361 lines
13 KiB
Markdown
361 lines
13 KiB
Markdown
---
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description: Get started with Docker Compose
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keywords: documentation, docs, docker, compose, orchestration, containers
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title: Get started with Docker Compose
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---
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On this page you build a simple Python web application running on Docker
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Compose. The application uses the Flask framework and maintains a hit counter in
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Redis. While the sample uses Python, the concepts demonstrated here should be
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understandable even if you're not familiar with it.
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## Prerequisites
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Make sure you have already installed both [Docker Engine](../get-docker.md)
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and [Docker Compose](install.md). You don't need to install Python or Redis, as
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both are provided by Docker images.
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## Step 1: Setup
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Define the application dependencies.
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1. Create a directory for the project:
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```console
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$ mkdir composetest
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$ cd composetest
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```
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2. Create a file called `app.py` in your project directory and paste this in:
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```python
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import time
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import redis
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from flask import Flask
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app = Flask(__name__)
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cache = redis.Redis(host='redis', port=6379)
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def get_hit_count():
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retries = 5
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while True:
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try:
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return cache.incr('hits')
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except redis.exceptions.ConnectionError as exc:
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if retries == 0:
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raise exc
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retries -= 1
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time.sleep(0.5)
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@app.route('/')
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def hello():
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count = get_hit_count()
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return 'Hello World! I have been seen {} times.\n'.format(count)
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```
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In this example, `redis` is the hostname of the redis container on the
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application's network. We use the default port for Redis, `6379`.
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> Handling transient errors
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>
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> Note the way the `get_hit_count` function is written. This basic retry
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> loop lets us attempt our request multiple times if the redis service is
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> not available. This is useful at startup while the application comes
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> online, but also makes our application more resilient if the Redis
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> service needs to be restarted anytime during the app's lifetime. In a
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> cluster, this also helps handling momentary connection drops between
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> nodes.
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3. Create another file called `requirements.txt` in your project directory and
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paste this in:
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```text
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flask
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redis
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```
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## Step 2: Create a Dockerfile
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In this step, you write a Dockerfile that builds a Docker image. The image
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contains all the dependencies the Python application requires, including Python
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itself.
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In your project directory, create a file named `Dockerfile` and paste the
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following:
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```dockerfile
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FROM python:3.7-alpine
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WORKDIR /code
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ENV FLASK_APP=app.py
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ENV FLASK_RUN_HOST=0.0.0.0
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RUN apk add --no-cache gcc musl-dev linux-headers
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COPY requirements.txt requirements.txt
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RUN pip install -r requirements.txt
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EXPOSE 5000
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COPY . .
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CMD ["flask", "run"]
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```
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This tells Docker to:
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* Build an image starting with the Python 3.7 image.
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* Set the working directory to `/code`.
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* Set environment variables used by the `flask` command.
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* Install gcc and other dependencies
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* Copy `requirements.txt` and install the Python dependencies.
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* Add metadata to the image to describe that the container is listening on port 5000
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* Copy the current directory `.` in the project to the workdir `.` in the image.
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* Set the default command for the container to `flask run`.
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For more information on how to write Dockerfiles, see the
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[Docker user guide](../develop/index.md)
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and the [Dockerfile reference](/engine/reference/builder/).
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## Step 3: Define services in a Compose file
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Create a file called `docker-compose.yml` in your project directory and paste
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the following:
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```yaml
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version: "{{ site.compose_file_v3 }}"
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services:
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web:
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build: .
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ports:
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- "5000:5000"
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redis:
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image: "redis:alpine"
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```
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This Compose file defines two services: `web` and `redis`.
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### Web service
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The `web` service uses an image that's built from the `Dockerfile` in the current directory.
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It then binds the container and the host machine to the exposed port, `5000`. This example service uses the default port for
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the Flask web server, `5000`.
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### Redis service
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The `redis` service uses a public [Redis](https://registry.hub.docker.com/_/redis/)
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image pulled from the Docker Hub registry.
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## Step 4: Build and run your app with Compose
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1. From your project directory, start up your application by running `docker-compose up`.
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```console
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$ docker-compose up
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Creating network "composetest_default" with the default driver
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Creating composetest_web_1 ...
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Creating composetest_redis_1 ...
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Creating composetest_web_1
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Creating composetest_redis_1 ... done
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Attaching to composetest_web_1, composetest_redis_1
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web_1 | * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)
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redis_1 | 1:C 17 Aug 22:11:10.480 # oO0OoO0OoO0Oo Redis is starting oO0OoO0OoO0Oo
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redis_1 | 1:C 17 Aug 22:11:10.480 # Redis version=4.0.1, bits=64, commit=00000000, modified=0, pid=1, just started
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redis_1 | 1:C 17 Aug 22:11:10.480 # Warning: no config file specified, using the default config. In order to specify a config file use redis-server /path/to/redis.conf
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web_1 | * Restarting with stat
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redis_1 | 1:M 17 Aug 22:11:10.483 * Running mode=standalone, port=6379.
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redis_1 | 1:M 17 Aug 22:11:10.483 # WARNING: The TCP backlog setting of 511 cannot be enforced because /proc/sys/net/core/somaxconn is set to the lower value of 128.
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web_1 | * Debugger is active!
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redis_1 | 1:M 17 Aug 22:11:10.483 # Server initialized
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redis_1 | 1:M 17 Aug 22:11:10.483 # WARNING you have Transparent Huge Pages (THP) support enabled in your kernel. This will create latency and memory usage issues with Redis. To fix this issue run the command 'echo never > /sys/kernel/mm/transparent_hugepage/enabled' as root, and add it to your /etc/rc.local in order to retain the setting after a reboot. Redis must be restarted after THP is disabled.
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web_1 | * Debugger PIN: 330-787-903
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redis_1 | 1:M 17 Aug 22:11:10.483 * Ready to accept connections
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```
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Compose pulls a Redis image, builds an image for your code, and starts the
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services you defined. In this case, the code is statically copied into the image at build time.
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2. Enter http://localhost:5000/ in a browser to see the application running.
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If you're using Docker natively on Linux, Docker Desktop for Mac, or Docker Desktop for
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Windows, then the web app should now be listening on port 5000 on your
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Docker daemon host. Point your web browser to http://localhost:5000 to
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find the `Hello World` message. If this doesn't resolve, you can also try
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http://127.0.0.1:5000.
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If you're using Docker Machine on a Mac or Windows, use `docker-machine ip
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MACHINE_VM` to get the IP address of your Docker host. Then, open
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`http://MACHINE_VM_IP:5000` in a browser.
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You should see a message in your browser saying:
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```console
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Hello World! I have been seen 1 times.
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```
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3. Refresh the page.
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The number should increment.
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```console
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Hello World! I have been seen 2 times.
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```
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4. Switch to another terminal window, and type `docker image ls` to list local images.
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Listing images at this point should return `redis` and `web`.
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```console
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$ docker image ls
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REPOSITORY TAG IMAGE ID CREATED SIZE
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composetest_web latest e2c21aa48cc1 4 minutes ago 93.8MB
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python 3.4-alpine 84e6077c7ab6 7 days ago 82.5MB
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redis alpine 9d8fa9aa0e5b 3 weeks ago 27.5MB
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```
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You can inspect images with `docker inspect <tag or id>`.
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5. Stop the application, either by running `docker-compose down`
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from within your project directory in the second terminal, or by
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hitting CTRL+C in the original terminal where you started the app.
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## Step 5: Edit the Compose file to add a bind mount
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Edit `docker-compose.yml` in your project directory to add a
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[bind mount](../storage/bind-mounts.md) for the `web` service:
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```yaml
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version: "{{ site.compose_file_v3 }}"
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services:
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web:
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build: .
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ports:
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- "5000:5000"
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volumes:
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- .:/code
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environment:
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FLASK_ENV: development
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redis:
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image: "redis:alpine"
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```
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The new `volumes` key mounts the project directory (current directory) on the
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host to `/code` inside the container, allowing you to modify the code on the
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fly, without having to rebuild the image. The `environment` key sets the
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`FLASK_ENV` environment variable, which tells `flask run` to run in development
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mode and reload the code on change. This mode should only be used in development.
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## Step 6: Re-build and run the app with Compose
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From your project directory, type `docker-compose up` to build the app with the updated Compose file, and run it.
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```console
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$ docker-compose up
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Creating network "composetest_default" with the default driver
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Creating composetest_web_1 ...
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Creating composetest_redis_1 ...
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Creating composetest_web_1
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Creating composetest_redis_1 ... done
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Attaching to composetest_web_1, composetest_redis_1
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web_1 | * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)
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...
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```
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Check the `Hello World` message in a web browser again, and refresh to see the
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count increment.
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> Shared folders, volumes, and bind mounts
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>
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> * If your project is outside of the `Users` directory (`cd ~`), then you
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need to share the drive or location of the Dockerfile and volume you are using.
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If you get runtime errors indicating an application file is not found, a volume
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mount is denied, or a service cannot start, try enabling file or drive sharing.
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Volume mounting requires shared drives for projects that live outside of
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`C:\Users` (Windows) or `/Users` (Mac), and is required for _any_ project on
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Docker Desktop for Windows that uses [Linux containers](../docker-for-windows/index.md#switch-between-windows-and-linux-containers).
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For more information, see [File sharing](../docker-for-mac/index.md#file-sharing) on Docker
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for Mac, and the general examples on how to
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> [Manage data in containers](../storage/volumes.md).
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>
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> * If you are using Oracle VirtualBox on an older Windows OS, you might encounter an issue with shared folders as described in this [VB trouble
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ticket](https://www.virtualbox.org/ticket/14920). Newer Windows systems meet the
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requirements for [Docker Desktop for Windows](../docker-for-windows/install.md) and do not
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need VirtualBox.
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{: .important}
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## Step 7: Update the application
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Because the application code is now mounted into the container using a volume,
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you can make changes to its code and see the changes instantly, without having
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to rebuild the image.
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Change the greeting in `app.py` and save it. For example, change the `Hello World!`
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message to `Hello from Docker!`:
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```python
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return 'Hello from Docker! I have been seen {} times.\n'.format(count)
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```
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Refresh the app in your browser. The greeting should be updated, and the
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counter should still be incrementing.
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## Step 8: Experiment with some other commands
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If you want to run your services in the background, you can pass the `-d` flag
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(for "detached" mode) to `docker-compose up` and use `docker-compose ps` to
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see what is currently running:
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```console
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$ docker-compose up -d
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Starting composetest_redis_1...
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Starting composetest_web_1...
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$ docker-compose ps
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Name Command State Ports
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-------------------------------------------------------------------
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composetest_redis_1 /usr/local/bin/run Up
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composetest_web_1 /bin/sh -c python app.py Up 5000->5000/tcp
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```
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The `docker-compose run` command allows you to run one-off commands for your
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services. For example, to see what environment variables are available to the
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`web` service:
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```console
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$ docker-compose run web env
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```
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See `docker-compose --help` to see other available commands. You can also install [command completion](completion.md) for the bash and zsh shell, which also shows you available commands.
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If you started Compose with `docker-compose up -d`, stop
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your services once you've finished with them:
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```console
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$ docker-compose stop
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```
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You can bring everything down, removing the containers entirely, with the `down`
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command. Pass `--volumes` to also remove the data volume used by the Redis
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container:
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```console
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$ docker-compose down --volumes
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```
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At this point, you have seen the basics of how Compose works.
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## Where to go next
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- Next, try the [Sample apps with Compose](samples-for-compose.md)
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- [Explore the full list of Compose commands](reference/index.md)
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- [Compose configuration file reference](compose-file/index.md)
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- To learn more about volumes and bind mounts, see [Manage data in Docker](../storage/index.md)
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