Swarm 101 - docker-compose.yml

Third and last post of the tutorial Swarm for beginners. After creating a Swarm cluster and learning the the basis of services management, it's time for deploying our services in the cluster using docker-compose.yml as we use it with docker.

You can use this simple project to follow the tutorial. All the images and versions in this post point to mymac project. To deploy with docker-compose.yml,

First of all, we create a file called docker-compose.yml; it will have the same structure as the well known docker-compose.yml, and it will work seamlessly with docker swarm.

Create a file called docker-compose.yml in server1. If you are using the simple mymac project, you can copy and paste directly. Otherwise, change whatever you need to start your project. The file should be something like:

version: "3"

    image: registry.gitlab.com/yamila/python-back:22528ebd5efb936249bd7bd8f3b6cf6b3113bdc3
       - 8000:8000
    command: gunicorn --bind wsgi:app
       replicas: 8

In version “3” we can add the deploy section where we specify the options for creating or updating a docker service. In the above example, for instance, we have declared 8 replicas (the same as –replicas 8). Let's create this service:

(server1)$ docker stack deploy --compose-file docker-compose.yml mystack
(server1)$ docker service ps mystack
  • docker stack deploy is the main command
  • –compose-file is the param which points to the docker-compose.yml file.
  • mystack which is the name for the stack (a bundle of services).

Now, we can test this. Go to in a browser and press “F5” several times; you'll see how the message changes with different macs and ips. Cool!

If we need to update the image for our service in the stack, we can just change the image reference to this one:

image: registry.gitlab.com/yamila/python-back:53d64b4f1b12ede003531b7aecb61c5fcf2613b7

Time to update:

(server1)$ docker stack deploy --compose-file docker-compose.yml mystack
(server1)$ watch docker service ps mystack_mymac

Same as before, this command will show you how the system is stopping a container and launching a new one with the new version. While the system is being updated, you can check in the browser, pressing “F5”: sometimes, you'll have the old backend (“Hello, my hostname is xxxx and my ip is zzzz”) and sometimes, you'll see the new backend (“Hello, my hostname is xxxx”). Eventually, all the requests will return the new backend, which means that the system is completely updated.

Congratulations! You get to the end of the tutorial. Following the three posts, you've learnt the basis of creating a cluster of Swarm, managing the services manually, and how to manage services and stacks with docker-compose.yml file. Besides, you've learnt lots of concepts that will apply the same to other orchestrators. Grab a cup of tea or go for a walk, you deserve it!!!

If you liked the tutorial, or you have any questions, don't hesitate and use the comments sections. I hope it's been both useful and interesting. Happy hacking!

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