Deploy a stack to a swarm

Estimated reading time: 7 minutes

When running Docker Engine in swarm mode, you can use docker stack deploy to deploy a complete application stack to the swarm. The deploy command accepts a stack description in the form of a Compose file.

The docker stack deploy command supports any Compose file of version “3.0” or above. If you have an older version, see the upgrade guide.

To run through this tutorial, you will need:

  1. A Docker Engine of version 1.13.0 or later, running in swarm mode. If you’re not familiar with swarm mode, you might want to read Swarm mode key concepts and How services work.

    Note: If you’re trying things out on a local development environment, you can put your engine into swarm mode with docker swarm init.

    If you’ve already got a multi-node swarm running, keep in mind that all docker stack and docker service commands must be run from a manager node.

  2. Docker Compose version 1.10 or later.

Set up a Docker registry

Because a swarm consists of multiple Docker Engines, a registry is required to distribute images to all of them. You can use the Docker Hub or maintain your own. Here’s how to create a throwaway registry, which you can discard afterward.

  1. Start the registry as a service on your swarm:

    $ docker service create --name registry --publish 5000:5000 registry:2
    
  2. Check its status with docker service ls:

    $ docker service ls
    
    ID            NAME      REPLICAS  IMAGE                                                                               COMMAND
    l7791tpuwkco  registry  1/1       registry:2@sha256:1152291c7f93a4ea2ddc95e46d142c31e743b6dd70e194af9e6ebe530f782c17
    

    Once it reads 1/1 under REPLICAS, it’s running. If it reads 0/1, it’s probably still pulling the image.

  3. Check that it’s working with curl:

    $ curl http://localhost:5000/v2/
    
    {}
    

Create the example application

The app used in this guide is based on the hit counter app in the Get started with Docker Compose guide. It consists of a Python app which maintains a counter in a Redis instance and increments the counter whenever you visit it.

  1. Create a directory for the project:

    $ mkdir stackdemo
    $ cd stackdemo
    
  2. Create a file called app.py in the project directory and paste this in:

    from flask import Flask
    from redis import Redis
    
    app = Flask(__name__)
    redis = Redis(host='redis', port=6379)
    
    @app.route('/')
    def hello():
        count = redis.incr('hits')
        return 'Hello World! I have been seen {} times.\n'.format(count)
    
    if __name__ == "__main__":
        app.run(host="0.0.0.0", port=8000, debug=True)
    
  3. Create a file called requirements.txt and paste these two lines in:

    flask
    redis
    
  4. Create a file called Dockerfile and paste this in:

    FROM python:3.4-alpine
    ADD . /code
    WORKDIR /code
    RUN pip install -r requirements.txt
    CMD ["python", "app.py"]
    
  5. Create a file called docker-compose.yml and paste this in:

    version: '3'
    
    services:
      web:
        image: 127.0.0.1:5000/stackdemo
        build: .
        ports:
          - "8000:8000"
      redis:
        image: redis:alpine
    

    Note that the image for the web app is built using the Dockerfile defined above. It’s also tagged with 127.0.0.1:5000 - the address of the registry created earlier. This will be important when distributing the app to the swarm.

Test the app with Compose

  1. Start the app with docker-compose up. This builds the web app image, pull the Redis image if you don’t already have it, and create two containers.

    You will see a warning about the Engine being in swarm mode. This is because Compose doesn’t take advantage of swarm mode, and deploys everything to a single node. You can safely ignore this.

    $ docker-compose up -d
    
    WARNING: The Docker Engine you're using is running in swarm mode.
    
    Compose does not use swarm mode to deploy services to multiple nodes in
    a swarm. All containers will be scheduled on the current node.
    
    To deploy your application across the swarm, use `docker stack deploy`.
    
    Creating network "stackdemo_default" with the default driver
    Building web
    ...(build output)...
    Creating stackdemo_redis_1
    Creating stackdemo_web_1
    
  2. Check that the app is running with docker-compose ps:

    $ docker-compose ps
    
          Name                     Command               State           Ports
    -----------------------------------------------------------------------------------
    stackdemo_redis_1   docker-entrypoint.sh redis ...   Up      6379/tcp
    stackdemo_web_1     python app.py                    Up      0.0.0.0:8000->8000/tcp
    

    You can test the app with curl:

    $ curl http://localhost:8000
    Hello World! I have been seen 1 times.
    
    $ curl http://localhost:8000
    Hello World! I have been seen 2 times.
    
    $ curl http://localhost:8000
    Hello World! I have been seen 3 times.
    
  3. Bring the app down:

    $ docker-compose down --volumes
    
    Stopping stackdemo_web_1 ... done
    Stopping stackdemo_redis_1 ... done
    Removing stackdemo_web_1 ... done
    Removing stackdemo_redis_1 ... done
    Removing network stackdemo_default
    

Push the generated image to the registry

To distribute the web app’s image across the swarm, it needs to be pushed to the registry you set up earlier. With Compose, this is very simple:

$ docker-compose push

Pushing web (127.0.0.1:5000/stackdemo:latest)...
The push refers to a repository [127.0.0.1:5000/stackdemo]
5b5a49501a76: Pushed
be44185ce609: Pushed
bd7330a79bcf: Pushed
c9fc143a069a: Pushed
011b303988d2: Pushed
latest: digest: sha256:a81840ebf5ac24b42c1c676cbda3b2cb144580ee347c07e1bc80e35e5ca76507 size: 1372

The stack is now ready to be deployed.

Deploy the stack to the swarm

  1. Create the stack with docker stack deploy:

    $ docker stack deploy --compose-file docker-compose.yml stackdemo
    
    Ignoring unsupported options: build
    
    Creating network stackdemo_default
    Creating service stackdemo_web
    Creating service stackdemo_redis
    

    The last argument is a name for the stack. Each network, volume and service name is prefixed with the stack name.

  2. Check that it’s running with docker stack services stackdemo:

    $ docker stack services stackdemo
    
    ID            NAME             MODE        REPLICAS  IMAGE
    orvjk2263y1p  stackdemo_redis  replicated  1/1       redis:3.2-alpine@sha256:f1ed3708f538b537eb9c2a7dd50dc90a706f7debd7e1196c9264edeea521a86d
    s1nf0xy8t1un  stackdemo_web    replicated  1/1       127.0.0.1:5000/stackdemo@sha256:adb070e0805d04ba2f92c724298370b7a4eb19860222120d43e0f6351ddbc26f
    

    Once it’s running, you should see 1/1 under REPLICAS for both services. This might take some time if you have a multi-node swarm, as images need to be pulled.

    As before, you can test the app with curl:

    $ curl http://localhost:8000
    Hello World! I have been seen 1 times.
    
    $ curl http://localhost:8000
    Hello World! I have been seen 2 times.
    
    $ curl http://localhost:8000
    Hello World! I have been seen 3 times.
    

    Thanks to Docker’s built-in routing mesh, you can access any node in the swarm on port 8000 and get routed to the app:

    $ curl http://address-of-other-node:8000
    Hello World! I have been seen 4 times.
    
  3. Bring the stack down with docker stack rm:

    $ docker stack rm stackdemo
    
    Removing service stackdemo_web
    Removing service stackdemo_redis
    Removing network stackdemo_default
    
  4. Bring the registry down with docker service rm:

    $ docker service rm registry
    
  5. If you’re just testing things out on a local machine and want to bring your Docker Engine out of swarm mode, use docker swarm leave:

    $ docker swarm leave --force
    
    Node left the swarm.
    
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