What's the simplest way to connect two containers without Docker compose?

Question

Considering the fact I cannot use Docker compose and a very simple Docker setup:

• a NodeJs app (container A)
• a service exposing a REST api (container B)

How can I connect to B from A to query the API without SSHing but just in appending stuff to docker run... ?

I've succeeded in getting internal ip of B (using docker container inspect B) and requesting from A something like fetch("http://ip-of-b/api"). It works but it's not well-down. Indeed if I make another deploy, B ip changes.

Is there a very simple way to make a production bridge between these 2 containers? I've read about --add-host=host.docker.internal:host-gateway but I can't understand how it works/it's relevant.

PS: in the future, I might have a third container (C) that must also be able to connect to A.

Answer 1

I've succeeded in getting internal ip of B (using docker container inspect B) and requesting from A something like fetch("http://ip-of-b/api").

That solution is problematic because container ips are dynamic; if you were to restart one of the two containers, you would no longer be able to connect. You want to use container names.

It's important to realize that Docker Compose is just a convenient wrapper around the docker API; you can accomplish exactly the same thing using docker run. When you bring up a Docker Compose application stack, Compose starts by creating a user-defined network, and then it subsequently attaches all the containers to that network. That's important because containers on a user defined network are able to refer to eachother by name.

We can do the same thing with docker run. First create a network:

docker network create mynetwork

Then launch one container attached to that network:

docker run --name app1 --network=mynetwork app1-image

And then launch a second container:

docker run --name app2 --network=mynetwork app2-image

Now within the app1 container you can use the hostname app2 to refer to the second container, and in the app2 container you can use the hostname app1 to refer to the first container. Here's a simple, runnable example:

docker network create mynetwork
docker run -d --name app1 --network mynetwork docker.io/alpinelinux/darkhttpd
docker run -d --name app2 --network mynetwork docker.io/alpinelinux/darkhttpd
docker run -d --name app3 --network mynetwork docker.io/alpinelinux/darkhttpd

Taking container app1 as an example, we can successfully access our services the appropriate container names. The following commands all run successfully:

docker exec app1 wget -O- http://app1:8080
docker exec app1 wget -O- http://app2:8080
docker exec app1 wget -O- http://app3:8080

You can read more about this in the Docker networking documentation.