What are asynchronous resources? A brief analysis of Node's method of realizing asynchronous resource context sharing

How does Node.js implement asynchronous resource context sharing? The following article will introduce to you how Node implements asynchronous resource context sharing. Let’s talk about the use of asynchronous resource context sharing for us. I hope it will be helpful to everyone!

Asynchronous resource context sharing means sharing context data within a network request life cycle or asynchronous resource call chain.

Before answering this question, we must first understand what asynchronous resources are.

Asynchronous resources

Asynchronous resources can be understood as objects with callbacks, such as but not limited to Promises, Timeouts, TCPWrap, UDP, etc. See Asynchronous resource type list for details.

The official definition is as follows:

An asynchronous resource represents an object with an associated callback. This callback may be called multiple times, such as the 'connection' event in net.createServer( ), or just a single time like in fs.open(). A resource can also be closed before the callback is called.

AsyncLocalStorage

here Introducing Node.js AsyncLocalStorage, the officially provided asynchronous context sharing solution. This feature was still an experimental feature before 16.4.0 and has been stable since 16.4.0.

AsyncLocalStorage can share data in asynchronous operation chains.

AsyncLocalStorage instance asyncLocalStorage has the following main methods:

• disable() disables asyncLocalStorage;
• getStore() returns the current context store, which must pass asyncLocalStorage .run() or asyncLocalStorage.enterWith() to perform asynchronous context initialization;
• enterWith(store) passes the context store through this method, and the store can be obtained in all subsequent asynchronous calls;

Example:

const store = { id: 1 };
// Replaces previous store with the given store object
asyncLocalStorage.enterWith(store);
asyncLocalStorage.getStore(); // Returns the store object
someAsyncOperation(() => {
  asyncLocalStorage.getStore(); // Returns the same object
});

• run(store, callback[, ...args]) Use run to specify the context store and its effective callback function. The store will only be in is obtained from the callback function.
• exit(callback[, ...args])

asyncLocalStorage.run() The first parameter of the function is to store the shared data we need to access in the asynchronous call. The second parameter is an asynchronous function.

The following is an example to demonstrate how to use AsyncLocalStorage to implement asynchronous resource context sharing:

Output:

runA 8f19ebef-58d7-4b1a-8b9b-46d158beb5d2 2022/5/24 20:26:17 this is a log message
runB 8f19ebef-58d7-4b1a-8b9b-46d158beb5d2 2022/5/24 20:26:17 this is a log message

Through asyncLocalStorage.run In the same asynchronous function running, function runA and function runB will be run, and runA and runB can access the same context data.

Performance issues

AsyncLocalStorage provides a large traversal for us to easily implement asynchronous resource context sharing in Node.js, but every asynchronous resource operation will trigger Async Hooks , will inevitably have a certain impact on the performance of our Node application. So how big is the impact?

According to an actual measurement by Kuzzle, using AsyncLocalStorage will cause about 8% additional performance loss. Of course, different business scenarios may have different performance. If you are concerned about this part of performance, you can also add comparative testing to your business to test the specific performance impact.

----	Log with AsyncLocalStorage	Log classic	difference
req/s	2613	2842	~8%

Application Scenario

In other multi-threaded languages, each HTTP creates a new thread, and each thread has its own memory. You can store global state in thread memory and retrieve global state from anywhere in your code.

In Node.js, because Node.js is single-threaded and shares memory among all HTTP requests, each HTTP request cannot hold mutually isolated global state.

AsyncLocalStorage can effectively isolate the status between different asynchronous operations, and plays a very important role in scenarios such as HTTP request tracking, APM tools, context log tracking, and request-based full-link log tracking.

For more node-related knowledge, please visit: nodejs tutorial!

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