Docker and mongo-go-driver "server selection error"

Question

I have a MongoDB replica set up and running using Docker and I can access through console, or Robo3T client, in order to run my queries.

These are the containers:

$ docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                      NAMES
efe6ae03323d        mongo               "docker-entrypoint.s…"   10 minutes ago      Up 10 minutes       0.0.0.0:30001->27017/tcp   mongo1
57d2701c8a43        mongo               "docker-entrypoint.s…"   10 minutes ago      Up 10 minutes       0.0.0.0:30002->27017/tcp   mongo2
7553966b9ff5        mongo               "docker-entrypoint.s…"   10 minutes ago      Up 10 minutes       0.0.0.0:30003->27017/tcp   mongo3

The problem is an error when I try to make a ping using the mongo-go-driver (I tried with version 1.0.0 and 1.0.2)

// Create MongoDB client    
client, err := mongo.NewClient(options.Client().ApplyURI("mongodb://localhost:30001"))
if err != nil {
    t.Fatalf("Exit error: %v", err)
}

ctx, cancel := context.WithTimeout(context.Background(), time.Minute)
defer cancel()
err = client.Connect(ctx)
if err != nil {
    t.Fatalf("Exit error: %v", err)
}

ctx, cancel = context.WithTimeout(context.Background(), time.Minute)
defer cancel()

// Ping
err = client.Ping(ctx, readpref.Primary())
if err != nil {
    t.Fatalf("Exit error Ping: %v", err)
}

the error is the following:

Exit error Ping: server selection error: server selection timeout
        current topology: Type: ReplicaSetNoPrimary
        Servers:
        Addr: mongo2:27017, Type: Unknown, State: Connected, Average RTT: 0, Last error: dial tcp: lookup mongo2: no such host
        Addr: mongo3:27017, Type: Unknown, State: Connected, Average RTT: 0, Last error: dial tcp: lookup mongo3: no such host
        Addr: mongo1:27017, Type: Unknown, State: Connected, Average RTT: 0, Last error: dial tcp: lookup mongo1: no such host

Answer 1

This is due to unresolved hostname from Docker host. In Docker, the instances mongo1, mongo2, and mongo3 are reachable by those names. However, these names are not reachable from the Docker host. This is evident by this line:

Addr: mongo2:27017, Type: Unknown, State: Connected, Average RTT: 0, Last error: dial tcp: lookup mongo2: no such host

MongoDB driver will attempt server discovery from given a replica set member(s); it will find all of other nodes within the replica set (via rs.conf). The problem here is the replica set is set with name mongo<N>, the driver (run in Docker host) would not be able to resolve these names. You can confirm this by trying to ping mongo1 from Docker host.

You can either try running the application from another Docker instance sharing the same Docker network as the replica set. Or, modify the Docker networking as such to allow resolvable hostnames.

UPDATE:

Regarding your comment on why using mongo shell, or PyMongo works.

This is due to the difference in connection mode. When specifying a single node, i.e. mongodb://node1:27017 in shell or PyMongo, server discovery are not being made. Instead it will attempt to connect to that single node (not as part as a replica set). The catch is that you need to connect to the primary node of the replica set to write (you have to know which one). If you would like to connect as a replica set, you have to define the replica set name.

In contrast to the mongo-go-driver, by default it would perform server discovery and attempt to connect as a replica set. If you would like to connect as a single node, then you need to specify connect=direct in the connection URI. See also Example Connect Direct