"Server Selection Timeout Error" MongoDB Go Driver with Docker

Question

I'm working on a very basic (I thought) starter program in Go using MongoDB and Docker. Trying to get a handle on these before we start using them at work. I've got my MongoDB running in a docker container, just using my local host, using the official Docker image. This is running fine, I can connect to it through MongoDB Compass and modify the DB.

My next task was to build a separate Docker container that is able to read and write to the DB. I'm using MongoDB-Go-Driver (https://godoc.org/github.com/mongodb/mongo-go-driver/mongo) for this as mgo is no longer kept up.

This is my code, I'm just following the numerous tutorials online to make a simple connection and then ping the DB to ensure connectivity.

client, err := mongo.Connect("mongodb://localhost:27017")

if err != nil {
    log.Fatal("error ", err)
}

// Check the connection
err = client.Ping(context.TODO(), nil)

if err != nil {
    log.Fatal("error2 ", err)
}

fmt.Println("Connected to MongoDB!")

It always fails on doing any operation on the DB (Find, FindOne, Ping, etc.) with error2 server selection timeout

This is my docker-compose file I'm running.

version: "3"

services:
  datastore:
    image: mongo
    ports: 
      - "27017:27017"
    networks: 
      - maccaptionNet
    volumes: 
      - .:/go/src/maccaption_microservice/dbdata
  jobservice:
    image: jobservicemaccaption:1.0
    networks:
      - maccaptionNet
    depends_on:
      - "datastore"


networks: 
  maccaptionNet:
    driver: bridge

I'm brand new to MongoDB and after hours of research haven't made any progress on this. I've read through https://docs.mongodb.com/manual/core/read-preference-mechanics/ https://docs.mongodb.com/manual/replication/

Can anyone point me in the right direction for this? I haven't been able to find a lot on this specific issue.

Thanks!

Answer 1

When you running the service and mongodb in docker you can't use localhost since the service is in a different container than mongodb, and from docker point of view it's under a different ip address.

You can connect with the service name you specify in docker-compose datastore

mongo.Connect("mongodb://datastore:27017")

Edit:

from: https://docs.docker.com/compose/networking/

By default Compose sets up a single network for your app. Each container for a service joins the default network and is both reachable by other containers on that network, and discoverable by them at a hostname identical to the container name

Meaning that if you run multiple containers via compose, you can access one container from the other by the container name,

Basically when docker-compose starts, it sets up the network, and each container in the compose joins the network under its container name. For a container's point if view, localhost is just the container itself, while he can search for other container's name and get back the container’s IP address.

Assuming that the docker is running on your localhost, you can set the name in etc/hosts file like this:

127.0.0.1 datastore

(if not just replace 127.0.0.1 with the docker ip)

And in the app you will connect with mongodb://datastore:27017

So you will be able to run the service both in the docker and from outside, if you'll decide to run only the db in docker

docker-compose start datastore

Answer 2

If you are connecting to one docker from another (like it is written in your docker-compose file, and using bridge network mode, you have to change your localhost to the hostname, like datastore

client, err := mongo.Connect("mongodb://datastore:27017")

When your go script uses localhost, it expects the database to located in the same docker

Answer 3

I think my answer might be unrelated but still, I was getting the same error and it was because my IP address was not listed in the IP whitelist tab in MongoDB atlas, so make sure you have your IP address there before trying to connect.

Answer 4

I had the same problem but found another way to address this issue. You can just pass network parameter while running docker image and this way docker points to correct localhost.

docker run --network="host" ....

Source for this solution