async context

Async CTX manager

Asynchronous Context Tracking

Imagine you're building a complex system where multiple tasks, called "async operations," are running at the same time. Each async operation needs to know about the state of the system at the time it started.

Async context tracking is a tool that allows you to store and access this state information. It's like having a "snapshot" of the system when an async operation begins, so that the operation can use the correct information.

How it Works

Async context tracking works by creating a "context object" for each async operation. This object contains the state information that the operation needs.

For example, let's say you have an online store that processes orders asynchronously. When a customer orders a product, the order is processed in the background.

The async operation that processes the order needs to know information about the customer, such as their name and address. This information is stored in the context object.

Code Example

Here's a simplified code example that shows how to use async context tracking:

const async_hooks = require("async_hooks");

// Create a context object for the current thread.
const context = async_hooks.createHook();

// Register a listener to the "init" event, which is triggered when an async operation starts.
context.init((asyncId, type, triggerAsyncId, resource) => {
  // Store the customer information in the context object.
  async_hooks.setCurrentHookContext({
    customerName: "John Doe",
    customerAddress: "123 Main Street",
  });
});

// Trigger an async operation, such as a database query or HTTP request.
const asyncOperation = () => {
  // Get the customer information from the context object.
  const customer = async_hooks.currentHookContext();
  console.log(customer.customerName); // Output: "John Doe"
};

// Start the async operation.
asyncOperation();

Potential Applications

Async context tracking has many potential applications in the real world, including:

Logging and tracing: Track the state of the system at the time an error occurs to help diagnose the problem.
Security: Ensure that only authorized users have access to sensitive information.
Performance monitoring: Track the time spent in async operations to identify performance bottlenecks.
Distributed systems: Coordinate communication between different parts of a distributed system.

Simplified Explanation of AsyncLocalStorage and AsyncResource from nodejs's async-context module:

Imagine you have a box where you can store things, like toys or secrets. In the world of JavaScript, this box is called AsyncLocalStorage. It's a special kind of box that can store data for each individual function that's running.

Now, when you run a function, it's like opening a new toy box. You can put things in that box that you only want that function to see. And when the function is done running, you can close the box and everything inside is gone.

But sometimes, you want to share things between different functions. That's where AsyncResource comes in. It's like a big box that all the toy boxes are stored in. When you put something in the big box, it's like you're sharing it with all the other toy boxes.

Real-World Examples:

Example 1: Storing User Data in AsyncLocalStorage

Imagine you're building a website that shows different pages to users depending on who they are. You can store the user's ID in AsyncLocalStorage, so that each function that runs on that page can access the user's ID without having to pass it around as an argument.

const asyncLocalStorage = new AsyncLocalStorage();

function getUserID() {
  // Get the user ID from the async local storage
  const userID = asyncLocalStorage.getStore();
  return userID;
}

Example 2: Sharing a Database Connection with AsyncResource

Imagine you're building an API that uses a database. You can create an AsyncResource for the database connection, and then each function that uses the database can access the connection without having to pass it around as an argument.

const asyncResource = new AsyncResource("Database Connection");

asyncResource.runInAsyncScope(() => {
  // Create a new database connection
  const connection = createDatabaseConnection();

  // Store the connection in the async resource
  asyncResource.emitBefore("createConnection", undefined, connection);

  // Do something with the database connection

  // Emit an after event to indicate that the connection is closed
  asyncResource.emitAfter("closeConnection", undefined, connection);
});

Potential Applications:

Storing user-specific data in web applications.
Sharing resources between functions, such as database connections.
Tracking execution time of asynchronous operations.
Debugging asynchronous code.

What is `AsyncLocalStorage`?

Imagine you have a box full of different-colored balls. Each ball represents a different asynchronous operation, like a database query or a network request. As these operations run, they can change the state of your program, like adding new balls or changing the colors of existing balls.

AsyncLocalStorage is like a special box that keeps track of these changes and makes sure that each ball always knows the state of the operation it represents. This way, when you look inside the box, you can always tell which balls belong to which operations and what state they're in.

How to use `AsyncLocalStorage`

To use AsyncLocalStorage, you first create an instance of the class:

const storage = new AsyncLocalStorage();

Then, you can use the run() method to execute a function within a specific context. The context is like a snapshot of the current state of the program, including any changes made by previous asynchronous operations.

// Increment the counter in the current context
storage.run(() => {
  const counter = storage.getStore();
  storage.setStore(counter + 1);
});

The getStore() method retrieves the current value of the context, while the setStore() method sets a new value.

Real-World Applications

AsyncLocalStorage is useful in any situation where you need to keep track of state across asynchronous operations. Here are a few examples:

Logging: You can use AsyncLocalStorage to assign unique identifiers to HTTP requests and track them throughout their execution. This makes it easier to debug issues and identify which requests are causing problems.
Caching: You can use AsyncLocalStorage to cache data for specific users or sessions. This can improve performance by avoiding unnecessary database queries.
Security: You can use AsyncLocalStorage to store sensitive data, such as user credentials, in a secure context. This helps to protect the data from unauthorized access.

Code Example

Here's a complete code example that shows how to use AsyncLocalStorage to log HTTP requests:

const http = require("http");
const { AsyncLocalStorage } = require("async_hooks");

// Create an instance of AsyncLocalStorage
const storage = new AsyncLocalStorage();

// Create an HTTP server
const server = http.createServer((req, res) => {
  // Assign a unique ID to the request
  storage.run(() => {
    const id = Math.random();
    storage.setStore(id);

    // Log the request start
    console.log(`Request started: ${id}`);

    // Simulate an asynchronous operation
    setTimeout(() => {
      // Log the request end
      console.log(`Request ended: ${id}`);
    }, 1000);
  });

  // Send the response
  res.end();
});

// Start the server
server.listen(3000);

When you run this code, you'll see the following output in your console:

Request started: 0.123456
Request ended: 0.123456
Request started: 0.678910
Request ended: 0.678910

Each request has its own unique ID, and the start and end times are logged in the correct order. This demonstrates how AsyncLocalStorage can be used to track state across asynchronous operations.

What is AsyncLocalStorage?

AsyncLocalStorage is like a storage box that can hold variables and data while you run async code (code that takes a while to finish). It's special because it makes sure that different parts of your code that are running in parallel can all access the same data.

How does AsyncLocalStorage work?

You can imagine AsyncLocalStorage as a secret locker. You can only put things in and take things out when you have the key to open the locker. The key is the "context".

When you call asyncLocalStorage.run(), you create a new secret locker and lock it with a key. All the code that runs inside the run() block will have access to the secret locker using the same key.

You can also create a new key using asyncLocalStorage.enterWith(). This is like making a copy of the key. All the code that runs after the enterWith() call will have access to the secret locker using the copy of the key.

Real-world example:

Let's say you have a function that does some async work, like fetching data from a database. You want to be able to access the database credentials from any part of the code that runs inside the async function.

You can use AsyncLocalStorage to store the database credentials in a secret locker. When the async function starts running, you can create a new key and lock the locker with that key. Then, all the code that runs inside the async function will have access to the database credentials through the secret locker.

Potential applications:

AsyncLocalStorage can be used in many different scenarios, such as:

Storing user-specific data in web applications
Tracking the progress of async operations
Sharing data between different parts of a distributed system

Simplified version of the code snippet:

const asyncLocalStorage = require('async_hooks').AsyncLocalStorage();

asyncLocalStorage.run(async () => {
  // Store data in the secret locker
  asyncLocalStorage.getStore().data = 'some data';

  // Do some async work
  const result = await someAsyncFunction();

  // Access the data from the secret locker
  console.log(asyncLocalStorage.getStore().data);  // 'some data'
});

Simplified Explanation:

Static Method: AsyncLocalStorage.bind(fn)

Imagine a situation where you have multiple threads running in your program, each thread having its own set of data (like user sessions, database connections, etc.). To keep track of this data and ensure that each thread uses the correct set, you use AsyncLocalStorage.

AsyncLocalStorage.bind(fn) allows you to bind a function to the current execution context. The execution context refers to the current thread and its set of data. By binding the function, you ensure that whenever it's called, it uses the data from the current context.

Code Snippet:

const asyncLocalStorage = require("async_hooks").AsyncLocalStorage();

// Bind a function to the current execution context
asyncLocalStorage.bind(() => {
  // This function will always have access to the data from the current context
});

Real-World Implementation:

Suppose you have a web application that uses multiple threads to handle user requests. Each user has a unique session, represented by data stored in the async local storage. When a request is received, you want to ensure that the correct session data is used to process the request.

By binding the request handling function to the current execution context, you can guarantee that it has access to the session data for the current user. This ensures that the request is processed using the correct user-specific data.

Potential Applications:

User Authentication and Authorization: Ensure that the correct user session is used for each request.
Database Transactions: Maintain per-thread database connections to avoid concurrency issues.
Logging and Tracing: Associate logs and traces with the current execution context for easier debugging and analysis.

Static Method: AsyncLocalStorage.snapshot()

Simplified Explanation:

Imagine you have a secret box that holds information about the current situation of your program. AsyncLocalStorage.snapshot() lets you take a snapshot of this box, so you can access its contents later, even if the program's situation has changed.

Detailed Explanation:

AsyncLocalStorage stores information about the current execution context, like the user ID or request ID. AsyncLocalStorage.snapshot() creates a new function that remembers the current execution context. When you call this function later, it retrieves the stored context and uses it within the call.

Code Snippet:

// Create an instance of AsyncLocalStorage
const storage = new AsyncLocalStorage();

// Store a value in the storage
storage.run(123, () => {
  // Take a snapshot of the current execution context
  const snapshot = AsyncLocalStorage.snapshot();

  // Create a function that uses the snapshot
  const getStoredValue = snapshot((fn) => {
    // Retrieve the stored value using the function passed as an argument
    return fn(() => storage.getStore());
  });

  // Call the function later, using the stored execution context
  const result = storage.run(321, () => getStoredValue());

  // Expected output: 123
  console.log(result);
});

Real-World Applications:

Tracking user sessions: Store the user's ID in AsyncLocalStorage and use it in all requests to identify the user.
Logging: Store the request ID in AsyncLocalStorage and include it in all log messages to correlate them with the request.
Error handling: Store the error context in AsyncLocalStorage and use it to provide more information when an error occurs.

Potential Improvements to Example:

// Create an instance of AsyncLocalStorage
const storage = new AsyncLocalStorage();

// Store a value in the storage
storage.run(123, () => {
  // Take a snapshot of the current execution context
  const snapshot = AsyncLocalStorage.snapshot();

  // Create a function to retrieve the stored value
  function getValue() {
    // Use the snapshot to retrieve the stored value
    return snapshot(() => storage.getStore());
  }

  // Call the function later, using the stored execution context
  storage.run(321, () => console.log(getValue()));
});

In this improved example, the function getValue is defined and used directly within the storage.run callback, which simplifies the code and makes it easier to read.

`asyncLocalStorage.disable()`

Simplified Explanation:

Imagine asyncLocalStorage as a special box that stores information for different tasks running in your code. When you disable it with asyncLocalStorage.disable(), it's like closing the lid of the box and saying, "No more storing information, please!"

Detailed Explanation:

asyncLocalStorage is a built-in Node.js module that allows you to store and access information (called "context") across different tasks running asynchronously (at the same time). By disabling it with asyncLocalStorage.disable(), you're preventing any new information from being stored.

Code Example:

// Disable the asyncLocalStorage instance
asyncLocalStorage.disable();

// Now, `asyncLocalStorage.getStore()` will always return `undefined` until you call `asyncLocalStorage.run()` or `asyncLocalStorage.enterWith()` again.
asyncLocalStorage.getStore(); // Returns `undefined`

Real-World Application:

Suppose you have a web server handling multiple requests simultaneously. You want to log the user ID for each request so that you can track their activities. You can use asyncLocalStorage to store the user ID for each request in a "context". However, once all requests are processed, you can disable asyncLocalStorage to prevent any new information from being stored. This ensures that the memory used by asyncLocalStorage is released back to the system.

Potential Applications:

Tracking user sessions and activities in web servers
Managing database connections for multiple parallel tasks
Maintaining application-specific configuration across asynchronous operations

`asyncLocalStorage.getStore()`

Explanation: In JavaScript, asyncLocalStorage is a built-in Node.js module that allows you to share data across asynchronous functions. Asynchronous functions are functions that don't finish their execution immediately and instead return a promise to complete later.

The asyncLocalStorage.getStore() method returns the current store, which is an object that holds the shared data. Think of it as a box where you can store and access data from within asynchronous functions.

Syntax:

asyncLocalStorage.getStore();

Return Value:

The return value is the current store, which is an object. If you call asyncLocalStorage.getStore() outside of an asynchronous context, it will return undefined.

Usage:

To use asyncLocalStorage.getStore(), you first need to call asyncLocalStorage.run() or asyncLocalStorage.enterWith(). These methods initialize an asynchronous context and attach the store to it. Once you have an active asynchronous context, you can access the store by calling asyncLocalStorage.getStore().

Here's an example of how to use it:

asyncLocalStorage.run(async () => {
  // Get the store within the asynchronous context
  const store = asyncLocalStorage.getStore();

  // Set a value in the store
  store.set("myData", "Hello world!");

  // Later, retrieve the value from the store
  const myValue = store.get("myData");

  // Note: The store is only accessible within the asynchronous context
});

Real-World Applications:

asyncLocalStorage and getStore() are useful in various real-world applications, including:

Request tracking: Sharing information about an HTTP request across asynchronous handlers.
Transaction management: Managing data associated with a database transaction.
Context propagation: Passing context information between nested asynchronous functions.

By using asyncLocalStorage and getStore(), you can avoid the pitfalls of global variables and improve the readability and maintainability of your asynchronous code.

async-context module

async-local-storage.enterWith(store)

async-local-storage stores the passed store object and the store object can be accessed in the subsequent async calls.
asyncLocalStorage.enterWith(store) method sets the store object in the current synchronous execution context and it persists through the subsequent asynchronous calls.

Example:

//  create a store object
const store = { id: 1 };

//  set the store object using asyncLocalStorage.enterWith(store)
asyncLocalStorage.enterWith(store);

//  get the store object using asyncLocalStorage.getStore()
asyncLocalStorage.getStore(); // Returns the store object

//  perform some asynchronous operation
someAsyncOperation(() => {
  //  get the store object in the async operation
  asyncLocalStorage.getStore(); // Returns the same object
});

Potential applications in real world:

Storing user-specific data in a web application.
Tracking the state of a long-running process.
Maintaining context information across multiple asynchronous calls.

async-context

The async-context module in Node.js provides a way to store and retrieve data in an asynchronous context. This is useful for storing data that needs to be accessed across multiple asynchronous operations, such as a user ID or a request ID.

`asyncLocalStorage.run(store, callback[, ...args])`

The run() method runs a function synchronously within a context and returns its return value. The store is not accessible outside of the callback function. The store is accessible to any asynchronous operations created within the callback.

The optional args are passed to the callback function.

If the callback function throws an error, the error is thrown by run() too. The stacktrace is not impacted by this call and the context is exited.

Example:

const store = { id: 2 };
try {
  asyncLocalStorage.run(store, () => {
    asyncLocalStorage.getStore(); // Returns the store object
    setTimeout(() => {
      asyncLocalStorage.getStore(); // Returns the store object
    }, 200);
    throw new Error();
  });
} catch (e) {
  asyncLocalStorage.getStore(); // Returns undefined
  // The error will be caught here
}

Real-world example:

The async-context module can be used to store data that needs to be accessed across multiple asynchronous operations, such as a user ID or a request ID. This can be useful for tracking users across multiple requests or for debugging purposes.

Potential applications:

Tracking users across multiple requests
Debugging asynchronous code
Storing data that needs to be accessed across multiple asynchronous operations

asyncLocalStorage.exit(callback[, ...args])

This method runs a function outside the current async context, meaning that the function won't have access to the async storage.

How does it work?

Imagine you have a stack of boxes, and each box represents a different async context. When you call asyncLocalStorage.exit, you're taking the top box off the stack and placing it aside. Inside the callback function, you won't be able to reach into that box to access its contents.

Parameters:

callback: A function that you want to run outside the async context.
...args: Any additional arguments that you want to pass to the callback function.

Return value:

The return value of the callback function. If the callback function throws an error, the error is thrown by exit() too.

Example:

asyncLocalStorage.run(async () => {
  // Inside async context
  asyncLocalStorage.getStore(); // Returns the store object or value

  await asyncLocalStorage.exit(() => {
    // Outside async context
    asyncLocalStorage.getStore(); // Returns undefined

    // Do something that doesn't require access to the async storage
  });

  // Back inside async context
  asyncLocalStorage.getStore(); // Returns the same object or value
});

Real-world applications:

Running code that doesn't require access to the async storage. For example, if you want to log an error, you can do it outside the async context to avoid polluting the async storage with unnecessary data.
Testing code that uses async storage. You can use exit to mock the async storage and test your code in isolation.

Usage with async/await

Simplified Explanation:

When using async/await in your code, you can use the asyncLocalStorage.run() method to create a context for your asynchronous operations. This context will allow you to store and access data within that context, even when the function or method is paused and resumed later.

Detailed Explanation:

The asyncLocalStorage.run() method takes two arguments: a map (used to store data) and a callback function. The callback function is where you'll perform your asynchronous operations and store or access data.

Code Snippet:

async function myAsyncFunction() {
  // Create a map to store data
  const myMap = new Map();

  // Run the context
  await asyncLocalStorage.run(myMap, async () => {
    // Store a key-value pair in the context
    myMap.set("key", "value");

    // Perform asynchronous operation (e.g., a database query)
    await fetch("https://example.com/api");

    // The stored value can be accessed later within the context
    console.log(myMap.get("key"));
  });
}

myAsyncFunction();

Real-World Application:

This pattern can be used in various real-world scenarios, such as:

Database transactions: Maintaining a consistent database state across multiple asynchronous operations.
User authentication: Tracking user information across different requests.
Logging and tracing: Storing context-specific information for debugging and monitoring purposes.

Potential Applications:

Maintaining session data in web applications: Store user-specific information (e.g., preferences, shopping cart) and access it across different pages and requests.
Managing database connections in microservices: Ensure that database transactions are isolated and consistent within a specific context.
Tracking user activity in analytics: Record user actions and interactions within a specific context (e.g., a page visit or a session).

AsyncLocalStorage: A Tool to Keep Track of Data in Asynchronous Code

Imagine you're playing a game with multiple players, and each player has their own set of items. To keep track of who has what, you could use a blackboard where each player's name is written next to their items.

Now, imagine your game is asynchronous, meaning the actions happening in the game take place over time. As the game progresses, players may pick up or drop items while taking their turns. To keep track of who has what, you would need a way to update the blackboard every time a player's items change.

This is where AsyncLocalStorage comes in. It's like a blackboard for asynchronous code, allowing you to store data for a specific piece of code and access it later from another part of the code.

Context Loss: When the Blackboard Gets Lost

Usually, AsyncLocalStorage works smoothly. However, in some cases, the data on the blackboard (the "execution context") may be lost. This can happen if you're not careful when using callback-based code (like old-style JavaScript functions) or custom thenable implementations (a type of promise).

Callback-Based Code: Promisify It

Promises are a newer way of writing asynchronous code that makes it easier to keep track of things. To convert a callback-based function into a promise, you can use the [util.promisify()][] function. This makes your code work with native JavaScript promises, which are more reliable and won't lose context.

// Old-style callback-based function
fs.readFile("./myfile.txt", (err, data) => {
  if (err) {
    // Handle error
  }

  // Use the data in the current context
});

// Using promisify to convert to a promise
const readFilePromise = util.promisify(fs.readFile);

// Now, you can use this like a promise
readFilePromise("./myfile.txt").then((data) => {
  // Use the data in the current context
});

Custom Thenable Implementations: Use AsyncResource

If you need to use callback-based code or have a custom thenable implementation, you can use the [AsyncResource][] class. This class associates the asynchronous operation with the correct execution context.

// Create an AsyncResource object
const asyncResource = new AsyncResource({
  // The name of the asynchronous operation (optional)
  name: "My Asynchronous Operation",
});

// Run the asynchronous operation, passing asyncResource as an argument
asyncResource.run(() => {
  fs.readFile("./myfile.txt", (err, data) => {
    if (err) {
      // Handle error
    }

    // Use the data in the current context
  });
});

Real-World Applications

AsyncLocalStorage and AsyncResource can be useful in various real-world applications, such as:

Logging and Debugging: Keep track of the execution context to identify where errors occur and for debugging purposes.
Authentication and Authorization: Store user-specific information (such as session ID) in each execution context to enable authentication and authorization checks.
Transaction Management: Associate data with a specific transaction to ensure that all operations related to the transaction are executed in the same context.
Resource Management: Keep track of resources allocated in a particular execution context to ensure proper cleanup and release.

Conclusion

AsyncLocalStorage and AsyncResource are powerful tools for managing data in asynchronous code. By understanding how to use them correctly, you can prevent context loss and ensure your code runs smoothly and reliably.

AsyncResource Class

What is it?

The AsyncResource class allows you to track and control the lifecycles of your own asynchronous resources, like custom timers or custom event emitters.

Key Features:

Lifetime tracking: The class helps you track the creation and destruction of your resources.
Context establishment: It allows you to execute code within the context of a specific resource, making it easy to track resources related to that code.
Event triggering: You can use the class to trigger events when a resource is created or destroyed.

How to use it:

1. Create a new AsyncResource:

const asyncResource = new AsyncResource({
  type: "MyCustomResource",
  triggerAsyncId: executionAsyncId(),
});

type is a string that identifies the type of your resource, e.g., MyCustomResource.
triggerAsyncId is the ID of the AsyncResource that triggered the creation of this resource. If omitted, the current execution context's asyncId will be used.

2. Run code in the context of the resource:

asyncResource.runInAsyncScope(() => {
  // Code executed within the context of the resource
});

This will establish the resource's context, trigger the before callbacks in the AsyncHooks hooks, execute the provided code, trigger the after callbacks, and restore the original context.

3. Destroy the resource:

asyncResource.emitDestroy();

This will call the destroy callbacks in the AsyncHooks hooks.

4. Other methods:

asyncId(): Returns the unique ID assigned to the AsyncResource instance.
triggerAsyncId(): Returns the ID of the AsyncResource that triggered the creation of this resource.

Potential Applications:

Tracking custom resources for debugging and performance analysis.
Managing resources that require manual cleanup.
Correlating events related to specific resources.

AsyncResource Object

An AsyncResource is an abstract class that represents an asynchronous event. Asynchronous events are things like I/O operations, timers, and promises. Each AsyncResource has a type, which is a string that identifies the type of event. For example, the type of an I/O operation might be "fs.read".

Creating an AsyncResource: To create an AsyncResource, you use the new keyword. The first argument to the constructor is the type of the event. The second argument is an optional object that contains options for the AsyncResource. One of the options is triggerAsyncId, which is the ID of the execution context that created the event. If you don't specify triggerAsyncId, it will be set to the current execution context's ID.

Using an AsyncResource: Once you have created an AsyncResource, you can use it to track the progress of the event. You can add listeners to the AsyncResource to be notified when the event completes or fails. You can also use the AsyncResource to retrieve information about the event, such as the triggerAsyncId.

Example: Here is an example of how to create and use an AsyncResource to track the progress of an I/O operation:

const fs = require("fs");

// Create an AsyncResource for the file read
const asyncResource = new AsyncResource("fs.read");

// Add a listener to the AsyncResource to be notified when the read completes
asyncResource.addListener("complete", (err, data) => {
  if (err) {
    // Handle the error
  } else {
    // Do something with the data
  }
});

// Read the file
fs.readFile("file.txt", (err, data) => {
  if (err) {
    // Handle the error
  } else {
    // Do something with the data
  }

  // The read operation is complete, so emit the 'complete' event
  asyncResource.emitComplete();
});

Applications: AsyncResources can be used for a variety of purposes, such as:

Tracking the performance of asynchronous events
Debugging asynchronous code
Correlating asynchronous events with other events
Canceling asynchronous events

Static method: `AsyncResource.bind(fn[, type[, thisArg]])`

The bind method takes a function and binds it to the current execution context, which means that when the function is called, it will have access to the same execution context as the code that called bind.

The type parameter is optional and can be used to specify a name for the AsyncResource that will be created when the function is called. The thisArg parameter is also optional and can be used to specify the value of the this keyword when the function is called.

For example, the following code creates a function that will print the value of the this keyword:

const fn = AsyncResource.bind(function() {
  console.log(this);
});

When the fn function is called, it will print the value of the this keyword from the code that called bind.

fn();
// { [Function: bind]
//   [Symbol(nodejs.util.promisify.custom)]: [Getter] }

The bind method can be useful for creating functions that can be reused in different contexts. For example, the following code creates a function that can be used to log messages to a file:

const logMessage = AsyncResource.bind(function(message) {
  fs.appendFile('log.txt', message, (err) => {
    if (err) throw err;
  });
});

The logMessage function can be called from anywhere in the code and it will always log messages to the same file.

Real-world applications

The bind method can be used in a variety of real-world applications, including:

Creating functions that can be reused in different contexts. The bind method can be used to create functions that can be passed to other functions or used in different modules without having to worry about the execution context.
Logging messages to a file. The bind method can be used to create a function that can be used to log messages to a file from anywhere in the code.
Tracking the execution time of code. The bind method can be used to create a function that can be used to track the execution time of code. This can be useful for debugging performance issues.

`asyncResource.bind(fn[, thisArg])`

fn {Function} The function to bind to the current AsyncResource.
thisArg {any} (optional)

Description

Binds the given function to execute in the scope of the current AsyncResource. When the function is called, it will have access to the asyncResource property, which will reference the AsyncResource that bound it.

Example

Consider an AsyncResource representing an HTTP request:

const asyncResource = new AsyncResource("HTTP request");

function requestHandler(req, res) {
  asyncResource.bind(function () {
    // This function will have access to the `asyncResource` property
    // referencing the HTTP request.
  })();
}

Potential Applications

Binding functions to AsyncResources allows for easy propagation of context information across asynchronous operations. This can be useful for debugging, tracing, and performance monitoring.

For example, a logging framework could use AsyncResources to associate log messages with the HTTP request they were generated during. This would make it easier to identify the source of log messages and to correlate them with other events that occurred during the request.

Real-World Complete Code Implementation

The following is a complete example of how to use asyncResource.bind():

const asyncResource = new AsyncResource("HTTP request");

function requestHandler(req, res) {
  asyncResource.bind(function () {
    // This function will have access to the `asyncResource` property
    // referencing the HTTP request.
    // ...
  })();
}

// Create an HTTP server and attach the request handler
const server = http.createServer(requestHandler);
server.listen(3000);

In this example, the requestHandler function is bound to the asyncResource for the HTTP request. This means that any functions called within requestHandler will also have access to the asyncResource property. This can be useful for propagating context information across asynchronous operations, such as logging or performance monitoring.

Simplified Explanation:

Imagine your code as a play with different actors and scenes. Async resources are like actors that can perform actions asynchronously, like sending emails or reading files.

asyncResource.runInAsyncScope lets you execute a function in the "context" of a specific async resource. This means that when the function runs, it's treated as if it was executed by that resource.

Detailed Explanation:

When you call asyncResource.runInAsyncScope, you're basically saying:

"Hey async resource, pretend that this function is running inside you. Do your normal things before and after, and let me know when it's done."

This has a few advantages:

Contextualization: It ensures that the function has access to any resources or information that the async resource normally has.
Hooks: It triggers any hooks or callbacks that the async resource has registered (we'll get to this later).

Real-World Example:

Let's say you have a function that logs a message after reading a file asynchronously. You can use asyncResource.runInAsyncScope to ensure that the logging happens as if the file reading was done by the async resource itself.

const fs = require("fs");
const asyncContext = require("async-context");

const readFileAsync = async (path) => {
  // Create an async resource for the file system operation
  const asyncResource = asyncContext.create("fs");

  // Call the function in the context of the async resource
  return asyncResource.runInAsyncScope(() => {
    return new Promise((resolve, reject) => {
      fs.readFile(path, (err, data) => {
        if (err) {
          reject(err);
        } else {
          resolve(data);
        }
      });
    });
  });
};

Potential Applications:

Logging: Ensure that log messages are associated with the correct async resource for debugging.
Metrics: Track metrics specific to an async resource, such as request latency.
Security: Control access to sensitive data based on the async resource that's requesting it.

What is asyncResource.emitDestroy()?

Imagine you're building a program that does a lot of asynchronous operations, like making HTTP requests. These operations might take a while to finish, and in the meantime, you want to keep track of them so you can clean up when they're done.

asyncResource.emitDestroy() is a method that lets you do just that. It's like a signal that tells the program "Hey, this asynchronous operation is finished, you can clean up now."

How does it work?

When you call asyncResource.emitDestroy(), it triggers a series of "cleanup functions" that you can register. These functions are called "destroy hooks."

When you register a destroy hook, you're telling the program "When this asynchronous operation finishes, run this function to clean up any resources."

Why is it important?

Calling asyncResource.emitDestroy() is important because it allows you to properly clean up resources that are associated with asynchronous operations. If you don't call it, the resources might not be cleaned up, which can lead to problems like memory leaks.

Real-world example

Here's a simple example of how you might use asyncResource.emitDestroy():

const asyncResource = new AsyncResource('my-async-operation');

asyncResource.run(() => {
  // Do some asynchronous operation...
});

// When the asynchronous operation is finished, call emitDestroy() to trigger the cleanup functions.
asyncResource.emitDestroy();

Potential applications

asyncResource.emitDestroy() can be used in any situation where you need to keep track of and clean up asynchronous operations. Here are a few examples:

Tracking HTTP requests in a web server
Managing database connections
Cleaning up after long-running background tasks

asyncResource.asyncId() method

Description:

The asyncId() method of the async-context module returns the unique identifier (asyncId) assigned to the resource. This ID is used to track the resource across asynchronous operations.

Syntax:

asyncResource.asyncId(): number;

Parameters:

None

Return Value:

A number representing the unique asyncId of the resource.

Real-World Example:

Suppose you have a web application that performs asynchronous tasks, such as sending HTTP requests. You can use the asyncId() method to track the progress of these tasks and ensure that they are completed in the correct order.

Here is a simple example:

const asyncContext = require("async-context");

const request = asyncContext.run(() => {
  // Make an HTTP request
  return fetch("https://example.com");
});

// Get the asyncId of the request
const asyncId = asyncContext.current().asyncId();

// Log the asyncId
console.log(`Async ID: ${asyncId}`);

In this example, the asyncId() method is used to obtain the unique identifier of the asynchronous HTTP request. This ID can then be used to track the progress of the request and ensure that it is completed in the correct order.

Potential Applications:

Tracking the progress of asynchronous tasks
Ensuring that asynchronous tasks are completed in the correct order
Debugging asynchronous code

What is asyncResource.triggerAsyncId()?

Every asynchronous operation in Node.js has an associated asyncId. When that operation is triggered, the triggerAsyncId is the asyncId of the operation that caused it to be triggered.

For example, if you have an HTTP request that triggers a database query, the triggerAsyncId of the database query would be the asyncId of the HTTP request.

How do I use asyncResource.triggerAsyncId()?

You can get the triggerAsyncId of an asynchronous operation by calling the triggerAsyncId() method on the AsyncResource object associated with that operation.

const asyncResource = new AsyncResource({});

asyncResource.triggerAsyncId(); // Returns undefined

Why would I want to use asyncResource.triggerAsyncId()?

You can use the triggerAsyncId() method to track the lineage of asynchronous operations. This can be useful for debugging purposes, or for tracking the performance of asynchronous operations.

Real-world example

Here is a real-world example of how you can use the triggerAsyncId() method:

const asyncResource = new AsyncResource({});

asyncResource.runInAsyncScope(() => {
  // This code will be executed in a new scope with a new asyncId.
  setTimeout(() => {
    console.log(`The asyncId of this timeout is ${asyncResource.triggerAsyncId()}`);
  }, 1000);
});

Output:

The asyncId of this timeout is undefined

In this example, the asyncResource.triggerAsyncId() method returns undefined because the timeout is not triggered by another asynchronous operation.

Applications in the real world

The asyncResource.triggerAsyncId() method can be used in a variety of real-world applications, including:

Debugging asynchronous operations
Tracking the performance of asynchronous operations
Identifying the source of errors in asynchronous operations

AsyncResource

AsyncResource is a class in the Node.js async_hooks module that provides a way to track asynchronous resources and their execution context. This allows developers to associate metadata with asynchronous operations and track their execution flow.

Using AsyncResource for a Worker Thread Pool

In Node.js, Worker threads are used to offload computationally intensive tasks to separate threads, allowing the main thread to continue executing. When using Worker threads, it's important to properly provide asynchronous tracking to ensure that the execution context is correctly associated with the task. AsyncResource can be used to achieve this.

Example Code

The following code shows how to use AsyncResource for a Worker thread pool:

// Import the necessary modules.
const { AsyncResource } = require("async_hooks");
const { Worker } = require("worker_threads");

// Create an AsyncResource for the worker thread pool.
const poolResource = new AsyncResource("Worker Thread Pool");

// Create a worker thread pool.
const pool = new WorkerPool({
  size: 4,
  resource: poolResource,
});

// Create a task for the worker thread pool.
const task = {
  a: 1,
  b: 2,
};

// Run the task on the worker thread pool.
pool.runTask(task);

Explanation

In the code above, the poolResource AsyncResource is created for the worker thread pool. This resource is associated with all tasks that are executed on the worker thread pool. When a task is run on the worker thread pool, the poolResource resource is automatically activated, providing access to the execution context of the task.

The runTask method of the worker thread pool takes the task as an argument and executes it on a worker thread. When the task is executed, the poolResource resource is activated, and any asynchronous operations that are performed within the task will be associated with the poolResource. This ensures that the execution context of the task is correctly tracked and propagated through any asynchronous operations that are performed.

Real-World Applications

AsyncResource can be used in various real-world applications to track and manage asynchronous operations. Some examples include:

Tracking the execution time of asynchronous operations for performance monitoring.
Identifying the source of errors that occur during asynchronous operations.
Ensuring that resources are properly released after asynchronous operations are completed.

Potential Benefits

Using AsyncResource for a Worker thread pool provides several potential benefits:

Improved debugging capabilities by providing access to the execution context of tasks.
Enhanced performance monitoring by allowing tracking of the execution time of tasks.
Reduced resource leaks by ensuring that resources are properly released after tasks are completed.

Introduction to Worker Pool

Imagine you have a lot of tasks that need to be completed. Instead of doing them all yourself, you can hire a team of workers to help you out. This is essentially what a worker pool is - a team of workers that can be used to complete tasks concurrently.

Creating a Worker Pool

To create a worker pool, you first need to specify how many workers you want. In the example code provided, the numThreads parameter is used to specify the number of workers to create.

Once you have specified the number of workers, you can create the worker pool using the WorkerPool class. The WorkerPool class is a subclass of the EventEmitter class, which means that it can emit events. In this case, the WorkerPool class emits the kWorkerFreedEvent event whenever a worker becomes free.

Adding Workers to a Worker Pool

Once you have created a worker pool, you can add workers to it using the addNewWorker method. The addNewWorker method creates a new worker using the Worker class. The Worker class represents a thread of execution that can be used to execute tasks in parallel.

When a new worker is created, it is added to the workers list and the freeWorkers list. The workers list contains all of the workers in the pool, while the freeWorkers list contains only the workers that are currently available to execute tasks.

Executing Tasks in a Worker Pool

To execute a task in a worker pool, you can use the runTask method. The runTask method takes two parameters: the task to be executed and a callback function to be called when the task is completed.

The runTask method first checks to see if there are any free workers available. If there are no free workers, the task is added to a queue and will be executed when a worker becomes available.

If there are free workers available, the runTask method assigns the task to one of the free workers. The worker then executes the task and calls the callback function when the task is completed.

Closing a Worker Pool

When you are finished using a worker pool, you can close it using the close method. The close method terminates all of the workers in the pool and removes them from the workers list and the freeWorkers list.

Real-World Applications

Worker pools can be used in a variety of real-world applications, including:

Image processing: Worker pools can be used to process images in parallel, which can significantly improve performance.
Data analysis: Worker pools can be used to analyze data in parallel, which can help to identify trends and patterns more quickly.
Machine learning: Worker pools can be used to train machine learning models in parallel, which can reduce training time.

Conclusion

Worker pools are a powerful tool that can be used to improve the performance of your applications. By using a worker pool, you can execute tasks in parallel and take advantage of the power of multiple CPUs.

Worker Pool

Imagine you have a lot of tasks to do, but you don't have enough time or resources to do them all at once. That's where a worker pool comes in.

A worker pool is like a group of helpers who can do your tasks for you. You give the worker pool a task, and the worker pool assigns it to one of its helpers. The helper does the task and gives the result back to the worker pool, which then gives it to you.

This way, you can do a lot of tasks at the same time, even if you don't have enough time or resources to do them all yourself.

Implementation

Here's a simplified example of how you could implement a worker pool in Node.js:

const { Worker } = require('worker_threads');

const workerPool = [];

for (let i = 0; i < 4; i++) {
  const worker = new Worker('./task_processor.js');
  workerPool.push(worker);
}

const runTask = (task, callback) => {
  const worker = workerPool.shift();
  worker.postMessage(task);
  worker.on('message', (result) => {
    callback(null, result);
    workerPool.push(worker);
  });
  worker.on('error', (err) => {
    callback(err, null);
    workerPool.push(worker);
  });
};

This example creates a worker pool with 4 workers. When you want to run a task, you call the runTask function. The runTask function assigns the task to a free worker from the worker pool. The worker does the task and sends the result back to the runTask function, which then gives it to you.

Real-World Applications

Worker pools are used in a variety of real-world applications, including:

Image processing
Data analysis
Machine learning
Video encoding

Potential Applications

Here are some potential applications for worker pools in the real world:

You could use a worker pool to process a large batch of images.
You could use a worker pool to analyze a large dataset.
You could use a worker pool to train a machine learning model.
You could use a worker pool to encode a video file.

Topic: Tracking Tasks in a Worker Pool

Simplified Explanation:

Imagine a group of workers who are assigned different tasks. To keep track of who is doing what, we need to associate each task with the worker who is working on it.

Original Content:

import WorkerPool from "./worker_pool.js";
import os from "node:os";

const pool = new WorkerPool(os.availableParallelism());

let finished = 0;
for (let i = 0; i < 10; i++) {
  pool.runTask({ a: 42, b: 100 }, (err, result) => {
    console.log(i, err, result);
    if (++finished === 10) pool.close();
  });
}

Additional Explanation:

This code creates a pool of workers using the WorkerPool class. The runTask method is used to assign tasks to the workers. Each task is defined by an object containing input parameters.

The callback function passed to runTask is called when the task is completed. The callback receives three parameters:

err: Any error that occurred during the task
result: The result of the task
info: Information about the task, including the task ID and the worker that ran it

Improved Code Snippet:

import WorkerPool from "./worker_pool.js";
import os from "node:os";

const pool = new WorkerPool(os.availableParallelism());

const tasks = [
  { a: 42, b: 100 },
  { a: 2, b: 200 },
  // ... more tasks
];

const callback = (err, result, info) => {
  console.log(info.taskID, err, result);
  if (info.taskID === tasks.length - 1) pool.close();
};

tasks.forEach((task) => pool.runTask(task, callback));

Real-World Application:

This technique can be used in situations where multiple tasks need to be processed in parallel, such as:

Image processing
Data analysis
Video compression
Scientific simulations

By using a worker pool, these tasks can be distributed among multiple workers, improving overall performance and efficiency.

Integrating AsyncResource with EventEmitter

Event listeners triggered by an EventEmitter may be run in a different execution context than the one that was active when eventEmitter.on() was called.

The following example shows how to use the AsyncResource class to properly associate an event listener with the correct execution context. The same approach can be applied to a Stream or a similar event-driven class.

import { createServer } from "node:http";
import { AsyncResource, executionAsyncId } from "node:async_hooks";

const server = createServer((req, res) => {
  req.on(
    "close",
    AsyncResource.bind(() => {
      // Execution context is bound to the current outer scope.
    })
  );
  req.on("close", () => {
    // Execution context is bound to the scope that caused 'close' to emit.
  });
  res.end();
}).listen(3000);

In this example:

We create an HTTP server using the createServer() function.
We add an event listener to the 'close' event of the request object using the req.on() method. The first event listener is bound to the current outer scope using the AsyncResource.bind() function. This means that when the 'close' event is emitted, the execution context of the event listener will be the same as the execution context of the code that added the event listener.
We add a second event listener to the 'close' event of the request object using the req.on() method. This event listener is not bound to any specific execution context, so when the 'close' event is emitted, the execution context of the event listener will be the execution context of the code that emitted the event.

Real-world application:

This technique can be used to ensure that event listeners are always executed in the correct execution context. This is important for applications that need to maintain state across different execution contexts. For example, an application that uses a database connection pool may need to ensure that all database operations are executed within the same execution context so that the connection pool can be properly managed.

Previousassert Nextaddons

Async CTX manager

Simplified Explanation of AsyncLocalStorage and AsyncResource from nodejs's async-context module:

Real-World Examples:

Potential Applications:

What is AsyncLocalStorage?

How to use AsyncLocalStorage

Real-World Applications

Code Example

Static Method: AsyncLocalStorage.snapshot()

asyncLocalStorage.disable()

asyncLocalStorage.getStore()

asyncLocalStorage.run(store, callback[, ...args])

AsyncResource Object

Static method: AsyncResource.bind(fn[, type[, thisArg]])

Real-world applications

asyncResource.bind(fn[, thisArg])

Introduction to Worker Pool

Creating a Worker Pool

Adding Workers to a Worker Pool

Executing Tasks in a Worker Pool

Closing a Worker Pool

Real-World Applications

Conclusion

What is `AsyncLocalStorage`?

How to use `AsyncLocalStorage`

`asyncLocalStorage.disable()`

`asyncLocalStorage.getStore()`

`asyncLocalStorage.run(store, callback[, ...args])`

Static method: `AsyncResource.bind(fn[, type[, thisArg]])`

`asyncResource.bind(fn[, thisArg])`