Saturday, 13 July 2019

Introduction to Docker Compose

In this post, we will see how to manager multiple container lifecycle using docker-compose. Let us understand what is docker compose.

Introduction

Docker compose is a tool provided by docker to manage multiple containers within a single host. So in case you want to create / start  / stop/ remove / scale up / scale down multiple containers from a single command, docker compose comes handy. But if you are looking for managing multiple containers within multiple host, consider using docker swarm / kubernetes. Docker compose manages container lifecycle within a single host. If your containerized application is hosted on multiple node in non-clustered mode, then you need to use another copy of docker-compose on another host. However I would suggest to look for docker swarm or kubernetes for that to manage it from single point.

What docker compose can do for you?

Docker compose can automate your container deployment, re-deployment, undeployment. It is not a tool to solely create docker image (docker build used to create docker image)

Installation of Docker Compose
If you are using Windows or Mac, docker compose is already installed as it comes in Docker toolbox. But in case of Linux, We need to first install docker compose.

YAML Configuration file
Docker compose provides a configuration file docker-compose.yml where in we need to write yaml script to manager container lifecycle.  Here is the simple example of docker-compose.yml with instruction

Docker-compose Example
Docker Compose Example

Let me explain further on it.
version : It indicates compose version number
services: This indicates docker-compose that below is the list of services that needs to be containerized
<service-name>: Name of the service for reference purpose
build: Path to docker file from where image to be build to be used to create container
ports: port mapping from host to container
volumes: volume mapping from host to container
image: image name to be used to create container

Docker compose commands:

So you now have docker-compose.yml and want to manage lifecycle of containers through it. We can create, start, stop, destroy, scale using the same docker-compose.yml file. 
Here is the list of commands:
  • docker-compose up - It creates container (if required) and also run the container. Use it with this option ( -d ) to run this daemon in background
  • docker-compose down - Just opposite of up command, it stops all the containers and also removes them.
  • docker-compose start - It starts the container. Please note that if the container does not exist, it will not create a new container. It just starts the stopped container listed in docker-compose.yml
  • docker-compose stop - It stops the running container. It goes through each service mentioned in docker-compose.yml and tries to stop the started container.
  • docker-compose rm - It deletes the stopped container. use it with -f to force delete the container.
  • docker-compose scale - It set the number of containers per service. We can both scale up and scale down the number of containers per service using the same command
  • docker-compose exec - It run the command inside the container. You need to pass container id along with command. For eg. docker-compose exec -i <container_id> ls / home
  • docker-compose pause - It pause the services. This is different from stop in the way that it is like sleeping for sometime and when resume it continues from the point where it is paused. Stop service will kill the container running thread and it will start from the scratch. For example, you have application that prints 2 line of statement. if you pause after printing first line of statement, unpause will continue from there and it will print the second line. In case of stop after first line and restart, it will again print both first and second line of statement
  • docker-compose unpause - Resumes the paused services.
  • docker-compose port - It prints the public port for port binding
  • docker-compose build - It build or rebuild services
  • docker-compose bundle - It generates a docker bundle from compose file
  • docker-compose config - It validates the docker compose file
  • docker-compose create - It creates the services
  • docker-compose images - It list the docker images
  • docker-compose logs - View container output
  • docker-compose top - It views the running container
  • docker-compose version - It displays the version of docker compose installed on the host
  • docker-compose help - It gets help on a command
Thats all for short introduction to docker compose. If you are using Ansible, you can manage containers lifecycle using docker_compose module. For any query, please mention in comments section. Thanks!!

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