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The Compose is a tool for defining and running multi-container Docker applications. With Compose, you use a YAML file to configure your application’s services. Then, with a single command, you create and start all the services from your configuration.
Compose works in all environments: production, staging, development, testing, as well as CI workflows.
Using Docker Compose is basically a three-step process:
1. Define your app’s environment with a Dockerfile so it can be reproduced anywhere.
2. Define the services that make up your app in docker-compose.yml so they can be run together in an isolated environment.
3. Run docker-compose up and Compose starts and runs your entire app.
Common use cases of Docker Compose:
Compose can be used in many different ways. Some common use cases are outlined below.
-
Development environments
When you’re developing software, the ability to run an application in an isolated environment and interact with it is crucial. The Compose command-line tool can be used to create the environment and interact with it.
The compose-file provides a way to configure all of the application’s service dependencies (databases, queues, caches, web service APIs, etc). Using the Compose command-line tool you can create and start one or more containers for each dependency with a single command ( docker-compose up).
Together, these features provide a convenient way for developers to get started on a project. Compose can reduce a multi-page “developer getting started guide” to a single machine-readable compose-file and a few commands.
-
Automated testing environments
An important part of any Continuous Deployment or Continuous Integration process is the automated test suite. Automated end-to-end testing requires an environment in which to run tests. Compose provides a convenient way to create and destroy isolated testing environments for your test suite. By defining the full environment in a compose-file, you can create and destroy these environments in just a few commands.
-
Single host deployments
Compose has traditionally been focused on the development and testing workflows, but with each release, docker-compose is making progress on more production-oriented features.
Let’s build a web application by using docker-compose;
We actually can run two containers with a single command in a single host that is shown above and below;
Let’s prepare our structure step by step;
1.Step: Spin up an AWS EC2 instance that will provide permission to SSH from 22 port and also open 6379 and 5000 ports from the security groups.
sudo yum update -y
sudo amazon-linux-extras install docker -y
sudo systemctl start docker
sudo systemctl enable docker
sudo usermod -a -G docker ec2-user
3.Step: For Linux systems, after installing Docker, you need to install Docker Compose separately. But, Docker Desktop App for Mac and Windows includes “Docker Compose” as a part of those desktop installs. We will download the current stable release of the “Docker Compose” executable.
sudo curl -L
"https://github.com/docker/compose/releases/download/1.26.2/docker-compose-$(uname -s)-$(uname -m)" \
-o /usr/local/bin/docker-compose
Apply executable permissions to the binary.
sudo chmod +x /usr/local/bin/docker-compose
Check if the “Docker Compose” is working.
docker-compose --version
If you are getting the same result that is shown below, you successfully downloaded Docker Compose.
4.Step: Building a web application by using Docker Compose; create a folder for the project and work under that folder.
5. Step: Create a file called `app.py` in your project folder and paste the following python code in it. In this example, the application uses the Flask framework and maintains a hit counter in Redis, and `redis` is the hostname of the `Redis container` on the application’s network. We will use the default port for Redis, `6379`.
import timeimport redis
from flask import Flaskapp = Flask(__name__)
cache = redis.Redis(host='redis', port=6379)def get_hit_count():
retries = 5
while True:
try:
return cache.incr('hits')
except redis.exceptions.ConnectionError as exc:
if retries == 0:
raise exc
retries -= 1
time.sleep(0.5)@app.route('/')
def hello():
count = get_hit_count()
return 'Hello World! I have been seen {} times.\n'.format(count)
6. Step: Create another file called `requirements.txt` in your project folder, add `flask` and `redis` as package list.
echo '
flask
redis
' > requirements.txt
7. Step: Create a Dockerfile which builds a Docker image in your project folder.
The image contains all the dependencies for the application, including Python itself.:
* Build an image starting with the Python 3.7 image.
* Set the working directory to `/code`.
* Set environment variables used by the flask command.
* Install gcc and other dependencies
* Copy `requirements.txt` and install the Python dependencies.
* Add metadata to the image to describe that the container is listening on port 5000
* Copy the current directory `.` in the project to the workdir `.` in the image.
* Set the default command for the container to flask run.
echo '
FROM python:3.7-alpine
WORKDIR /code
ENV FLASK_APP app.py
ENV FLASK_RUN_HOST 0.0.0.0
RUN apk add --no-cache gcc musl-dev linux-headers
COPY requirements.txt requirements.txt
RUN pip install -r requirements.txt
EXPOSE 5000
COPY . .
CMD ["flask", "run"]
' > Dockerfile
8.Step: Create a file called “docker-compose.yml” in your project folder.
echo '
version: "3"
services:
web:
build: .
ports:
- "5000:5000"
redis:
image: "redis:alpine"
' > docker-compose.yml
9. Step: Finally you will have just four yaml file that is shown below.
10.Step: Everything is ready and now is action time. Type the command “docker-compose up” on your terminal.
11.Step: Enter http://”ec2-host-name”5000/ in a browser to see the application is running.Whenever you refresh your browser, the counter will increase the result number.That means that your service in the container is up and running.
12.Step: Press `Ctrl+C` to stop containers, and run `docker ps -a` to see containers created by Docker Compose.
13.Step: Remove the containers with “docker-compose down “ command.
Useful commands for Docker-Compose:
1. Verify the installation with the command:
docker-compose version
2. Print list of information about running containers from current docker-compose.yaml
docker-compose ps
3. Terminal into the docker container
docker exec -it :container_id bash
4. Run the docker container in debug mode
docker-compose run --service-ports web
5. Start the docker container
docker-compose up
6. Start the docker container in the background
docker-compose up -d
7. Remove all docker containers in the repository
docker-compose down
8. Remove a specific docker container
docker kill :container_id
Wrap
That’s the conceptual framework. I clarified some of the terms you’ll see in the Docker-compose ecosystem.
Hopefully this overview has helped you better understand the Docker-compose landscape. I also hope it has also opened your eyes to the value of metaphors in understanding new technologies.
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