Mongodb useUnifiedTopology error

index.js

// mongodb driver 3.5.3
// mongodb instance 3.6.15 and 4.2.0
const MongoClient = require('mongodb').MongoClient
const Work = class Work {
  constructor(db_instance) {
    this.db_instance = db_instance
    this.collection = 'data'
  }

  async run (index) {
    console.info('working', index)
    let opts = {
      readPreference: 'secondary'
    }
    let q = {
      name: 'potato adams',
      foo: 'bar'
    }
    let collection = this.db_instance.collection(this.collection)
    await collection.find(q, opts).toArray()
    console.info('finished', index)
  }
}

const DB = class DB {
  constructor () {
    this.url = 'mongodb://localhost:27017,localhost:27018,localhost:27019/some-db?replicaSet=rs0'
    this.db_name = 'some-db'
  }

  connect() {
    return new Promise((resolve, reject) => {
      const ops = {
        useNewUrlParser: true,
        poolSize: 25,
        useUnifiedTopology: true
      }
      console.log('Connecting to database')

      this.client = new MongoClient(this.url, ops)
      this.client.connect((err) => {
        if (err) {
          console.error('err')
          return reject(err)
        }
        this.db = this.client.db(this.db_name)
        return resolve()
      })
    })
  }
}

const Processor = class Processor {
  constructor() {
    this.db_instance = new DB()
  }

  async run( ) {
    try {
      await this.db_instance.connect()
      let prom = []
      let count = 20
      for (let index = 0; index < count; index++) {
          let w = new Work(this.db_instance.db)
          prom.push(w.run(index))
      }
      await Promise.all(prom)
    } catch (ex) {
      throw ex
    }
  }
}

let processor = new Processor()
processor.run()
.then(() => {
  console.log('finished')
  process.exit(0)
})
.catch((err) => {
  console.error('err', err)
  process.exit(1)
})

output-1-true.txt

Connecting to database
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err { MongoServerSelectionError: Server selection timed out after 30000 ms
    at Timeout.waitQueueMember.timer.setTimeout [as _onTimeout] (/Users/paulrobinson/workspace/akero/investigate/mongoose/native_secondary/node_modules/mongodb/lib/core/sdam/topology.js:428:30)
    at ontimeout (timers.js:436:11)
    at tryOnTimeout (timers.js:300:5)
    at listOnTimeout (timers.js:263:5)
    at Timer.processTimers (timers.js:223:10)
  name: 'MongoServerSelectionError',
  reason:
   TopologyDescription {
     type: 'ReplicaSetWithPrimary',
     setName: 'rs0',
     maxSetVersion: 3,
     maxElectionId: null,
     servers:
      Map {
        'localhost:27017' => [ServerDescription],
        'localhost:27018' => [ServerDescription],
        'localhost:27016' => [ServerDescription] },
     stale: false,
     compatible: true,
     compatibilityError: null,
     logicalSessionTimeoutMinutes: 30,
     heartbeatFrequencyMS: 10000,
     localThresholdMS: 15,
     commonWireVersion: 6 },
  [Symbol(mongoErrorContextSymbol)]: {} }

output-2-true.txt

Connecting to database
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err { MongoServerSelectionError: Server selection timed out after 30000 ms
    at Timeout.waitQueueMember.timer.setTimeout [as _onTimeout] (/Users/paulrobinson/workspace/akero/investigate/mongoose/native_secondary/node_modules/mongodb/lib/core/sdam/topology.js:428:30)
    at ontimeout (timers.js:436:11)
    at tryOnTimeout (timers.js:300:5)
    at listOnTimeout (timers.js:263:5)
    at Timer.processTimers (timers.js:223:10)
  name: 'MongoServerSelectionError',
  reason:
   TopologyDescription {
     type: 'ReplicaSetWithPrimary',
     setName: 'rs0',
     maxSetVersion: 3,
     maxElectionId: null,
     servers:
      Map {
        'localhost:27017' => [ServerDescription],
        'localhost:27018' => [ServerDescription],
        'localhost:27016' => [ServerDescription] },
     stale: false,
     compatible: true,
     compatibilityError: null,
     logicalSessionTimeoutMinutes: 30,
     heartbeatFrequencyMS: 10000,
     localThresholdMS: 15,
     commonWireVersion: 6 },
  [Symbol(mongoErrorContextSymbol)]: {} }

output-3-false.txt

Connecting to database
(node:31692) DeprecationWarning: current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
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finished