Node Async Context

It works like thread-local storage in threaded programming, but is based on async hooks that were added into NodeJS in v8.1.0.

Warning: async hooks are marked as experimental feature; therefore, node-async-context shouldn't be used for storing any critical data in production

Use case

Keep context per request in an express app

• Correlation ID / Request ID - API gateway assign you correlation ID that you want to attach to any log entry, error report or as header when calling an upstream service related to the request
• metrics - collect metrics related to request, time spent waiting for response from DB, upstream service, etc

Usage

Simple express middleware that initialize context and put there Correlation ID received from API gateway:

const { enable, context } = require("@onelogin/node-async-context");

enable();

module.exports = (req, res, next) => {
  // creates a new context and bind it to current execution (request)
  context.create();
  // reads correlation ID from request header and store it into context
  context.set("correlation-id", req.get("X-Correlation-ID"));
  next();
};

Configure Winston logger to read Correlation ID from context and make it part of every log entry:

const winston = require("winston");
const { context } = require("@onelogin/node-async-context");

const formatter = entry => {
  return `${entry.level}: ${entry.message} - CID: ${context.get(
    "correlation-id"
  )}`;
};

module.exports = () => {
  const logger = new winston.Logger({
    transports: [
      new winston.transports.Console({
        formatter
      })
    ]
  });

  return logger;
};

Read Correlation ID from context and setting it as HTTP header when calling an upstream service:

const superagent = require("superagent");
const { context } = require("@onelogin/node-async-context");

module.exports = {
  listUsers: () => {
    return (
      superagent
        // send GET request to /users via superagent HTTP client
        .get(`/users`)
        // set X-Correlation-ID header based on correlation ID from shared context
        .set("X-Correlation-ID", context.get("correlation-id"))
    );
  }
};

API:

node-async-context exports:

enable()

enables async hooks that powers context; you app will start to listen to async hooks; without calling enable() context won't work

context

object with following methods:

• create() creates an empty context and bind it to current execution (set current execution as root of async executions tree which share the same context; in other words, it sets scope of created context to start from this point).
• set(key, value) set value into current context (key-value store) under given key
• get(key) => value get value from current context (key-value store) for given key

disable()

cleanup method - it disable async hooks and clean storage with contexts