express-rtracer

express-rtracer

Express Request Tracer - a middleware for CLS-based request id generation, batteries included. An out-of-the-box solution for adding request id into your logs.

Automatically generates a UUID value as the id for each request and stores it in Continuation-Local Storage (CLS, see cls-hooked). If the request contains X-Request-Id header, uses its value instead. Allows to obtain the generated id anywhere in your routes later and use it for logging or any other purposes.

How to use it

Install:

npm install --save express-rtracer

Use the middleware provided by the library before the first middleware that needs to have access to request ids. Note that some middlewares, e.g. body-parser or express-jwt, may cause CLS context to get lost. To avoid such issues, you should use any third party middleware that does not need access to request ids before you use this middleware.

const express = require('express')
const rTracer = require('express-rtracer')

const app = express()
// any third party middleware that does not need access to request ids goes here
// ...

app.use(rTracer.middleware())
// you can override default middleware config:
// app.use(rTracer.middleware({
//   headerName: 'X-Your-Request-Header'
// }))

// all code starting from here has access to request ids

Obtain request id in middlewares on the incoming request:

// an example middleware for a generic find entity endpoint
app.get('/api/v1/entity/{id}', (req, res, next) => {
  entityService.find(req.params.id)
    .then((entity) => {
      // you can obtain the request id here
      const requestId = rTracer.id()
      logger.log(`requestId: ${requestId}`)
      
      res.json(entity)
    })
    .catch(next)
})

You can access the same request id from code that does not have access to the Express req object.

// an imaginary entity-service.js
async function find (entityId) {
  // you can obtain the request id here
  const requestId = rTracer.id()
  // ...
}

Integration with loggers

The main use case for this library is request id generation and logging automation. You can integrate with any logger library in a single place and get request ids in logs across your Express application.

Without having a request id, as a correlation value, in your logs, you will not be able to determine which log entries belong to the process of handling the same request. You could generate a request id manually and store it in the Express req object, but then you will have to explicitly pass req into all other modules on the route. And express-rtracer comes to the rescue!

Let's consider integration with winston, one of most popular logging libraries.

const { createLogger, format, transports } = require('winston')
const { combine, timestamp, printf } = format

// a custom format that outputs request id
const rTracerFormat = printf((info) => {
  const rid = rTracer.id()
  return rid
    ? `${info.timestamp} [request-id:${rid}]: ${info.message}`
    : `${info.timestamp}: ${info.message}`
})

const logger = createLogger({
  format: combine(
    timestamp(),
    rTracerFormat
  ),
  transports: [new transports.Console()]
})

A complete example is available in /examples/winston.js file.

Middleware configuration

These are the available config options for the middleware(options) function. Options are optional.

{
  // Respect request header flag (default: true).
  // If set to true, the middleware will be using a value from the specified header (if the value is present).
  useHeader: true,
  // Request header name, case insensitive (default: X-Request-Id).
  // Used if useHeader is set to true.
  headerName: 'X-Request-Id'
}

Troubleshooting

To avoid weird behavior with Express:

• Make sure you require express-rtracer as the first dependency in your app. Some popular packages may use async which breaks CLS.

For Node 10 users:

• Node 10.0.x-10.3.x is not supported. That's because V8 version 6.6 introduced a bug that breaks async_hooks during async/await. Node 10.4.x uses V8 v6.7 where the bug is fixed. See: https://github.com/nodejs/node/issues/20274.

License

Licensed under MIT.