overleaf/services/document-updater/test/cluster_failover/js/test_blpop_failover.js

let listenInBackground, sendPings
const redis = require('@overleaf/redis-wrapper')
const rclient1 = redis.createClient({
  cluster: [
    {
      port: '7000',
      host: 'localhost',
    },
  ],
})

const rclient2 = redis.createClient({
  cluster: [
    {
      port: '7000',
      host: 'localhost',
    },
  ],
})

let counter = 0
const sendPing = function (cb) {
  if (cb == null) {
    cb = function () {}
  }
  return rclient1.rpush('test-blpop', counter, error => {
    if (error != null) {
      console.error('[SENDING ERROR]', error.message)
    }
    if (error == null) {
      counter += 1
    }
    return cb()
  })
}

let previous = null
const listenForPing = cb =>
  rclient2.blpop('test-blpop', 200, (error, result) => {
    if (error != null) {
      return cb(error)
    }
    let [, value] = Array.from(result)
    value = parseInt(value, 10)
    if (value % 10 === 0) {
      console.log('.')
    }
    if (previous != null && value !== previous + 1) {
      error = new Error(
        `Counter not in order. Got ${value}, expected ${previous + 1}`
      )
    }
    previous = value
    return cb(error, value)
  })

const PING_DELAY = 100
;(sendPings = () => sendPing(() => setTimeout(sendPings, PING_DELAY)))()
;(listenInBackground = () =>
  listenForPing(error => {
    if (error) {
      console.error('[RECEIVING ERROR]', error.message)
    }
    return setTimeout(listenInBackground)
  }))()