NodeJS has a neat API called AsyncLocalStorage
. It’s used to share information across callbacks and promise chains.
For example, it is useful to share information for all the code executed when serving a web request. I also found it very useful to keep trace information to easily keep track of which item was being processed during a batch process.
The only caveat in my opinion, is that you need to wrap your code in yet another callback.
How to use AsyncLocalStorage
:
First, create an instance of
AsyncLocalStorage
:Then, use the
run
method to wrap your code:Finally, use the
getStore
method to retrieve the shared state:
Real-world example
The previous example is very simple because it only shows how to use it.
Here’s a more elaborate example that will hopefully show why this feature is so useful:
import { AsyncLocalStorage } = from "node:async_hooks";
async function process() {
try {
await Promise.all([foo(), bar()]);
} catch (err) {
handleError(err);
}
}
async function foo() {
const { index } = asyncLocalStorage.getStore();
console.log(`processing foo ${index}`);
}
async function bar() {
const { index } = asyncLocalStorage.getStore();
console.log(`processing bar ${index}`);
if (Math.random() < 0.1) {
// simulate a random failure
throw new Error("A random failure happened");
}
}
async function handleError(err) {
const { index } = asyncLocalStorage.getStore();
console.log(`Handling error when processing ${index}: ${err}`);
}
const asyncLocalStorage = new AsyncLocalStorage();
// process 20 things, asynchronously
const tasks = [];
for (let i = 0; i < 20; i++) {
tasks.push(
asyncLocalStorage.run({ index: i }, process)
);
}
If you execute this code, it will clearly log for which items the failures occured, because the required information is available in the shared state exposed by AsyncLocalStorage
.
Example of the output:
processing foo 0
processing bar 0
processing foo 1
processing bar 1
...
Handling error when processing 3: Error: A random failure happened
Handling error when processing 8: Error: A random failure happened
Handling error when processing 14: Error: A random failure happened
Top comments (0)