var db = client.db('test');
 var largeObj = {};
 for(var i = 0; i < 10000; i++){
 largeObj['aaaaaaaaaaaaaaaa' + i] = Math.random();
 }
var session = client.startSession();
var v = Date.now();
console.log('init v', v);
var collection = db.collection('test');
//OP1
collection.updateOne({_id: 1}, {$set: Object.assign({v: v + 1}, largeObj)}, {session, upsert: true}, function(){ console.log(new Date(), 'a'); });
//OP2
collection.findOne({_id: 1}, {projection: {v: 1}, session}, function(err, result){ console.log(new Date(), err, result)});
//OP3
collection.updateOne({_id: 1}, {$set: {v: v+ 2}}, {session}, function(){ console.log(new Date(), 'b'); });
//OP4
collection.findOne({_id: 1}, {projection: {v: 1}, session}, function(err, result){ console.log(new Date(), err, result)});
setTimeout( () => {
 db.collection('test').findOne({_id: 1}, {projection: {v: 1}, session}, console.log);
 }, 2000);