This section will show how to use Docker Compose with some small exercises and with a simple Node/Redis app. Before starting, you'll need to have Docker Compose installed.
Compose is a tool for defining and running multi-container Docker applications. With Compose, you use a Compose file to configure your application’s services. Then, using a single command, you create and start all the services from your configuration. To learn more about all the features of Compose see the list of features.
Using Compose is basically a three-step process:
- Define your app’s environment with a
Dockerfileso it can be reproduced anywhere. - Define the services that make up your app in
docker-compose.ymlso they can be run together in an isolated environment. - Lastly, run
docker-compose upand Compose will start and run your entire app.
Check the available commands of Docker Compose. Type in your terminal:
docker-compose
- Whenever you don't remember a command, just type docker-compose
- For more info, type
docker-compose help COMMAND(e.g.docker-compose help build)
The docker-compose.yml file is a YAML file defining services, networks and volumes. The default path for a Compose file is ./docker-compose.yml.
A service definition contains configuration which will be applied to each container started for that service, much like passing command-line parameters to docker run. Likewise, network and volume definitions are analogous to docker network create and docker volume create.
Options specified in the Dockerfile (e.g., CMD, EXPOSE, VOLUME, ENV) are respected by default - you don’t need to specify them again in docker-compose.yml.
We've already created a simple app in code/guestbook-node that uses node.js with express and redis.
-
Go to
code/guestbook-nodefolder -
Review
Dockerfile:
FROM node:7.7.0-alpine
RUN mkdir /code
WORKDIR /code
ADD package.json /code/
RUN npm install
ADD . /code/
CMD ["node", "main.js"]
We use the base image of node:7.7.0-alpine. It is the official image in Alpine, a minimal OS.
The Dockerfile then creates the directory where our code will be stored, /code, and it copies the package.json so it can install the node dependencies.
Afterwards it copies all the code we have in the host machine and runs the command that will keep the container running.
- Review
docker-compose.yml:
version: '2'
services:
redis:
image: redis:alpine
web:
build: .
ports:
- "80:3000"
depends_on:
- redis
The docker-compose.yml file describes the services that make your app. In this example those services are a web server and database. The compose file also describes which Docker images these services use, how they link together, any volumes they might need mounted inside the containers. Finally, the docker-compose.yml file describes which ports these services expose. See the docker-compose.yml reference for more information on how this file works.
In this case, we defined two services, redis that uses redis:alpine and web, our nodejs app. We linked the two of them, and web depends on redis as you can see in depends_on. Also, our nodejs app listens the port 3000 so we linked host's port 80 to the docker container 3000 port.
With docker-compose we can build all the images at once running:
docker-compose build
The docker-compose build reads docker-compose.yml and build all the services defined in there.
We can run a one-time command against a service. For example, the following command starts the web service and runs sh as its command.
docker-compose run web sh
Commands you use with run start in new containers with the same configuration as defined by the service's configuration. This means the container has the same volumes, links, as defined in the configuration file. There two differences though.
First, the command passed by run overrides the command defined in the service configuration. For example, if the web service configuration is started with node, then docker-compose run web sh overrides it with sh.
The second difference is the docker-compose run command does not create any of the ports specified in the service configuration. This prevents the port collisions with already open ports.
We can run docker-compose up that builds, (re)creates, starts, and attaches to containers for a service. Unless they are already running, this command also starts any linked services.
Type in your terminal:
docker-compose up
This instructs Compose to run the services defined in the docker-compose.yml in containers, using the redis image and the web service's image and configuration.
The docker-compose up command aggregates the output of each container. When the command exits, all containers are stopped.
If we want, we can run the containers in background with -d flag:
docker-compose up -d
At this point, your Node app should be running at port 8088 on your Docker host. If you are using a Docker Machine VM, you can use the docker-machine ip MACHINE_NAME to get the IP address.
We can see the log output from services running:
docker-compose logs
If we want to review the logs of a specific service, e.g. web:
docker-compose logs web
We can run ps like in docker ps to list containers and their status:
docker-compose ps
docker-compose stop
Stops running containers without removing them. They can be started again with docker-compose start.
If we want we can stop only one container:
docker-compose stop web
Starts existing containers for a service, e.g. web:
docker-compose start web
docker-compose rm
The previous command removes stopped service containers.
If we want to stop and remove them:
docker-compose down
Update the title of guestbook app adding your name.
The goal of this exercise is:
- Understand the development process
- Understand how to update the code and deploy it
Add a new service like web in the docker-compose.yml to have another application that connects to the existing redis and expose a different port.
The goal of this exercise is:
- Understand how
docker-compose.ymlworks - Understand how to add a new service
- Understand how to expose ports
- Be able to run at the same time the two
webcontainers
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This workshop was prepared by harbur.io, under MIT License. Feel free to fork and improve.