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@psenger/async-context-id

0.1.2 • Public • Published

async-context-id

[!TAG] 0.1.1

A lightweight, powerful correlation ID tracking and context store written for Node.js that automatically propagates context through async operations using Node's async_hooks. Perfect for distributed tracing, request tracking, and logging correlation in micro services architectures.

Table of Contents

Features

  • Automatic context propagation through async operations
  • Thread-local-like storage for Node.js
  • UUID v4 based correlation IDs
  • ️ Thread-safe context isolation
  • Deep cloning of context data
  • Automatic cleanup of contexts
  • Zero dependencies (uses only Node.js built-ins)

Installation

NPM

npm install @psenger/async-context-id --save

YARN

yarn add @psenger/async-context-id

How It Works

This library uses Node.js's async_hooks module to track async operations:

stateDiagram-v2
    [*] --> AsyncContextId: Create Instance

    state AsyncContextId {
        state "Async Hooks Setup" as setup {
            [*] --> Hook
            Hook --> Init: New Async Operation
            Hook --> PromiseResolve: Promise Resolved
            Hook --> Destroy: Operation Complete
        }

        state "Context Operations" as ops {
            state "Context Store (Map)" as store {
                SetContext --> Store: Update Context
                GetContext --> Store: Retrieve Context
                SetCorrelationId --> Store: Update ID
                GetCorrelationId --> Store: Get/Generate ID
                Clear --> Store: Delete Context
            }
        }

        state fork_state <<fork>>

        Init --> fork_state
        fork_state --> CopyParentContext: Has Parent Context
        fork_state --> CreateNewContext: No Parent Context

        CopyParentContext --> Store: Set New AsyncID
        CreateNewContext --> Store: Generate New ID

        PromiseResolve --> Store: Copy Context to New AsyncID
        Destroy --> Store: Delete Context for AsyncID
    }

    state "Example Flow" as example {
        Request --> SetCorrelationId: Upstream ID
        SetCorrelationId --> ProcessData: Async Operation
        ProcessData --> GetContext: Get Results
        GetContext --> Clear: Cleanup
    }

    AsyncContextId --> BeforeExit: Process Exit
    BeforeExit --> [*]: Cleanup Hook & Store

Flow Diagram Explanation

Instance Creation

  • Program starts by creating a singleton AsyncContextId instance
  • Sets up async hooks and initializes the context store (Map)

Async Hooks Setup

  • Hook listens for three main events:
    • init: When new async operations are created
    • promiseResolve: When promises are resolved
    • destroy: When async operations complete

Context Operations

Custom data is stored in the meta attribute. All context / meta operations use deep cloning to ensure isolation between async operations.

  • Main operations on the context store:
    • setContext: Updates context for current async ID
    • getContext: Retrieves context for current async ID
    • setCorrelationId: Sets/updates correlation ID
    • getCorrelationId: Gets or generates correlation ID
    • clear: Removes context for current async ID

Context Propagation

  • When a new async operation starts:
    • If parent context exists, it's copied to new async ID
    • If no parent context, new context is created with generated ID
  • When promises resolve, context is copied to new async ID
  • When operations complete, context is cleaned up

Cleanup

  • On process exit:
    • Disables async hooks
    • Clears context store
    • Removes event listeners

Example Flow

  • Shows typical request handling:
    • Set correlation ID from upstream
    • Process data asynchronously
    • Retrieve context for logging
    • Clear context when done

API

Modules

Module Description
@psenger/async-context-id

A lightweight, powerful correlation ID tracking and context store.

Classes

Global Description
AsyncContextId

A singleton class that tracks correlation IDs across asynchronous operations in Node.js. Uses async_hooks to automatically propagate correlation context across async boundaries.
LruMap ⇐ Map

A Map extension implementing Least Recently Used (LRU) caching strategy. Automatically removes oldest entries when size limit is reached.
TimedMap ⇐ Map

A Map extension that automatically deletes entries after a specified time-to-live (TTL).

@psenger/async-context-id

A lightweight, powerful correlation ID tracking and context store.

AsyncContextId

A singleton class that tracks correlation IDs across asynchronous operations in Node.js. Uses async_hooks to automatically propagate correlation context across async boundaries.

Kind: global class
Properties

Name Type Description
contextStore Map Storage for async context data
hook AsyncHook The async_hooks instance for tracking async operations
cleanUpFn function Cleanup function registered for process exit

new AsyncContextId([options])

This class provides context propagation across async operations in Node.js applications. It maintains correlation IDs and metadata throughout the async execution chain.

Returns: AsyncContextId - The singleton instance

Param Type Default Description
[options] Object {} Configuration options
[options.store] Map new Map() Optional Map instance for context storage, this package includes both a LRU ( Least Recently Used ) Map and a Timed Map.
[options.correlationIdFn] fn Optional function to override default UUID generation. Should return a string

Example

// Using default Map and UUID generation
const tracker = new AsyncContextId();

Example

// Using custom LRU Map and correlation ID generator
const tracker = new AsyncContextId({
  store: new LruMap(1000),
  correlationIdFn: () => `custom-${Date.now()}`
});

asyncContextId.cleanUpFn()

remove the listener of itself to prevent memory leaks

Kind: instance method of AsyncContextId

asyncContextId.getCorrelationId() ⇒ string

Retrieves the correlation ID for the current async context. Creates a new context with generated ID if none exists.

Kind: instance method of AsyncContextId
Returns: string - The current correlation ID
Throws:

  • Error If async hooks are not enabled

Example

const correlationId = tracker.getCorrelationId();
res.setHeader('x-correlation-id', correlationId);

asyncContextId.setCorrelationId(correlationId)

Sets the correlation ID for the current async context. Creates a new context if none exists.

Kind: instance method of AsyncContextId
Throws:

  • Error If the correlationId is not a string
  • Error If async hooks are not enabled
Param Type Description
correlationId string The correlation ID to set

Example

const upstreamId = req.headers['x-correlation-id'];
if (upstreamId) {
  tracker.setCorrelationId(upstreamId);
}

asyncContextId.getContext() ⇒ Object | string | number | Object

Retrieves the complete context object for the current async operation. Creates a new context if none exists.

Kind: instance method of AsyncContextId
Returns: Object - The correlation contextstring - context.correlationId - The correlation IDnumber - context.startTime - Unix timestamp of context creationObject - context.metadata - Custom metadata object
Throws:

  • Error If async hooks are not enabled

Example

const context = tracker.getContext();
console.log({
  correlationId: context.correlationId,
  duration: Date.now() - context.startTime,
  metadata: context.metadata
});

asyncContextId.setContext([context])

Updates the context for the current async operation. Creates a new context if none exists. Preserves existing correlationId and startTime unless explicitly overridden.

Kind: instance method of AsyncContextId
Throws:

  • Error If async hooks are not enabled
  • Error If context is not an object
Param Type Default Description
[context] Object {} The context object to merge
[context.correlationId] string Optional correlation ID override
[context.metadata] Object Optional metadata to merge

Example

// Add request context
tracker.setContext({
  metadata: {
    operation: 'processData',
    requestId: req.id,
    userId: req.user.id
  }
});

asyncContextId.clear()

Removes the correlation context for the current async operation.

Kind: instance method of AsyncContextId
Throws:

  • Error If async hooks are not enabled

Example

try {
  await processRequest(data);
} finally {
  tracker.clear();
}

LruMap ⇐ Map

A Map extension implementing Least Recently Used (LRU) caching strategy. Automatically removes oldest entries when size limit is reached.

Kind: global class
Extends: Map

new LruMap(maxSize)

Creates an LRU cache with specified maximum size.

Param Type Description
maxSize number Maximum number of entries

Example

const cache = new LruMap(3);
cache.set('a', 1).set('b', 2).set('c', 3);
cache.set('d', 4); // Removes 'a', now contains b,c,d

lruMap.set(key, value) ⇒ this

Sets a value, removing oldest entry if size limit reached.

Kind: instance method of LruMap
Returns: this - The LruMap instance for chaining

Param Type Description
key * The key to set
value * The value to store

Example

const cache = new LruMap(2);
cache.set('key1', 'value1')
     .set('key2', 'value2')
     .set('key3', 'value3'); // Removes key1

TimedMap ⇐ Map

A Map extension that automatically deletes entries after a specified time-to-live (TTL).

Kind: global class
Extends: Map

new TimedMap(ttl)

Creates a new TimedMap instance.

Param Type Description
ttl number Time-to-live in milliseconds for each key-value pair

Example

const cache = new TimedMap(5000); // 5 second TTL
cache.set('key1', 'value1');
console.log(cache.get('key1')); // 'value1'
// After 5 seconds:
console.log(cache.get('key1')); // undefined

Example

// Resetting TTL on value update
const cache = new TimedMap(2000);
cache.set('user', { name: 'Alice' });
// 1 second later:
cache.set('user', { name: 'Alice', age: 30 }); // Resets the 2-second timer

timedMap.set(key, value) ⇒ this

Sets a value in the map with an automatic deletion timer. If the key already exists, its timer is reset.

Kind: instance method of TimedMap
Returns: this - The TimedMap instance for chaining

Param Type Description
key * The key to set
value * The value to store

Example

const cache = new TimedMap(10000);
cache.set('apiKey', 'xyz123')
     .set('timestamp', Date.now());

timedMap.delete(key) ⇒ boolean

Deletes a key-value pair and its associated timer.

Kind: instance method of TimedMap
Returns: boolean - True if the element was deleted, false if it didn't exist

Param Type Description
key * The key to delete

Example

const cache = new TimedMap(5000);
cache.set('temp', 'data');
cache.delete('temp'); // Manually delete before TTL expires

Usage

Enhanced Logging with Console Monkey Patching

This example demonstrates how to automatically inject correlation IDs into console logs using monkey patching. The code below intercepts standard console methods and prepends each log message with its log level and correlation ID.

eg LOG-LEVEL CORRELATION-ID

const fs = require('fs')
const path = require('path')
const util = require('util')
const {AsyncContextId} = require('../dist/index')
const TRACKER = new AsyncContextId()
const logFile = path.join(__dirname, 'app.log')
const original = {
  log: console.log,
  error: console.error,
  warn: console.warn,
  debug: console.debug,
  info: console.info
}
try {
  if (fs.existsSync(logFile)) {
    fs.writeFileSync(logFile, '')
  }
} catch (err) {
  console.error('Error handling log file:', err)
}
function formatApacheErrorTimestamp(date = new Date()) {
  const days = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat']
  const months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
  return `[${days[date.getDay()]} ${months[date.getMonth()]} ${String(date.getDate()).padStart(2, '0')} ` +
    `${String(date.getHours()).padStart(2, '0')}:${String(date.getMinutes()).padStart(2, '0')}:` +
    `${String(date.getSeconds()).padStart(2, '0')}.${String(date.getMilliseconds()).padStart(3, '0')} ` +
    `${date.getFullYear()}]`
}
Object.keys(original).forEach(method => {
  console[method] = function (...args) {
    const prefix = `${formatApacheErrorTimestamp()} [${method.toLowerCase()}] [${TRACKER.getContext().correlationId}] ${TRACKER.getContext()?.metadata?.fullName} `
    if (typeof args[0] === 'string') {
      args[0] = prefix + args[0]
    } else {
      args.unshift(prefix)
    }
    original[method].apply(console, args)
    fs.appendFileSync(logFile, util.format(...args) + '\n')
  }
})

Express Middleware

This Express middleware automatically manages correlation IDs across HTTP requests. It extracts the correlation ID from incoming request headers, propagates it through the request lifecycle, and includes it in the response headers.

Note: Register this middleware early in your Express application's middleware chain to ensure correlation IDs are available throughout the entire request lifecycle.

correlation-middleware.js

const {AsyncContextId} = require('../dist/index') // '@psenger/async-context-id'
const asyncContextId = new AsyncContextId()
const correlationMiddleware = () => (req, res, next) => {
  try {
    asyncContextId.clear()
    let correlationId = req.headers['x-correlation-id']
    correlationId = asyncContextId.setCorrelationId(correlationId)
    res.setHeader('x-correlation-id', correlationId)
    res.on('finish', () => {
      asyncContextId.clear()
    })
    next()
  } catch (error) {
    next(error)
  }
}
module.exports = correlationMiddleware

app.js

const express = require('express')
const app = express()
const router = express.Router()
const {systemTimer} = require('./timer')
const controller = require('./controller')
const correlationMiddleware = require('./correlation-middleware')
require('./monkey-patch-logs')
systemTimer()
app.use(express.json())
app.use(express.urlencoded({extended: false}))
app.use(correlationMiddleware())
router.post('/:id', controller)
app.use('/', router)
module.exports = app

simple-controller.js

const {AsyncContextId} = require('../dist/index') // '@psenger/async-context-id'
const TRACKER = new AsyncContextId()
module.exports = function (req, res) {
  const id = req.params.id
  const correlationId = TRACKER.getContext().correlationId
  console.log(`${correlationId} saw ${id} in controller`)
  TRACKER.setContext({
    metadata: {
      id,
    }
  })
  // do something else here which will expose meta upstream
}

Winston Logger Integration

const winston = require('winston')
const {AsyncContextId} = require('../dist/index') // '@psenger/async-context-id'

const TRACKER = new AsyncContextId()

const logger = winston.createLogger({
  format: winston.format.combine(
    winston.format.timestamp(),
    winston.format.json(),
    winston.format((info) => {
      const context = TRACKER.getContext();
      return {
        ...info,
        correlationId: context?.correlationId || 'no-correlation-id',
        metadata: context?.metadata || {}
      };
    })()
  ),
  transports: [
    new winston.transports.Console()
  ]
});

‼️ Caution - Memory Management Strategies

All scaling systems degrade when affected by memory leaks. This module hosts a single map and addresses such leaks through two configurable Map implementations for context tracking:

  • LRU (Least Recently Used) Map
  • Timed Map

These maps can be configured during AsyncContextId initialization. As AsyncContextId operates as a singleton, the map implementation must be set at creation and remains immutable.

‼️ Caution - Correlation ID with UUID

The default implementation of correlation ID uses non-cryptographic UUID generation to prevent recursive async hook triggers that would otherwise occur with Node's Crypto module (internal crypto operations would initiate new async hooks). Since this is based on UUID v4, it may be necessary for consumers to increase the complexity of the correlation ID. Therefore, this functionality has been exposed as an option ( correlationIdFn ).

e.g.

// Safe UUID v4 implementation without Crypto
generateCorrelationId() {
  if (this.correlationIdFn) {
    return this.correlationIdFn()
  }
  return "xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx".replace(/[xy]/g, (c) => {
    const r = (Math.random() * 16) | 0;
    const v = c === "x" ? r : (r & 0x3) | 0x8;
    return v.toString(16);
  });
}

‼️ Limitations

The async hooks implementation tracks context through EventEmitters, Timers, and Node.js core callbacks. However, as async hooks remain experimental, context propagation should be tested extensively in production like scenarios. In the event that something does not work as expected, there are some untested patterns and suggested work around. Remember it is better to error on the side of caution rather than create a memory leak.

Untested pattern - Process Next Tick 

process.nextTick(() => {
    // context may get lost
    const context = tracker.getContext(); // will create new context
});

Untested pattern - Node.js Core Module Callbacks (like fs, http, etc.):

const fs = require('fs');
fs.readFile('somefile.txt', (err, data) => {
    // context may get lost
    const context = tracker.getContext(); // will create new context
});

Solution to Untested pattern

Despite the untested patterns, these callbacks can be wrapped and bound. Word of caution, Arrow Functions can not be bound therefore, you must use function declaration.

class CorrelationTracker {
    bindCallback(fn) {
        const currentContext = this.getContext();
        return (...args) => {
            const asyncId = asyncHooks.executionAsyncId();
            this.correlationStore.set(asyncId, JSON.parse(JSON.stringify(currentContext)));
            try {
                return fn(...args);
            } finally {
                this.correlationStore.delete(asyncId);
            }
        };
    }
}

// Usage:
emitter.on('someEvent', tracker.bindCallback(() => {
    // context is preserved
    const context = tracker.getContext();
}));
  1. Or use the async_hooks executionAsyncResource when available:
const asyncHooks = require('async_hooks');
const { AsyncResource } = require('async_hooks');

class TrackedEmitter extends EventEmitter {
    emit(event, ...args) {
        const asyncResource = new AsyncResource(event);
        return asyncResource.runInAsyncScope(() => {
            return super.emit(event, ...args);
        });
    }
}

Contributing

Thanks for contributing! 😁 Here are some rules that will make your change to markdown-fences fruitful.

Rules

  • Raise a ticket to the feature or bug can be discussed
  • Pull requests are welcome, but must be accompanied by a ticket approved by the repo owner
  • You are expected to add a unit test or two to cover the proposed changes.
  • Please run the tests and make sure tests are all passing before submitting your pull request
  • Do as the Romans do and stick with existing whitespace and formatting conventions (i.e., tabs instead of spaces, etc)
    • we have provided the following: .editorconfig and .eslintrc
    • Don't tamper with or change .editorconfig and .eslintrc
  • Please consider adding an example under examples/ that demonstrates any new functionality

Commit Message

This module uses release-please which needs commit messages to look like the following Conventional Commits

<type>[optional scope]: <description>

[optional body]

type is typically fix, feat. When type ends with a ! or is BREAKING CHANGE it indicates this is a breaking change.

type should be followed by a short description,

optional body can have more detail

Testing

  • All tests are expected to work
  • Tests are based off of dist/index.js NOT your src code. Therefore, you should BUILD it first.
  • Coverage should not go down, and I acknowledge it is very difficult to get the tests to 100%

License

MIT License

Copyright (c) 2025 Philip A Senger

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Acknowledgments

This project directly uses the following open-source packages:

Dependencies

  • None

Development Dependencies

Readme

Keywords

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Install

npm i @psenger/async-context-id

Repository

github.com/psenger/async-context-id

Homepage

github.com/psenger/async-context-id#readme

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Version

0.1.2

License

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  • psenger
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