Performance implication of (mongo.Client).Ping

I’ve recently noticed some odd behavior while testing the performance of writing or reading to a MongoDB serveless cluster.

I don’t think is related to the serverless option, but as a context I’m using a new serverless cluster in AWS (Ireland region) which I interact with from my local machine.

Some demo code:

package main

import (
	"context"
	"fmt"
	"log"
	"time"

	"go.mongodb.org/mongo-driver/mongo"
	"go.mongodb.org/mongo-driver/mongo/options"
)

type Item struct {
	Value string `bson:"value"`
}

func main() {
	ctx := context.Background()
	client, err := mongo.Connect(ctx, options.Client().ApplyURI("add-url"))
	if err != nil {
		log.Fatalln(err)
	}

	err = client.Ping(ctx, nil)
	if err != nil {
		log.Fatalln(err)
	}

	collection := client.Database("test").Collection("test")
	item := Item{
		Value: "foo",
	}
	start := time.Now()
	_, err = collection.InsertOne(ctx, item)
	if err != nil {
		log.Fatalln(err)
	}

	fmt.Println("Item added in", time.Since(start))
}

This works fine, producing results of about 60ms consistently.

However, if I remove the err = client.Ping(ctx, nil) part the times jump to over 500ms consistently. Why does this happen?

Not much info I get from the docs.

From the documentation of NewClient method:

The NewClient function does not do any I/O and returns an error if the given options are invalid. The Client.Connect method starts background goroutines to monitor the state of the deployment and does not do any I/O in the main goroutine to prevent the main goroutine from blocking. Therefore, it will not error if the deployment is down.

The Client.Ping method can be used to verify that the deployment is successfully connected and the Client was correctly configured.

After you remove Ping instruction, InsertOne takes longer due to the overhead of creating the connection to the server. You may try to measure how long Ping command takes and it should be around 500ms.