Tips on docker-compose

Docker is a technology to create containers. What if we have multiple contains and need them to work together? Docker compose comes to rescue. The configuration is a yaml file and it's easy to follow. I'm going to write down some tips when I learned to use it.

Keep the Docker Container Running

The docker container is supposed to run only one process. The container stops when the process exits. So if we have a process that runs in the forground, it'll most likely exit because there is no console or input. For this kind of container, usually we run the command docker run -ti <image name> to keep the container running. In docker-compose world, you need to put this in the configuration for the service.

tty: true
stdin_open: true

Expose the Network Ports

In running docker, we can use docker -P to expose all the ports specified in the Dockerfile. We can also use docker -p [] to expose the specified ports in the command line. However, docker -P doesn't guarantee to map those ports from the container to the same ports on the host. The mapping is random. We can explicitly set those mapping using docker -p []. docker-compose by default will create a bridge for the specified containers. Depending on how we write it, we can expose the port only to that bridge network or also in the host.

expose:
- "8080"

This will only expose the ports to the linked services.

ports:
- "8080:80"

This also expose the ports on the host. It maps the port 8080 on host to the port 80 on the container.

Connect to the Linked Service in a Container

We specify the service in the configuration. docker-compose will bring up one or multiple container running the same thing for the service. Even if you don't specify the networks, docker-compose still creates a default one for you. You can find all the network by docker network ls. The name of the default network for your docker-compose containers is derived from the directory of the docker-compose.yml file. For example, docker-compose.yml is in the directory 'example'. The default network for those containers is 'example_default'. This may change but this is the behavior right now.

How do you connect to other containers brought up by docker-compose? There are two ways:

1. Use the service name. For example, in docker-compose.yml, you specify the service like

   services:
   foo:
   image: 'centos:latest'
   bar:
   image: 'ubuntu:latest'

   In the container that runs Centos, you can use bar to connect to the other container that runs Ubuntu.

2. Use the ip address.
   When you find the network for your containers, you can use docker network inspect <network name> to inspect the details of the network. From there, you can find the ip address of the containers. For example:

   {
       "Name": "example_default",
       "Containers": {
           "4a87be92f610b839c77f0fa87c7bdd13797ca4eac3c2061a4d8e66a1c5e9c867": {
               "Name": "example_foo_1",
               "EndpointID": "0752c01405904b702459560fcc1bc90ef0e24d3e769e8bc46d5edcf06944dba1",
               "MacAddress": "02:42:ac:15:00:03",
               "IPv4Address": "172.21.0.3/16",
               "IPv6Address": ""
           },
           "6244c44cab85df31d684abb5e0b2ce12ecfbef902e13c638d1ab50981b876142": {
               "Name": "example_bar_1",
               "EndpointID": "5b0edb758b080010f8c3fca92e5cb583724c9fefcc23c5148c8c3375517d0093",
               "MacAddress": "02:42:ac:15:00:02",
               "IPv4Address": "172.21.0.2/16",
               "IPv6Address": ""
           }
       },

By far, those are the things I think it's useful when using docker-compose. I spent some time on searching online. I hope this can help you to try out the way to manage containers.