Continuations in Node

app.js

const { createNamespace } = require('continuation-local-storage')
const createUUID = require('uuid/v1')

// Application
const logger = require('./logger')
const performSomeProcessing = require('./processing')

// Establish a new storage area for the continuation
const context = createNamespace('context')

function ingestMessageFromKafka(message) {
  context.set('uuid', `O-${createUUID()}`)
  
  // The logger automatically tags the message with the UUID
  logger.info("Doing something with the offer")
  
  // Methods called inside this method will also be aware of the context,
  // but the context is scoped within the call stack (this applies to async
  // callbacks, too)
  performSomeProcessing()
}

context.run(ingestMessageFromKafka, { type: "offer", id: 432 })

// Outputs:
// [info] [O-10726890-00a4-11e8-a730-1143f88b51b3] Doing something with the offer
// [info] [O-10726890-00a4-11e8-a730-1143f88b51b3] Performing some processing

logger.js

const { getNamespace } = require('continuation-local-storage')

function formatTags(...tags) {
  return tags.map(tag => `[${tag}]`).join(' ')
}

module.exports = {
  device: console,

  info(...args) {
    const uuid = getNamespace('context').get('uuid')
    const tags = ['info', uuid]
    this.device.log(formatTags(...tags), ...args)
  }
}

processing.js

const logger = require('./logger')

module.exports = function() {
  // Will still be tagged with the context, even though this file knows nothing about the tags
  logger.info("Performing some processing")
}