node-cls

Continuation Local Storage

The purpose with this module is to share contexts across async (and sync) calls. Contexts are accessed by keys and can be nested. It is an alternative to the deprecated domain. It is based on async_hooks that were introduced in node 8. Beware that that the async_hooks are still experimental in nodejs.

Usage

A typical scenario is when you need to share context in a request handler.

let http = require('http');
let cls = require('node-cls');

let server = http.createServer(function (request, response) {
    let context = cls.create('request-context');
    context.id = 1;
    context.request = request;
    context.response = response;   
    context.run(doWork);
})

server.listen(8080)

function doWork() {
    let context = cls.get('request-context');
    context.response.end(`End: ${context.id}`) //End: 1

}

Context is retained in async calls.

let cls = require('node-cls');

let context = cls.create('myContext');
context.run(() => {
    context.name = 'George';
    setTimeout(onTimeout, 300);
});

function onTimeout() {
    let context = cls.get('myContext');
    console.log(context.name); //George
}

Contexts can be nested, even on the same key.

let cls = require('node-cls');

let context = cls.create('myContext');
context.run(async () => {
    context.name = 'George';
    let context2 = cls.create('myContext');
    await context2.run(onNested);
    console.log(context.name) //George
});

async function onNested() {
    await Promise.resolve();
    let context = cls.get('myContext');
    console.log(context.name); //undefined
    context.name = 'John Nested';
    setTimeout(onTimeout, 300);
}

function onTimeout() {
    let context = cls.get('myContext');
    console.log(context.name); //John Nested
}

In node 12 and above you can start a context directly instead of wrapping it in the run function. The start function returns a promise. You can leave the current context by calling exit.

let cls = require('node-cls');

async function main() {
    let context = cls.create('myContext');
    context.name = 'George';
    await context.start();

    let context2 = cls.create('myContext');
    context2.name = 'John Nested';
    await context2.start();
    console.log(cls.get('myContext').name); //John Nested

    cls.exit('myContext');
    console.log(cls.get('myContext').name); //George

    cls.exit('myContext');
    console.log(cls.get('myContext')); //undefined
}

main();