For now a few days i'm trying to dockerize a new stack we are using at the company.  

But i'm now stuck at a point where this driver running in a docker container doesn't want to connect to a mongodb server that is also running in a docker container.

Here is the error i'm getting when i want to do any interaction with the mongodb server: 

2019/03/14 14:27:30 server selection error: server selection timeout
current topology: Type: Unknown
Servers:
Addr: localhost:27017, Type: Unknown, State: Connected, Avergage RTT: 0, Last error: dial tcp 127.0.0.1:27017: connect: connection refused

 This only happens when the app is inside a docker container.

This is the code I've used to test this: 

package mainpackage main
import (
 "context"
 "fmt"
 "log"
 "go.mongodb.org/mongo-driver/bson"
 "go.mongodb.org/mongo-driver/mongo"
 "go.mongodb.org/mongo-driver/mongo/options"
)

func C() context.Context {
 return context.Background()
}

func main() {
 connectionURL := "mongodb://mongo?authMechanism=SCRAM-SHA-1"
 fmt.Println("Connecting to mongodb server:", connectionURL)
 client, err := mongo.Connect(C(), options.Client().ApplyURI(connectionURL))
 if err != nil {
  log.Panicln(err)
 }
 DB := client.Database("idk")
 _, err = DB.Collection("test").Find(C(), bson.M{})
 if err != nil {
  log.Panicln(err)
 }
}

How i run the database to test this: 

docker run -p 27017:27017 --name mongo -it --rm mongo

Then to run the app i build this docker container:

FROM ubuntu
WORKDIR /
COPY ./testBinary /testBinary
CMD /testBinary

And to run it i do:

docker run --link mongo:mongo -it --rm test-binary

When running it hangs for around 30 seconds and after that it errors with the error specified above.

I've tested this with multiple ways that work.

• When running mongo-express in a docker container works:

  docker run --link mongo:mongo -it --rm -p 8081:8081 mongo-express

• When running the mongoDB server in a container and the app directly on the system (not in a docker container)
• when running the mongoDB server the normal way (no docker containers via systemctl) and the app directly on the system (also not in a docker container)

Some notes:

• This has nothing todo with --link because when running everything in a docker network it has the same issue.