Pronoy Chopra

Pronoy Chopra

Developer/Engineer

© Pronoy Chopra 2020

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Docker Notes - Part 3

Multi container applications. You need multiple docker containers to do your bidding because every component might need to be scaled. And obviously because you hate yourself so you resort to automating this stuff that takes you longer than it would if you were to use bare deployments using Ansible.

Anyway, so docker-compose. Multiple containers in one file and how they work with each other. Docker-compose is going to orchestrate these containers on a shared network. Here’s an example of a js app working with redis via one docker-compose.yml

version: '3'
services: 
    redis-server:
        image: 'redis' # use redis image

    visits:
        build: .
        ports:
            - "4001:8081" # host:container

Now to build them - docker-compose build and then to run them docker-compose up.

To daemonize them so that they run in background docker-compose up -d

To bring them down docker-compose down

Error handling within containers

Stuff crashes all the time so there has to be a way to restart the container if it fails for some reason.

So Docker has restart policies in place according to the exit code it receives.

'no': "Never attempt to restart"
always: "If container stops for any reason - restart it"
on-failure: Only restart if stopped with an error code
unless-stopped: always restart till forced to stop (manually)

We just add the restart key under the container to instate the police

version: '3'
services: 
    redis-server:
        image: 'redis' # use redis image

    visits:
        restart: always
        build: .
        ports:
            - "4001:8081" # host:container

This will restart no matter what.

Checking status

So just like docker ps we have docker-compose ps as well. But just like the other commands it needs to be run in the directory where you have docker-compose.yml for a targetted status

Docker compose build context

Oh dear lord. Why does Docker do this? One of the dumbest things I’ve seen in any util. Let us specify context manually - why is there a wall around where I can send the files from?

Here’s the directory structure:

    parent_directory
        src_code1
        src_code2
        src_code3
        docker_stuff
             project
                Dockerfile
                .dockerignore
                docker-compose.yml

Anyway, here’s how to specify context and a custom Dockerfile

version: '3'
services: 
    redis-server:
        image: 'redis' # use redis image
    visits:
        restart: always
        build:
            context: ../..
            dockerfile: ${PWD}/Dockerfile

        ports:
            - "4001:8081" # host:container

Apparently according to this post dockerfile is relative to context. If that were true this wouldn’t have worked $PWD/Dockerfile

One aspect of Docker is that it builds everything serially. However that can be sped up by using something called BuildKit

BuildKit brings concurrency into the game and it’s probably the best way to build containers. However, we need a bit of a hack to pass the actual BuildKit environment variable via the cli. That’s how to pass it through to docker-compose

COMPOSE_DOCKER_CLI_BUILD=1 DOCKER_BUILDKIT=1 docker-compose build