package main

import (
	"context"
	"go.mongodb.org/mongo-driver/bson"
	"go.mongodb.org/mongo-driver/mongo"
	"go.mongodb.org/mongo-driver/mongo/options"
	"log"
	"time"
)

func main() {
	opt := options.Client().ApplyURI("mongodb://xxx")
	opt.SetMaxPoolSize(10) // important
	client, err := mongo.NewClient(opt)

	ctx, _ := context.WithTimeout(context.Background(), 10*time.Second)
	if err = client.Connect(ctx); err != nil {
		log.Fatal(err)
	}

	for i := 0; true; i++ {
		startTime := time.Now()

		col := client.Database("xxx").Collection("xxx")
		filter := bson.M{}
		opt := options.FindOptions{}
		opt.SetLimit(1) // important
		ctx, _ = context.WithTimeout(context.Background(), 5*time.Second)
		cur, err := col.Find(ctx, filter, &opt)
		if err != nil {
			log.Fatal(err)
		}
		defer cur.Close(ctx)
		for cur.Next(ctx) {                  // important
		}

		log.Println(i, time.Since(startTime))
	}
}

// output:
//
// 2019/04/20 12:05:30 0 1.255561118s
// 2019/04/20 12:05:30 1 656.780938ms
// 2019/04/20 12:05:31 2 877.063852ms
// 2019/04/20 12:05:32 3 1.024375444s
// 2019/04/20 12:05:33 4 887.392948ms
// 2019/04/20 12:05:34 5 946.072635ms
// 2019/04/20 12:05:35 6 1.049663158s
// 2019/04/20 12:05:36 7 802.969367ms
// 2019/04/20 12:05:37 8 1.039775024s
// 2019/04/20 12:05:42 9 5.000851648s
// 2019/04/20 12:05:47 context deadline exceeded