DevExplorations

Getting per-request context in NodeJS with async_hooks

March 4, 2019 • ☕️ 6 min read

I recently got a problem when I was building a HTTP server in NodeJS. I’m logging things in a lot of places inside my codebase and I have a unique ID for each request. I want to append this ID to each of my log messages to trace what’s happening in each request. How to efficiently do this?

The simplest way is to pass this request ID as an argument in each of my functions. The problem with this is that it’s not maintainable: if I’m 5 functions deep in my stack and I want to log something, I need to edit 5 functions to add an argument and edit every function call.

You can make sure to always pass a “context” object in every one of your function but there is still a problem. I’m using a SQL lib which can be configured to run a function when a long query is detected but this function is called only with the query string. I cannot pass my request context to it.

If you are ready to use experimental NodeJS stuff, I have a really elegant solution for you thanks to async_hooks.

Some theory about async_hooks

The async_hooks module expose functions to track asynchronous resources. These hooks are called whenever a setTimeout, server listener or any other asynchronous task is created, started, finished and destroyed.

When an asynchronous resource is created, a new asyncId will be assigned to it and our init hook will be called with this id and the asyncId of the parent resource. This module also exposes a very useful executionAsyncId() method to get the current asyncId of our function execution.

Here’s how we can use it to simply log a message when the hooks are called:

const fs = require('fs');
const async_hooks = require('async_hooks');

const log = (str) => fs.writeSync(1, `${str}\n`);

async_hooks.createHook({
  init(asyncId, type, triggerAsyncId) {
    log(`INIT: asyncId: ${asyncId} / type: ${type} / trigger: ${triggerAsyncId}`);
  },
  destroy(asyncId) {
    log(`DESTROY: asyncId: ${asyncId}`);
  },
}).enable();

You may notice we are not using console.log here. It’s because console.log is an asynchronous operation and would trigger a hook which will call console.log which is an asynchronous operation and would trigger a hook… And this is a case of an infinite loop. The solution is to use fs.writeSync which is synchronous and will not trigger one of our hooks.

Let’s try these hooks with a simple setTimeout example in which we are logging the current asyncId outside and inside the setTimeout:

log(`>> Calling setTimeout: asyncId: ${async_hooks.executionAsyncId()}`);
setTimeout(() => {
  log(`>> Inside setTimeout callback: asyncId: ${async_hooks.executionAsyncId()}`);
}, 0);
log(`>> Called setTimeout: asyncId: ${async_hooks.executionAsyncId()}`);

When executing this code, we’ll get this output:

>> Calling setTimeout: asyncId: 1
INIT: asyncId: 2 / type: Timeout / trigger: 1
>> Called setTimeout: asyncId: 1
>> Inside setTimeout callback: asyncId: 2
DESTROY: asyncId: 2

Let’s see what happened here.

  • We started with an asyncId equal to 1 and called setTimeout.
  • It created a Timeout asynchronous resource and triggered our init hook with the newly created asyncId of 2 and its parent asyncId of 1.
  • We logged the end of our program with an asyncId still equals to 1
  • The Timeout resource callback was called and we logged the current asyncId which is equals to 2
  • The Timeout resource was destroyed and our destroy hook was triggered

There are also two other hooks: before and after. They can be used to monitor the timings of some asynchronous resources like external HTTP requests or SQL queries.

Okay, but what’s the point?

With executionAsyncId() and the init we can recreate a “stack” of our functions calls even if they were asynchronous.

Here’s a real example. We are creating a HTTP server, reading and sending the content of a test.txt file on each request.

const fs = require('fs');
const async_hooks = require('async_hooks');
const http = require('http');

const log = (str) => fs.writeSync(1, `${str}\n`);

async_hooks.createHook({
  init(asyncId, type, triggerAsyncId) {
    log(`asyncId: ${asyncId} / trigger: ${triggerAsyncId}`);
  },
}).enable();

const readAndSendFile = (res) => {
  fs.readFile('./test.txt', (err, file) => {
    log(`>> Inside readAndSendFile: execution: ${async_hooks.executionAsyncId()}`);
    res.end(file);
  });
}

const requestHandler = (req, res) => {
  log(`>> Inside request: execution: ${async_hooks.executionAsyncId()}`);
  readAndSendFile(res);
}

const server = http.createServer(requestHandler);

server.listen(8080);

Let’s execute this code and send two requests. I removed some useless lines from the output.

>> Inside request: execution: 6
asyncId: 9 / trigger: 6
asyncId: 11 / trigger: 9
asyncId: 12 / trigger: 11
asyncId: 13 / trigger: 12
>> Inside readAndSendFile: execution: 13
[...]
>> Inside request: execution: 31
asyncId: 34 / trigger: 31
asyncId: 36 / trigger: 34
asyncId: 37 / trigger: 36
asyncId: 38 / trigger: 37
>> Inside readAndSendFile: execution: 38

We see that our two requests were assigned two asyncId: 6 and 31. Reading our file created new async resources transparent to our code and then our readAndSendFile logged two asyncId: 13 and 38.

From the readAndSendFile function, we can get our original request asyncId by retracing our “async path”. For example, for our first request, we start with an asyncId equals to 13 and then we get 13 → 12 → 11 → 9 → 6.

Getting something useful

With all of these, we can create two functions to create and get a context object for each of our requests. This can also be used for any other usage, not only HTTP server.

const async_hooks = require('async_hooks');

const contexts = {};

async_hooks.createHook({
  init: (asyncId, type, triggerAsyncId) => {
    // A new async resource was created
    // If our parent asyncId already had a context object
    // we assign it to our resource asyncId
    if (contexts[triggerAsyncId]) {
      contexts[asyncId] = contexts[triggerAsyncId];
    }
  },
  destroy: (asyncId) => {
    // some cleaning to prevent memory leaks
    delete contexts[asyncId];
  },
}).enable();


function initContext(fn) {
  // We force the initialization of a new Async Resource
  const asyncResource = new async_hooks.AsyncResource('REQUEST_CONTEXT');
  return asyncResource.runInAsyncScope(() => {
    // We now have a new asyncId
    const asyncId = async_hooks.executionAsyncId();
    // We assign a new empty object as the context of our asyncId
    contexts[asyncId] = {}
    return fn(contexts[asyncId]);
  });
}

function getContext() {
  const asyncId = async_hooks.executionAsyncId();
  // We try to get the context object linked to our current asyncId
  // if there is none, we return an empty object
  return contexts[asyncId] || {};
};

module.exports = {
  initContext,
  getContext,
};

Let’s write a small test to check if it’s all working right.

const {initContext, getContext} = require('./context.js');

const logId = () => {
  const context = getContext();
  console.log(`My context id is: ${context.id}`);
}

initContext((context) => {
  context.id = 1;
  setTimeout(logId, 100);
  setTimeout(logId, 300);
});

initContext((context) => {
  context.id = 2;
  setTimeout(logId, 200);
  setTimeout(logId, 400);
});

By executing this we get:

My context id is: 1
My context id is: 2
My context id is: 1
My context id is: 2

What’s next?

With these two functions we implemented a simple but very useful way to create a context to store data between our function without having to pass these data as arguments.

A real-life usage would be creating a context for each HTTP request, generating a request ID and fetching this ID inside our logging function to print it on each line. I’ve also used it to run every database call of one HTTP request in the same SQL transaction.

You should be careful about how much information you include in your context. This should be kept as simple as possible to keep the data flow in your program simple to understand and prevent edge cases which can create bugs. Using this kind of context will also confuse tools like TypeScript which won’t be able to know what’s in your current context.

Keep in mind that the async_hooks is still experimental but if you like to live on the edge, go and try it!