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doc: improve AsyncLocalStorage sample
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PR-URL: #32757
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: Chengzhong Wu <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Gerhard Stöbich <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
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puzpuzpuz authored and BridgeAR committed Apr 28, 2020
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49 changes: 24 additions & 25 deletions doc/api/async_hooks.md
Original file line number Diff line number Diff line change
Expand Up @@ -876,41 +876,40 @@ chains. It allows storing data throughout the lifetime of a web request
or any other asynchronous duration. It is similar to thread-local storage
in other languages.

The following example builds a logger that will always know the current HTTP
request and uses it to display enhanced logs without needing to explicitly
provide the current HTTP request to it.
The following example uses `AsyncLocalStorage` to build a simple logger
that assigns IDs to incoming HTTP requests and includes them in messages
logged within each request.

```js
const { AsyncLocalStorage } = require('async_hooks');
const http = require('http');
const { AsyncLocalStorage } = require('async_hooks');

const kReq = 'CURRENT_REQUEST';
const asyncLocalStorage = new AsyncLocalStorage();

function log(...args) {
const store = asyncLocalStorage.getStore();
// Make sure the store exists and it contains a request.
if (store && store.has(kReq)) {
const req = store.get(kReq);
// Prints `GET /items ERR could not do something
console.log(req.method, req.url, ...args);
} else {
console.log(...args);
}
function logWithId(msg) {
const id = asyncLocalStorage.getStore();
console.log(`${id !== undefined ? id : '-'}:`, msg);
}

http.createServer((request, response) => {
asyncLocalStorage.run(new Map(), () => {
const store = asyncLocalStorage.getStore();
store.set(kReq, request);
someAsyncOperation((err, result) => {
if (err) {
log('ERR', err.message);
}
let idSeq = 0;
http.createServer((req, res) => {
asyncLocalStorage.run(idSeq++, () => {
logWithId('start');
// Imagine any chain of async operations here
setImmediate(() => {
logWithId('finish');
res.end();
});
});
})
.listen(8080);
}).listen(8080);

http.get('http://localhost:8080');
http.get('http://localhost:8080');
// Prints:
// 0: start
// 1: start
// 0: finish
// 1: finish
```

When having multiple instances of `AsyncLocalStorage`, they are independent
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