Optimistic Loop Mongo Node + throng

optimisticLoopMongo.js

import throng from 'throng';
import {MongoClient} from 'mongodb';

let dups = 0;

async function insertOptimisticLoop(doc, collection, retry=100){
  const cursor = await collection // find greatest __t progrssive number
    .find( { __t: { $exists: true } }, { __t: 1, __id: -1} )
    .sort( { __t: -1 } ).limit(1);
  const next = await cursor.hasNext() ? (await cursor.next()).__t + 1 : 1;   // calculate next __t
  doc.__t = next;
  let result; try { result = await collection.insert(doc); } catch (error) {
    if( error.code == 11000 /* dup key */ ) {
      dups++;
      return new Promise((resolve,reject)=>setTimeout(()=>resolve(
        insertOptimisticLoop(doc, collection, retry*(Math.random()+1)))
      , retry));
    }
    else console.error(error);
  }
  return result;
}

throng(async function(workerNo){
  console.log('worker', workerNo, 'up');
  const db = await MongoClient.connect('mongodb://localhost:27017/test')
  const collection = db.collection('test');
  await collection.ensureIndex({__t: 1}, {name: '__t', unique: true});
  await Promise.all([insertOptimisticLoop({workerNo, x: 1}, collection),
  insertOptimisticLoop({workerNo, x: 2}, collection),
  insertOptimisticLoop({workerNo, x: 3}, collection),
  insertOptimisticLoop({workerNo, x: 4}, collection),
  insertOptimisticLoop({workerNo, x: 5}, collection),
  insertOptimisticLoop({workerNo, x: 6}, collection),
  insertOptimisticLoop({workerNo, x: 7}, collection),
  insertOptimisticLoop({workerNo, x: 8}, collection),
  insertOptimisticLoop({workerNo, x: 9}, collection),
  insertOptimisticLoop({workerNo, x: 10}, collection)]);
  await db.close();
  console.log('worker', workerNo, 'finish', dups);
})//(1).then(()=>process.exit(0))