How to run a command once in Docker compose

Question

So I'm working on a docker compose file to deploy my Go web server. My server uses mongo, so I added a data volume container and the mongo service in docker compose. Then I wrote a Dockerfile in order to build my Go project, and finally run it.

However, there is another step that must be done. Once my project has been compiled, I have to run the following command: ./my-project -setup

This will add some necessary information to the database, and the information only needs to be added once. I can't however add this step on the Dockerfile (in the build process) because mongo must already be started.

So, how can I achieve this? Even if I restart the server and then run again docker-compose up I don't want this command to be executed again.

I think I'm missing some Docker understanding, because I don't actually understand everything about data volume containers (are they just stopped containers that mount a volume?). Also, if I restart the server, and then run docker-compose up, which commands will be run? Will it just start the same container that was now stopped with the given CMD?

In any case, here is my docker-compose.yml:

version: '2'
services:
  mongodata:
    image: mongo:latest
    volumes:
      - /data/db
    command: --break-mongo
  mongo:
    image: mongo:latest
    volumes_from:
      - mongodata
    ports:
      - "28001:27017"
    command: --smallfiles --rest --auth
  my_project:
    build: .
    ports:
      - "6060:8080"
    depends_on:
      - mongo
      - mongodata
    links:
      - mongo

And here is my Dockerfile to build my project image:

FROM golang

ADD . /go/src/my_project
RUN cd /go/src/my_project && go get
RUN go install my_project
RUN my_project -setup
ENTRYPOINT /go/bin/my_project

EXPOSE 8080

Answer 1

I suggest to add an entrypoint-script to your container; in this entrypoint-script, you can check if the database has been initialized, and if it isn't, perform the required steps.

As you noticed in your question, the order in which services / containers are started should not be taken for granted, so it's possible your application container is started before the database container, so the script should take that into account.

As an example, have a look at the official WordPress image, which performs a one-time initialization of the database in it's entrypoint-script. The script attempts to connect to the database (and retries if the database cannot be contacted (yet)), and checks if initialization is needed; https://github.com/docker-library/wordpress/blob/df190dc9c5752fd09317d836bd2bdcd09ee379a5/apache/docker-entrypoint.sh#L146-L171

NOTE

I notice you created a "data-only container" to attach your volume to. Since docker 1.9, docker has volume management, including naming volumes. Because of this, you no longer need to use "data-only" containers.

You can remove the data-only container from your compose file, and change your mongo service to look something like this;

  mongo:
    image: mongo:latest
    volumes:
      - mongodata:/data/db
    ports:
      - "28001:27017"
    command: --smallfiles --rest --auth

This should create a new volume, named mongodata if it doesn't exist, or re-use the existing volume with that name. You can list all volumes using docker volume ls and remove a volume with docker volume rm <some-volume> if you no longer need it