How to connect 2 docker compontens within the same docker-compose.yaml

Question

I am new to the docker world and I have some issues regarding how to connect 2 docker services tougher.

I am using https://memgraph.com/ as my database and when I am running it locally I am running it like this

docker run -it -p 7687:7687 -p 3000:3000 memgraph/memgraph-platform

I wrote my program which is going to connect to the database using mgclient and when I am running it locally everything is working fine.

Now I am trying to put in inside the docker container and running it using docker-compose.yaml

My docker-compose.yaml is:

version: "3.5"
services:
  memgraph:
    image: memgraph/memgraph-platform:2.1.0
    container_name: memgraph_container
    restart: unless-stopped
    ports:
      - "7687:7687"
      - "3000:3000"
  my_app:
    image: memgraph_docker
    container_name: something
    restart: unless-stopped
    command: python main.py

and when I am trying to run it with this command:

docker-compose up

I am getting an error regarding the connection to the server. Could anyone tell me what I am missing regarding the docker-compose.yaml?

Answer 1

How does your my_app connect to the database?

Are you using a connection string of the form localhost:7687 (or perhaps localhost:3000)? This would work locally because you are publishing (--publish=7687:7687 --publish=3000:3000) the container's ports 7687 and 3000 to the host port's (using the same ports).

NOTE You can remap ports when your docker run. For example, you could --publish=9999:7686 and then you would need to use port 9999 on your localhost to access the container's port 7687.

When you combine the 2 containers using Docker Compose, each container is given a name that matches the service name. In this case, your Memgraph database is called memgraph (matching the service name).

Using Docker Compose, localhost takes on a different mean. From my_app, localhost is my_app. So, using localhost under Docker Compose, my_app would try connecting to itself not the database.

Under Docker Compose, for my_app (the name for your app), you need to refer to Memgraph by its service name (memgraph). The ports will be unchanged as both 7687 and 3000 (whichever is correct).

NOTE The ports statement in your Docker Compose config is possibly redundant *unless you want to be able to access the database from your (local)host (which you may for debugging). From a best practice standpoint, once my_app is able to access the database correctly, you don't need to expose the database's ports to the host.

Update

It is good practice to externalize configuration (from your app). So that you can configure your app dynamically. An easy way to do this is to use environment variables.

For example:

main.py:

import os


conn = connect(
    host=os.getenv("HOST"),
    port=os.getenv("PORT"),
)

Then, when you run under e.g. Docker, you need to set these values:

docker run ... --env=HOST="localhost" --env=PORT="7687" ...

And under Docker Compose, you can:

version: "3.5"
services:
  memgraph:
    image: memgraph/memgraph-platform:2.1.0
    container_name: memgraph_container
    restart: unless-stopped

  my_app:
    image: memgraph_docker
    container_name: something
    restart: unless-stopped
    command: python main.py
    environment:
      HOST: memgraph
      PORT: 7687