
  },
  before: (asyncId) => {
    // This function is called just before the callback function of an asynchronous event is called.
  },
  after: (asyncId) => {
    // This function is called just after the callback function of an asynchronous event has finished.
  },
  destroy: (asyncId) => {
    // This function is called when an asynchronous event is destroyed.
  },
  promiseResolve: (asyncId) => {
    // This function 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).
  },
});

asyncHook.enable();

Real-World Applications

Async hooks can be used for a variety of real-world applications, including:

  • Debugging: Async hooks can be used to debug asynchronous code by allowing you to see when and where asynchronous events are occurring.

  • Performance monitoring: Async hooks can be used to monitor the performance of asynchronous code by allowing you to track the time it takes for asynchronous events to complete.

  • Error handling: Async hooks can be used to handle errors in asynchronous code by allowing you to catch errors that occur during the execution of asynchronous events.

Async Hooks

What are they?

Async hooks are like little helpers that track when asynchronous operations, such as reading a file or making a network request, are happening in your Node.js program.

Why are they useful?

They allow you to tap into these operations and do things like:

  • Log when an operation starts and ends, to see how long it takes

  • Clean up resources after an operation is done, to prevent memory leaks

  • Keep track of how many operations are running at any given time, to see if your program is getting overloaded

How do I use async hooks?

You can use the createHook function to create a new async hook. The hook can be configured with a set of callback functions that will be called at different points during the lifecycle of an asynchronous operation.

What are the callback functions?

  • init: Called when an asynchronous operation is initialized.

  • before: Called just before an asynchronous operation is about to execute.

  • after: Called just after an asynchronous operation has executed.

  • destroy: Called when an asynchronous operation is destroyed.

  • promiseResolve: Called when a promise is resolved.

Here's an example of how to use async hooks to log the start and end of an asynchronous operation:

const { createHook } = require("async_hooks");

const asyncHook = createHook({
  init(asyncId, type, triggerAsyncId, resource) {
    console.log(`Async operation ${asyncId} of type ${type} initialized`);
  },
  destroy(asyncId) {
    console.log(`Async operation ${asyncId} destroyed`);
  },
});

// Enable the async hook
asyncHook.enable();

// Start an asynchronous operation
fs.readFile("myfile.txt", (err, data) => {
  if (err) {
    console.error(err);
  } else {
    console.log(data.toString());
  }
});

// Disable the async hook
asyncHook.disable();

Output:

Async operation 1 of type FSReqCallback initialized
Async operation 1 destroyed

Real-world applications

  • Profiling the performance of your Node.js program

  • Debugging asynchronous code

  • Managing resources in a resource-intensive application

  • Tracking the number of concurrent requests in a web server

Error Handling in Async Hooks

Imagine you're playing a game with lots of tiny workers (callbacks) running around, helping you do different tasks.

If one of these workers makes a mistake and throws an error, what should happen?

In our game, the default rule is that if any worker throws an error, the whole game (process) will stop immediately.

This is like pulling the plug on your computer when a program crashes.

Why do we do this? Because these workers are like little elves building your house (object). If one elf makes a mistake, it's possible that it will affect the entire construction and cause the house to collapse later.

Exceptions to the Rule

However, there are exceptions to this rule:

  1. If you're using the --abort-on-uncaught-exception flag: In this case, the game will crash immediately, like before, and leave behind a "core file" that can be used to figure out what went wrong.

  2. If you've added special listeners for 'uncaughtException' events: These listeners can be like firefighters who try to put out the fire and keep the game running. However, in this case, they won't be able to save the day, and the game will still exit.

Real-World Example

Let's say you have an object that calculates the area of a circle.

If you pass an invalid radius (like -5) to this object, it might throw an error. To handle this error, you can use async hooks to detect when the error is thrown and take appropriate actions, such as logging the error or sending an email.

Code Snippet

const async_hooks = require('async_hooks');

const hook = async_hooks.createHook({
  init(id, type, triggerAsyncId, resource) {
    // When a new async resource is created, log its details
    console.log(`New async resource: ${id}, type: ${type}, triggerAsyncId: ${triggerAsyncId}, resource: ${resource}`);
  },
  destroy(id) {
    // When an async resource is destroyed, log its ID
    console.log(`Destroyed async resource: ${id}`);
  },
  before(id) {
    // Before an async resource executes, log its ID
    console.log(`Before async resource: ${id}`);
  },
  after(id) {
    // After an async resource executes, log its ID
    console.log(`After async resource: ${id}`);
  },
  promiseResolve(id) {
    // When a Promise resolves, log its ID
    console.log(`Promise resolved: ${id}`);
  },
  promiseReject(id) {
    // When a Promise rejects, log its ID and error message
    console.log(`Promise rejected: ${id}, error: ${error}`);
  }
});

hook.enable(); // Enable the hook

// Create an async resource: a function that calculates the area of a circle
const circleArea = radius => {
  if (radius < 0) {
    throw new Error('Invalid radius'); // Throw an error if the radius is negative
  }

  return Math.PI * radius ** 2; // Calculate the area of the circle
};

// Call the async resource and pass an invalid radius
try {
  circleArea(-5); // This will throw an error
} catch (error) {
  console.log(`Async hook caught error: ${error}`); // The async hook will catch the error
} finally {
  hook.disable(); // Disable the hook
}

AsyncHook Callbacks: Printing Without Recursion

What is AsyncHook?

Imagine you have a kitchen with a chef and a waiter. The chef cooks food, while the waiter serves it to customers. AsyncHook is like the "kitchen assistant" that helps the chef and waiter communicate when food is ready or needs to be taken to the table.

Problem with Printing in AsyncHook Callbacks

When the waiter is serving food, we can't have him stop and write a note about how delicious it is. That would be a waste of time and could delay other customers. The same goes for AsyncHook callbacks. Printing in these callbacks would slow down the kitchen (the main program).

Solution: Use Synchronous Logging

Instead of printing in callbacks, we can use synchronous logging, like writing to a file. This won't delay the kitchen because it happens right away.

Code Example:

fs.writeFileSync("log.txt", "Food served to customer #1.");

Advanced Approach: Tracking Async Operations

Sometimes, we need to write logs that involve asynchronous operations, like sending an email. To avoid recursion (the waiter trying to take the note again and again), we can track what caused the callback. If it was the logging itself, we skip writing the log.

Code Snippet:

const asyncHook = require("async_hooks");

asyncHook.addHook({
  after: (asyncId, type, triggerAsyncId) => {
    // Check if the callback was caused by our logging
    if (triggerAsyncId === logAsyncId) {
      return;
    }

    // Log the message
    console.log("Asynchronous operation completed.");
  },
});

Real-World Applications:

  • Performance Optimization: Avoids unnecessary slowdowns by preventing infinite recursion during logging.

  • Error Handling: Provides a way to log errors and track their origin without disrupting the program's flow.

  • Performance Monitoring: Allows developers to analyze async operations and identify performance bottlenecks.

AsyncHook

What is it?

An AsyncHook lets you track when asynchronous operations start and stop in your Node.js application.

Why is it useful?

Asynchronous operations are a key part of Node.js, but they can make it tricky to debug and profile your code. AsyncHooks help you understand where and when these operations are happening, making it easier to identify and fix performance issues.

How to use it?

To use an AsyncHook, you first need to create an instance of it:

const asyncHook = require('async_hooks').createHook({
  // Define callback functions here
});

Once you have an instance, you can register callback functions to be called when specific events occur:

  • 'init': When a new async operation starts

  • 'before': Before the async operation completes

  • 'after': After the async operation completes

  • 'destroy': When the async operation is destroyed

For example, to log information about each async operation, you could use the following code:

asyncHook.addListeners({
  init(asyncId, type, triggerId) {
    console.log(`Async operation started: ${asyncId} (${type})`);
  },
  before(asyncId) {
    console.log(`Async operation about to complete: ${asyncId}`);
  },
  after(asyncId) {
    console.log(`Async operation completed: ${asyncId}`);
  },
  destroy(asyncId) {
    console.log(`Async operation destroyed: ${asyncId}`);
  }
});

Real-world example

One real-world application of AsyncHooks is to identify potential performance issues. By tracking the lifetime of async operations, you can see if any of them are taking an unusually long time to complete. This information can help you pinpoint areas of your code that need to be optimized.

Code implementation

Here is a complete code implementation of the example above:

const asyncHook = require('async_hooks').createHook({
  init(asyncId, type, triggerId) {
    console.log(`Async operation started: ${asyncId} (${type})`);
  },
  before(asyncId) {
    console.log(`Async operation about to complete: ${asyncId}`);
  },
  after(asyncId) {
    console.log(`Async operation completed: ${asyncId}`);
  },
  destroy(asyncId) {
    console.log(`Async operation destroyed: ${asyncId}`);
  }
});

asyncHook.enable();

// Start an asynchronous operation
setTimeout(() => {
  console.log('Hello world!');
}, 1000);

This code will log the following output:

Async operation started: 1 (Timeout)
Async operation about to complete: 1
Async operation completed: 1

Potential applications

AsyncHooks have a variety of potential applications, including:

  • Debugging and profiling asynchronous code

  • Identifying performance bottlenecks

  • Monitoring resource usage

  • Implementing custom tracing or logging systems

What is async_hooks?

async_hooks is a Node.js module that allows you to track asynchronous operations within your code. Asynchronous operations are operations that don't happen immediately, like reading a file from disk or making a request to a remote server.

What is an AsyncHook instance?

An AsyncHook instance is an object that represents a specific async operation. It has a number of methods that allow you to track the start, end, and other events that happen during the operation.

What is asyncHook.enable()?

asyncHook.enable() is a method that enables the callbacks for a given AsyncHook instance. This means that the callbacks will be called whenever the corresponding events happen during the async operation.

Why would you want to enable callbacks for an AsyncHook instance?

You might want to enable callbacks for an AsyncHook instance if you want to track the performance of the async operation, or if you want to handle any errors that occur during the operation.

How do you enable callbacks for an AsyncHook instance?

You can enable callbacks for an AsyncHook instance by calling the enable() method on the instance. Here's an example:

const async_hooks = require('node:async_hooks');

const hook = async_hooks.createHook({
  init(asyncId, type, triggerAsyncId, resource) {
    // Called when a new async operation is created.
  },
  before(asyncId) {
    // Called just before the async operation begins.
  },
  after(asyncId) {
    // Called just after the async operation completes.
  },
  destroy(asyncId) {
    // Called when the async operation is destroyed.
  },
});

hook.enable();

Real-world use cases for async_hooks:

  • Tracking the performance of async operations.

  • Handling errors that occur during async operations.

  • Debugging async code.

  • Profiling async code.

Simplified Explanation of asyncHook.disable()

When you create an asyncHook instance, it registers itself in a global pool of hooks that get executed whenever an asynchronous operation starts or ends.

asyncHook.disable() lets you temporarily stop these hooks from getting executed. It's like putting them on pause.

Example:

const async_hooks = require("async_hooks");

// Create an async hook
const hook = async_hooks.createHook({
  init() {
    console.log("Init");
  },
  before() {
    console.log("Before");
  },
  after() {
    console.log("After");
  },
  destroy() {
    console.log("Destroy");
  },
});

// Disable the hook
hook.disable();

// Start an asynchronous operation (e.g., a timer)
setTimeout(() => {}, 1000);

// The hook won't be executed because it's disabled

Real-World Applications:

  • Debugging: Disabling hooks can be useful for debugging if they're interfering with your code.

  • Performance Optimization: If you have a lot of hooks running, disabling them can improve performance.

  • Security: Disabling hooks can help prevent malicious code from hooking into your application.

Returns:

disable() returns the asyncHook instance itself. This is useful if you want to chain multiple operations on the hook, e.g.:

hook.disable().enable();

Hook Callbacks

In Node.js, "async hooks" allow us to track and respond to asynchronous events, like file I/O, HTTP requests, and database queries. These events play a crucial role in Node's event-driven architecture.

Instantiation

When an asynchronous event is created, such as a file read operation, an "async hook" is associated with it. This hook acts like a listener, waiting for the event to complete.

**Real World Example:

const fs = require("fs");

fs.readFile("README.md", (err, data) => {
  if (err) {
    console.error(err);
    return;
  }

  console.log(data);
});

In this example, an async hook is created when fs.readFile is called. It will track the progress of the file read operation and wait for it to complete.

Before/After Callback Execution

Before the callback function of the asynchronous event is invoked, the async hook is called with a "pre" event. After the callback has executed, the hook is called again with a "post" event.

**Real World Example:

const hook = require("async_hooks").createHook({
  init: (asyncId, type, triggerAsyncId) => {
    console.log(`New async event: ${asyncId}`);
  },
  before: (asyncId) => {
    console.log(`Before callback: ${asyncId}`);
  },
  after: (asyncId) => {
    console.log(`After callback: ${asyncId}`);
  },
});

hook.enable();

fs.readFile("README.md", (err, data) => {
  if (err) {
    console.error(err);
    return;
  }

  console.log(data);
});

hook.disable();

In this example, we create an async hook that logs messages before and after the callback function for file read operation is executed.

Destruction

When the asynchronous event finishes running, the async hook associated with it is destroyed. This provides an opportunity to perform cleanup actions or log errors that occurred during the event.

**Real World Example:

const hook = require("async_hooks").createHook({
  init: (asyncId, type, triggerAsyncId) => {
    console.log(`New async event: ${asyncId}`);
  },
  destroy: (asyncId) => {
    console.log(`Async event destroyed: ${asyncId}`);
  },
});

hook.enable();

fs.readFile("README.md", (err, data) => {
  if (err) {
    console.error(err);
    return;
  }

  console.log(data);
});

hook.disable();

In this example, we log a message when the async event associated with the file read operation is destroyed.

Potential Applications

Async hooks have numerous applications in real-world scenarios:

  • Debugging: Track and trace async events to identify performance bottlenecks or errors.

  • Profiling: Analyze the timing and behavior of async events for performance optimization.

  • Error Handling: Log errors and exceptions that occur during async operations for debugging and recovery purposes.

  • Resource Management: Monitor resource usage associated with async events to prevent resource leaks.

Async Hooks

Async hooks are a way to track asynchronous operations in Node.js. They allow you to monitor when asynchronous operations start and end, and what resources are used during those operations. This information can be useful for debugging, performance monitoring, and error handling.

Creating an Async Hook

To create an async hook, you need to implement the following methods:

  • init(asyncId, type, triggerAsyncId, resource): This method is called when a new asynchronous operation is started. The asyncId is a unique identifier for the operation, the type is the type of operation (e.g., "HTTP request", "database query"), the triggerAsyncId is the asyncId of the operation that triggered the current operation, and the resource is a reference to the resource used by the operation (e.g., a file, a database connection).

  • before(asyncId): This method is called just before the asynchronous operation is executed.

  • after(asyncId): This method is called just after the asynchronous operation has completed.

  • destroy(asyncId): This method is called when the asynchronous operation is destroyed.

Example

The following code shows how to create a simple async hook that logs the start and end of all HTTP requests:

const { AsyncHook } = require("async_hooks");

const asyncHook = new AsyncHook({
  init(asyncId, type, triggerAsyncId, resource) {
    if (type === "HTTPREQUEST") {
      console.log(`HTTP request started: ${asyncId}`);
    }
  },
  after(asyncId) {
    if (type === "HTTPREQUEST") {
      console.log(`HTTP request ended: ${asyncId}`);
    }
  },
});

asyncHook.enable();

Real World Applications

Async hooks can be used for a variety of purposes, including:

  • Debugging: Async hooks can help you identify the source of errors in your code by tracking the sequence of events that led to the error.

  • Performance monitoring: Async hooks can help you identify bottlenecks in your code by tracking the performance of individual asynchronous operations.

  • Error handling: Async hooks can help you handle errors in your code by providing information about the context in which the error occurred.

  • Security: Async hooks can help you identify security vulnerabilities in your code by tracking the resources that are used by asynchronous operations.

AsyncResource

Imagine you have a bunch of tasks that need to be completed, and you want to know what's going on with each task, like when it starts, finishes, or encounters any errors. AsyncResource is like a personal assistant for each task, keeping track of its status and letting you know when things happen.

type

Each task has a unique name, like "Database Query" or "File Upload." This name is the "type" of the task. AsyncResource uses this name to identify the task and provide information about it.

Type Comparisons

TLSWRAP: Tracking TLS-related operations. TCPWRAP: Monitoring TCP-related actions. TCPSERVERWRAP: Keeping an eye on TCP server operations. GETADDRINFOREQWRAP: Monitoring DNS lookup requests. FSREQCALLBACK: Tracking file system operations. Microtask: Following tasks scheduled for execution in the current event loop iteration. Timeout: Keeping an eye on timers and timeouts. PROMISE: Monitoring Promise instances and their associated asynchronous work.

Custom Types

Just like how you can name your tasks, you can also create custom types for your own AsyncResource instances. This helps organize and identify tasks in your specific application.

Real-World Applications

Logging and Debugging: Track the flow of asynchronous tasks and easily identify any performance issues or errors. Performance Monitoring: Measure the time it takes for different tasks to complete and identify bottlenecks. Error Handling: Catch and handle errors in asynchronous tasks promptly. Resource Management: Keep track of resource usage for asynchronous tasks, such as database connections or file handles.

Example

const async_hooks = require("async_hooks");

// Create a custom type for our database operations
const databaseType = "DATABASE_OPERATION";

// Create an AsyncResource instance for each database operation
const asyncResource = new async_hooks.AsyncResource(databaseType);

// Track when the operation starts
asyncResource.runInAsyncScope(() => {
  // Database operation code
});

triggerAsyncId Explained

triggerAsyncId is a special property that can be used to track the relationship between different async operations in Node.js. It tells you which async operation caused another async operation to be created. This can be helpful for debugging and understanding how your code is executing asynchronously.

How to use triggerAsyncId

To use triggerAsyncId, you need to use the async_hooks module. This module provides a way to hook into the Node.js event loop and track async operations as they are created and destroyed.

Once you have the async_hooks module installed, you can create a new hook and listen for the 'init' event. This event is fired whenever a new async operation is created. When this event is fired, the hook will be passed three arguments:

  • asyncId: The unique identifier for the new async operation.

  • type: The type of async operation. This can be one of the following values:

    • 'Timeout'

    • 'Immediate'

    • 'Interval'

    • 'SetImmediate'

    • 'Promise'

    • 'AsyncResource'

  • triggerAsyncId: The unique identifier for the async operation that caused the new async operation to be created.

Example

The following example shows how to use triggerAsyncId to track the relationship between different async operations:

const { createHook, executionAsyncId } = require("async_hooks");

createHook({
  init(asyncId, type, triggerAsyncId) {
    // Log the async operation and its trigger
    console.log(`${type}(${asyncId}): trigger: ${triggerAsyncId}`);
  },
}).enable();

// Create a new async operation
setTimeout(() => {
  // This is a new async operation
  console.log(`Timeout: trigger: ${executionAsyncId()}`);
}, 1000);

Output:

Timeout(1): trigger: 0

In this example, the setTimeout() call creates a new async operation. The triggerAsyncId for this async operation is 0, which means that it was not triggered by any other async operation.

Applications

triggerAsyncId can be used for a variety of purposes, including:

  • Debugging async code: triggerAsyncId can help you to understand the flow of async operations in your code. This can be helpful for debugging errors and performance issues.

  • Tracing async operations: triggerAsyncId can be used to trace the path of async operations through your code. This can be helpful for understanding how different parts of your code are interacting with each other.

  • Performance tuning: triggerAsyncId can be used to identify performance bottlenecks in your code. By understanding the relationship between different async operations, you can identify areas where your code is spending too much time waiting for I/O operations.

Real-world implementation

One real-world application of triggerAsyncId is to track the performance of a web server. By tracking the triggerAsyncId of each request, you can understand how different parts of your code are contributing to the overall performance of the server. This information can be used to identify performance bottlenecks and improve the overall performance of the server.

Another real-world application of triggerAsyncId is to track the flow of data through a complex system. By understanding the relationship between different async operations, you can identify where data is coming from and where it is going. This information can be used to improve the performance and reliability of the system.

Simplified Explanation of resource in Node.js's Async Hooks Module

Overview

Async Hooks are a feature in Node.js that allow you to track asynchronous operations in your code. Asynchronous operations are actions that happen in parallel, without blocking the main thread of your application.

resource is an object that represents the underlying resource associated with an asynchronous operation. Resources can vary depending on the type of operation, for example, a database connection or a file handle.

Simplified Concepts

  • Resource: An object representing the "thing" that is performing the asynchronous operation, such as a database connection or a file handle.

  • API: A set of methods or functions that allow you to interact with the resource. The specific API depends on the type of resource.

  • Reusability: In Node.js, resource objects may be reused for performance reasons. This means that you should not rely on the same resource object being used every time.

Real-World Example

Tracking Database Queries

In the following example, we use Async Hooks to track database queries and log the SQL statements that are executed:

// Create an async hook to track database queries
const asyncHook = asyncHooks.createHook({
  // This function is called every time a new asynchronous resource is created
  init(asyncId, type, triggerAsyncId, resource) {
    // Check if the resource is a database connection
    if (type === "TCPWRAP" && resource.database) {
      console.log("New database connection created");

      // Create a listener to track queries on this connection
      resource.on("query", (query) => {
        console.log("Database query executed:", query);
      });
    }
  },
});

// Enable the async hook
asyncHook.enable();

// Execute a database query
connection.query("SELECT * FROM users", (err, results) => {
  // ...
});

// Disable the async hook
asyncHook.disable();

Potential Applications

  • Performance monitoring: Track the time spent on asynchronous operations to identify potential bottlenecks.

  • Error handling: Log or handle errors that occur during asynchronous operations.

  • Security logging: Monitor sensitive database queries or file access operations.

  • Debugging: Trace the execution of asynchronous operations to debug issues.

Additional Notes

  • The API for accessing resources is not specified for Node.js internal resources. However, you can use the async_hooks module to access the resource object directly.

  • It is important to use resources carefully and avoid storing references to them for extended periods of time, as they may be reused.

  • Async Hooks can be used to track other types of asynchronous resources, such as timers, file system operations, and HTTP requests.

Asynchronous Context Tracking

Asynchronous programming involves executing code without blocking the main thread. JavaScript uses an event loop to handle asynchronous tasks, which can make it challenging to track the context in which asynchronous operations are executed.

AsyncLocalStorage

AsyncLocalStorage is a module in Node.js that provides a way to track and store context-specific data across asynchronous operations. It allows developers to associate data with the current execution context and retrieve it later, even if the context has changed.

Async Hooks

Async hooks are a lower-level API that allows developers to track asynchronous events in Node.js. They provide more control and flexibility than AsyncLocalStorage but require more manual management of context data.

Example:

Let's consider an example where we want to track the execution context of a database query.

AsyncLocalStorage Implementation:

const asyncLocalStorage = require("async_hooks");

// Create an AsyncLocalStorage instance
const storage = asyncLocalStorage.createAsyncLocalStorage();

// Define a function to execute the database query
const executeQuery = async () => {
  // Store the context in AsyncLocalStorage
  storage.run(async () => {
    storage.getStore().query = "SELECT * FROM users";

    // Execute the query
    // ...

    // Retrieve the context later
    console.log(storage.getStore().query);
  });
};

Async Hooks Implementation:

const asyncHooks = require("async_hooks");

// Create an async hook to track database queries
const hook = asyncHooks.createHook({
  init: (asyncId, type, triggerAsyncId) => {
    if (type === "query") {
      // Store the context in a map
      contextMap.set(asyncId, { query: "SELECT * FROM users" });
    }
  },
});

// Enable the async hook
hook.enable();

// Execute the database query
// ...

// Retrieve the context later
const query = contextMap.get(asyncHooks.executionAsyncId()).query;
console.log(query);

Real-World Applications:

  • Logging: Track the execution context of log messages to provide more context for debugging.

  • Profiling: Measure the performance of specific asynchronous operations by tracking the time spent in each context.

  • Transaction Management: Ensure that data is rolled back or committed based on the execution context of a transaction.

Summary:

  • AsyncLocalStorage provides a convenient way to track context data but is limited in flexibility.

  • Async Hooks offer more control and flexibility but require manual management of context data.

  • Both methods are useful for tracking asynchronous context in Node.js, with different trade-offs in simplicity and flexibility.

before(asyncId)

Concept:

Imagine you have a car race and want to know when each car is about to start or finish. The before(asyncId) callback in Node.js's async-hooks module acts like a starting line announcer for asynchronous operations.

Details:

When an asynchronous operation like a file read or server connection starts or ends, Node.js assigns it a unique ID (asyncId). Just before the operation's callback function runs, the before(asyncId) callback is triggered, giving you a heads-up.

Example:

// Listen for new connections on port 8080
const http = require("http");

const server = http.createServer();

// Triggered before the 'connection' event callback runs
server.on("before", (asyncId) => {
  // Log info about the upcoming connection
  console.log(`New connection about to start`, asyncId);
});

// Main 'connection' event handler
server.on("connection", (socket) => {
  // Socket-related logic goes here
});

server.listen(8080);

Real-World Applications:

  • Performance monitoring: Track the duration of asynchronous operations for performance analysis.

  • Resource management: Monitor and control the resources consumed by asynchronous operations.

  • Error handling: Identify and debug errors within asynchronous callbacks.

  • Context propagation: Share context information (e.g., user ID) across asynchronous operations for better data consistency.

async-hooks Module: after(asyncId) Method

Definition:

The after method in the async-hooks module is called immediately after the callback specified in the before method is completed.

Explanation:

Async hooks allow you to track asynchronous operations in your Node.js application. The after method is one of the four types of async hooks. It allows you to perform some action once an asynchronous operation has finished executing.

How it Works:

When you use the before method to register a callback for an asynchronous operation, you can also specify a callback for the after method. This callback will be called once the asynchronous operation is complete, regardless of whether or not it succeeded.

For example:

const async_hooks = require("async_hooks");

async_hooks
  .createHook({
    after: (asyncId) => {
      // Do something after the asynchronous operation is complete
    },
  })
  .enable();

// Perform some asynchronous operation
setTimeout(() => {
  // The 'after' callback will be called here
}, 1000);

Usage:

  • Logging and Profiling: You can use after to log information about completed asynchronous operations or to measure their performance.

  • Error Handling: If an error occurs during the execution of an asynchronous operation, after will still be called. You can use this to handle errors and ensure that resources are cleaned up properly.

  • Resource Management: You can use after to release resources that were allocated during the asynchronous operation.

Real-World Examples:

  • Monitoring Application Performance:

async_hooks
  .createHook({
    after: (asyncId) => {
      const duration = async_hooks.executionAsyncId(asyncId);
      console.log(`Asynchronous operation took ${duration}ms to complete`);
    },
  })
  .enable();

  • Ensuring Resource Cleanup:

async_hooks
  .createHook({
    after: (asyncId) => {
      const resource = async_hooks.getAsyncResource(asyncId);
      if (resource) {
        resource.cleanup();
      }
    },
  })
  .enable();

What is destroy(asyncId)?

When you create an asynchronous function in JavaScript (e.g., a function that takes a callback or uses async/await), Node.js creates an "async context" for it. This context tracks information about the function and its state.

destroy(asyncId) is a function that is called when the async context for an asynchronous function is destroyed. This can happen when the function completes or when it is explicitly destroyed by the embedder API emitDestroy().

Why is destroy(asyncId) important?

Some resources (e.g., memory) used by the async context may need to be cleaned up when the context is destroyed. If this cleanup doesn't happen, it can lead to memory leaks.

Potential applications in real world:

  • Detecting and preventing memory leaks caused by unmanaged resources in asynchronous functions.

  • Debugging memory allocation and usage patterns in asynchronous code.

Real-world example:

const asyncHooks = require("async_hooks");
const asyncHook = asyncHooks.createHook({
  destroy(asyncId) {
    console.log(`Async context ${asyncId} destroyed.`);
  },
});
asyncHook.enable();

// Create an asynchronous function
const fn = async () => {
  // Do something asynchronous...
};

// Call the asynchronous function
fn();

setTimeout(() => {
  // Explicitly destroy the async context for the function
  asyncHooks.emitDestroy(asyncHook.id);
}, 1000);

This code logs Async context <asyncId> destroyed. after the asynchronous function fn is called and after the embedder API emitDestroy() is called to explicitly destroy the async context.

Simplified Explanation:

What is promiseResolve(asyncId)?

When you create a Promise in JavaScript and call its resolve function, the promiseResolve(asyncId) function is triggered. It's like a signal that says, "Hey, the Promise is now resolved!"

What does it mean when a Promise is resolved?

A Promise is a way to handle actions that may or may not finish immediately, like fetching data from a server. When you resolve a Promise, you're saying that the action is complete and you can now do something with the result.

Why is promiseResolve(asyncId) useful?

It helps you track when Promises are resolved, which can be helpful for debugging or performance optimization. For example, if you're waiting for a bunch of Promises to resolve before performing a certain task, you can use promiseResolve(asyncId) to monitor their progress.

How to use promiseResolve(asyncId):

You don't need to call promiseResolve(asyncId) directly. It's automatically called when a Promise is resolved. You can use it as follows:

new Promise((resolve) => resolve(true)).then((a) => {
  // This callback will be executed when the `Promise` is resolved
  // by the `resolve` function above. You can access the result
  // of the `Promise` using the `a` argument.
});

Real-World Example:

Imagine you're building a website that loads data from a server. You can use Promise.resolve to handle the data fetching. Then, you can use promiseResolve(asyncId) to track when the data is available and update the UI accordingly.

// Fetch data from the server
const promise = new Promise((resolve) => {
  setTimeout(() => resolve('Data from server'), 1000);
});

// Update the UI when the data is available
promise.then((data) => {
  // `data` contains the data fetched from the server
  updateUI(data);
});

Potential Applications:

  • Debugging: Identifying when Promises are resolved can help you diagnose issues in your code.

  • Performance Optimization: Tracking Promise resolutions can help you optimize the performance of asynchronous code.

  • Concurrency Control: Controlling when Promises are resolved can help you manage concurrent operations and avoid conflicts.

async_hooks.executionAsyncResource()

Imagine you have a lot of tasks running in your computer, like watching a movie, downloading a file, and opening a website. Each of these tasks is like a thread or a separate stream of execution.

async_hooks is a module in Node.js that helps you keep track of these different tasks. It works by attaching a special object called an "async resource" to each task. This async resource is like a tag that carries information about the task, such as its current status or any data that you want to associate with it.

executionAsyncResource() is a function that returns the async resource attached to the current task that is running. This is useful if you want to access information about the current task or if you want to store data that is specific to that task.

For example, let's say you want to track the time it takes to execute a particular function. You can use executionAsyncResource() to store a timestamp at the beginning of the function and then retrieve it at the end to calculate the execution time.

Here's an example:

const { executionAsyncResource } = require("async_hooks");

function longRunningFunction() {
  const start = Date.now();

  // Do some long-running task

  const end = Date.now();

  // Store the execution time in the async resource
  executionAsyncResource().executionTime = end - start;
}

longRunningFunction();

// Retrieve the execution time from the async resource
const executionTime = executionAsyncResource().executionTime;

This way, you can easily track the execution time of any function without having to modify the function itself.

Real-world applications:

  • Performance monitoring: You can use async_hooks to monitor the performance of your application by tracking the execution time of different tasks. This can help you identify bottlenecks and areas for improvement.

  • Error handling: You can use async_hooks to handle errors that occur during task execution. By attaching an error handler to each task, you can ensure that errors are handled gracefully and that your application doesn't crash.

  • Data sharing: You can use async_hooks to share data between different tasks. This is useful for tasks that need to access data from a common source, such as a database or a cache.

Improved code snippets:

  • Tracking execution time:

const { executionAsyncResource } = require("async_hooks");

function longRunningFunction() {
  const asyncResource = executionAsyncResource();
  asyncResource.executionTime = Date.now();

  // Do some long-running task

  const executionTime = Date.now() - asyncResource.executionTime;
}

  • Error handling:

const { executionAsyncResource } = require("async_hooks");

function longRunningFunction() {
  const asyncResource = executionAsyncResource();
  asyncResource.error = null;

  try {
    // Do some long-running task
  } catch (err) {
    asyncResource.error = err;
  }
}

  • Data sharing:

const { executionAsyncResource } = require("async_hooks");

const sharedData = {
  // ...
};

function task1() {
  const asyncResource = executionAsyncResource();
  asyncResource.sharedData = sharedData;

  // Access shared data
}

function task2() {
  const asyncResource = executionAsyncResource();
  asyncResource.sharedData = sharedData;

  // Access shared data
}

async_hooks.executionAsyncId()

This function returns the unique identifier of the current asynchronous execution context. This identifier is useful for tracking the flow of asynchronous operations and identifying the context in which they were executed.

Simplified Explanation:

Imagine you have a bunch of tasks that need to be executed asynchronously, like making a network request or reading a file. Each task is executed in its own "execution context". executionAsyncId() allows you to get a unique number that identifies the execution context of the current task.

Code Snippet:

// Get the execution context ID of the current task
const executionId = async_hooks.executionAsyncId();

Real-World Application:

One common use case for executionAsyncId() is debugging asynchronous code. For example, you can use it to identify the context in which an error occurred or to track the performance of a specific asynchronous operation.

Complete Code Implementation:

// Create an async function that prints its execution context ID
const printExecutionId = async () => {
  console.log(`Execution ID: ${async_hooks.executionAsyncId()}`);
};

// Execute the async function
printExecutionId();

Output:

Execution ID: 1

In this example, the executionAsyncId() function returns the execution context ID of the printExecutionId function, which is 1.

What is async_hooks?

async_hooks is a module in Node.js that allows you to track asynchronous events in your code. This can be useful for debugging, performance monitoring, and other tasks.

triggerAsyncId()

The triggerAsyncId() function returns the ID of the resource that triggered the current asynchronous event. This can be useful for tracking the source of an event and understanding how different parts of your code are interacting.

Example

The following code shows how to use triggerAsyncId() to track the source of a setTimeout() event:

const async_hooks = require("async_hooks");

// Create an async hook to track setTimeout events
const hook = async_hooks.createHook({
  init(asyncId, type, triggerAsyncId) {
    // The 'init' event is called when a new async resource is created
    if (type === "Timeout") {
      console.log(`Timeout created with ID ${asyncId}`);
      console.log(`Triggered by resource with ID ${triggerAsyncId}`);
    }
  },
});

// Enable the hook
hook.enable();

// Create a setTimeout event
setTimeout(() => {
  // The callback function for the setTimeout event
  console.log("Timeout callback executed");
}, 100);

Output:

Timeout created with ID 1
Triggered by resource with ID 0
Timeout callback executed

As you can see, the triggerAsyncId() function returned the ID of the resource that created the setTimeout event (in this case, the global object, which has an ID of 0).

Real-world applications

async_hooks can be used for a variety of real-world applications, including:

  • Debugging: async_hooks can be used to debug asynchronous code and identify potential performance bottlenecks.

  • Performance monitoring: async_hooks can be used to track the performance of asynchronous operations and identify areas for improvement.

  • Security: async_hooks can be used to track the source of asynchronous events and identify potential security vulnerabilities.

Conclusion

async_hooks is a powerful tool that can be used to track asynchronous events in Node.js. The triggerAsyncId() function can be used to identify the source of an asynchronous event, which can be useful for debugging, performance monitoring, and other tasks.

Simplified Explanation:

In Node.js, there are different types of events (async_hooks.providers) that can occur when asynchronous operations (like timers, network requests) are used. To make it easier to track and handle these events, Node.js provides a way to get a unique numeric ID for each type of event.

asyncWrapProviders:

The async_hooks.asyncWrapProviders property is a map that links a provider type (like "TCPWrap" for network connections) to a numeric ID.

Real-World Code Example:

const async_hooks = require("async_hooks");

// Get the event type ID for TCP network connections
const tcpId = async_hooks.asyncWrapProviders.TCPWrap;

// Create an async hook to listen for events with the TCP ID
async_hooks.createHook({ init }).enable();

function init(asyncId, type, triggerId) {
  // Check if the event type is for a TCP connection
  if (type === tcpId) {
    console.log(`New TCP connection: ${asyncId}`);
  }
}

Applications in the Real World:

Performance Optimization: Identifying and optimizing slow asynchronous operations can be done by tracking the time taken for each event type.

Error Handling: By handling specific event types, customized error messages and recovery mechanisms can be implemented.

Security Analysis: Tracking asynchronous operations helps identify potential security vulnerabilities by monitoring suspicious activity or unauthorized access.

Debugging and Profiling: Asynchronous event types can be tracked to trace the flow of operations and identify bottlenecks.

Async Hooks for Promise Execution Tracking

Problem: By default, JavaScript promises don't track their execution or the resources that trigger them.

Solution: Async hooks allow you to track this information.

Enabling Promise Execution Tracking

import { createHook } from "node:async_hooks";

const hook = createHook({ init() {} });

// Enable tracking
hook.enable();

How It Works

  1. The init function is called whenever a new async resource (e.g., a promise) is created.

  2. Async hooks now track the execution and trigger information for promises.

Getting Execution and Trigger IDs

Promise.resolve(1729).then(() => {
  const executionId = executionAsyncId();
  const triggerId = triggerAsyncId();

  // executionId: ID of the current promise execution
  // triggerId: ID of the resource that triggered the promise callback

  console.log(`Execution ID: ${executionId}, Trigger ID: ${triggerId}`);
});

In the above example, executionId and triggerId provide insights into the execution context of the promise callback.

Subtleties

Only Chained Promises

Async hooks track only promises created by then() or catch(). Promises created by Promise.resolve() directly are not tracked.

Execution and Trigger IDs

Execution and trigger IDs are unique integers assigned to each async resource. They help identify the execution context and the resource that caused the execution.

Real-World Applications

  • Performance monitoring: Track the execution time of promises to identify performance bottlenecks.

  • Error handling: Log error details along with the execution and trigger IDs, providing more context for debugging.

  • Resource management: Identify which resources triggered promise callbacks, helping optimize resource allocation.

  • Security auditing: Track promise executions to detect suspicious or malicious activity.

Complete Code Example

import { createHook } from "node:async_hooks";

const hook = createHook({
  init(id, type, triggerId, resource) {
    /* ... */
  },
  before(id) {
    /* ... */
  },
  after(id) {
    /* ... */
  },
  destroy(id) {
    /* ... */
  },
});

hook.enable();

Promise.resolve(1729).then(() => {
  // ... Do something ...
});

This example shows a complete hook implementation. The init function is called when the promise is created, before and after when the promise callback is executed, and destroy when the promise is completed.

JavaScript Embedder API

Simplified Explanation:

Imagine you have a bunch of tasks running in your JavaScript code, and some of these tasks take a long time, like fetching data from the internet. When these tasks finish, you want to perform certain actions, like updating the screen or sending a message to the user.

The AsyncResource API in JavaScript helps you keep track of these tasks and make sure the right actions are taken when they're done. It's like having a little assistant that watches over your tasks and tells you when they're finished.

How it Works

Simplified:

  • You create an AsyncResource for each task that takes a long time.

  • The AsyncResource knows when the task starts and ends.

  • When the task ends, the AsyncResource triggers a "callback" function.

  • The callback function can do whatever you need it to do, like updating the screen or sending a message.

Real-World Example

Simplified:

Imagine you have a website that displays a list of products from a database. When a user clicks on a product, you want to show more details about it.

Code:

// Create an AsyncResource for the database query
const asyncResource = new AsyncResource("database-query");

// Start the database query
asyncResource.run(() => {
  // Get the product details from the database
  const productDetails = getFromDB(productId);

  // Show the product details on the screen
  showDetails(productDetails);
});

// Define the callback function to be triggered when the query finishes
asyncResource.on("before", (asyncId, type) => {
  // Show a loading spinner to the user
  showLoadingSpinner();
});

asyncResource.on("after", (asyncId, type) => {
  // Hide the loading spinner
  hideLoadingSpinner();
});

In this example, the AsyncResource ensures that the loading spinner is shown before the database query starts and hidden when it finishes, giving the user a better experience.

Potential Applications

  • Performance Monitoring: Identifying and optimizing tasks that take a long time.

  • Error Handling: Tracking and recovering from errors in asynchronous tasks.

  • Debugging and Profiling: Understanding and improving the behavior of asynchronous code.

  • Concurrency: Coordinating multiple asynchronous tasks to ensure they run smoothly.

  • Asynchronous Queuing: Managing a queue of tasks and executing them in a specific order.

What is AsyncResource?

AsyncResource is a class used to manage asynchronous resources. Asynchronous resources are operations that don't block the main thread, such as reading a file, sending an HTTP request, or waiting for a timer event.

Why use AsyncResource?

AsyncResource helps you track the execution state of asynchronous resources and perform certain operations when they complete, such as:

  • Create metrics: Count the number of active async resources, measure their execution time, etc.

  • Debug: Identify slow or problematic async operations.

  • Error handling: Catch and handle errors that occur during asynchronous operations.

  • Resource cleanup: Release resources allocated by async operations when they complete.

Simplified Example:

Suppose you have an asynchronous function that reads a file. You can use AsyncResource to track the file read operation like this:

const asyncResource = new AsyncResource("file-read");

asyncResource.runInAsyncScope(async () => {
  // This function runs in the async context
  const data = await readFile("file.txt");
});

asyncResource.on("complete", (resource) => {
  // This function runs when the file read operation completes
  console.log(`File read completed with data: ${data}`);
});

Real-World Applications:

  • Monitoring resource utilization: Track the number of active database connections or HTTP requests.

  • Debugging slow operations: Identify which async operations are taking too long and investigate the cause.

  • Error handling: Detect and handle errors that occur during database queries or network operations.

  • Resource cleanup: Ensure that file handles or database connections are closed properly when async operations complete.

Improved Version:

The following improved version of the example includes error handling:

asyncResource.runInAsyncScope(async () => {
  try {
    // This function runs in the async context
    const data = await readFile("file.txt");
    console.log(`File read completed with data: ${data}`);
  } catch (error) {
    console.error(`Error reading file: ${error}`);
  }
});

Class: AsyncLocalStorage

The AsyncLocalStorage class in Node.js enables you to store and retrieve values associated with the current execution context. This is useful for scenarios where you need to access data across asynchronous operations, such as in a middleware or a database transaction.

How to use:

To store a value in the AsyncLocalStorage, use the set() method:

asyncLocalStorage.set("user", "John Doe");

To retrieve a value, use the get() method:

const user = asyncLocalStorage.get("user");

The value stored in the AsyncLocalStorage is accessible throughout the current execution context, including any asynchronous operations that are spawned within that context. For example:

asyncLocalStorage.set("user", "John Doe");

setTimeout(() => {
  const user = asyncLocalStorage.get("user");
  console.log(user); // prints 'John Doe'
}, 1000);

Hook Callbacks:

AsyncLocalStorage can be used with Async Hooks to intercept asynchronous operations and access the stored values. For example, you can use the before() and after() callbacks to track the duration of an asynchronous operation:

const asyncHook = require("async_hooks");

asyncHook
  .createHook({
    init(asyncId, type, triggerAsyncId, resource) {
      if (type === "PROMISE") {
        asyncLocalStorage.set("startTime", Date.now());
      }
    },

    destroy(asyncId) {
      if (asyncLocalStorage.has("startTime")) {
        const startTime = asyncLocalStorage.get("startTime");
        const endTime = Date.now();
        console.log(
          `Promise with asyncId ${asyncId} took ${
            endTime - startTime
          }ms to execute.`
        );
      }
    },
  })
  .enable();

Real-World Applications:

  • Logging and tracing: You can use AsyncLocalStorage to store context-specific information (such as user ID or request ID) that can be used for logging or tracing purposes.

  • Request context: You can use AsyncLocalStorage to store request-specific data across multiple asynchronous middleware functions and controllers.

  • Database transactions: You can use AsyncLocalStorage to track the database connection associated with a particular request and ensure that all related operations are handled within the same transaction.

Improved Example:

Suppose you have an express middleware that sets the current user from a JWT token and you want to access the user information in subsequent asynchronous callbacks. Here's how you can use AsyncLocalStorage:

const asyncLocalStorage = require("async_hooks").createHook();
const express = require("express");

const app = express();

app.use(async (req, res, next) => {
  const token = req.headers["authorization"];
  const user = await validateToken(token);
  asyncLocalStorage.run(user, () => {
    next();
  });
});

app.get("/api/users", async (req, res) => {
  const user = asyncLocalStorage.getStore();
  // Handle the request using the user information
});

app.listen(3000);

In this example, the validateToken() function is responsible for validating the JWT token and retrieving the associated user information. The asyncLocalStorage.run() method ensures that the user information is accessible throughout the execution of the request handler and any asynchronous callbacks it invokes.

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