Using Docker Stacks for local development

While looking into the rather new healthcheck feature of Docker I noticed that docker-compose seems more and more like a legacy tool with docker stack acting as its replacement. So I set out to see, if I could use that for local development with the same ease I got from docker-compose.

Let’s work on a small example application here containing three components:

1. An application backend
2. A frontend application
3. Traefik as a reverse proxy (which makes it easier to add new services in the future)

You can find the whole setup on Github.

For this, we already have a simple docker-compose.yml file which we now want to use with Docker Stack:

version: '3'

services:
  backend:
    build: ./backend
    volumes:
      - ./backend/backend:/app/backend
    labels:
      - "traefik.backend=api"
      - "traefik.enable=true"
      - "traefik.frontend.rule=PathPrefixStrip:/api/"

  frontend:
    image: nginx:1.13.3-alpine
    volumes:
      - ./frontend:/usr/share/nginx/html
    labels:
      - "traefik.backend=frontend"
      - "traefik.enable=true"
      - "traefik.frontend.rule=PathPrefix:/"

  reverse:
    image: traefik:1.3.7-alpine
    command: "--configfile=/etc/traefik/traefik.toml"
    volumes:
      - ./traefik/traefik.toml:/etc/traefik/traefik.toml
      - /var/run/docker.sock:/var/run/docker.sock
    ports:
      - 8088:8080
      - 8080:80

Getting ready

The first thing you have to do, is to make your local machine act as a Docker Swarm manager. Just initialising a new swarm locally does the trick here:

$ docker swarm init

After that you can create a new docker-compose.yml-based stack and deploy it onto your local Docker Swarm with the following command:

$ docker stack deploy demo \
  --compose-file docker-compose.yml

Moving from docker-compose

Most of our original file should work out of the box with Docker’s new stack feature, except for one little detail:

You can no longer only define a build property, which was used to tell docker-compose where to find a Dockerfile for your custom container. If you still use that, you will receive something like that as error:

$ docker stack deploy demo \
  --compose-file ../docker-compose.yml
Ignoring unsupported options: build

Creating network demo_default
Creating service demo_reverse
Creating service demo_backend
failed to create service demo_backend: Error response from daemon: rpc error: code = InvalidArgument desc = ContainerSpec: image reference must be provided

Instead, you will have to create the image beforehand and just use the image property:

services:
  backend:
    image: docker-stack-backend:latest
    labels:
      - "traefik.backend=api"
      - "traefik.enable=true"
      - "traefik.frontend.rule=PathPrefixStrip:/api/"

Now we can start the stack again and it should work:

$ cd backend
$ make docker
$ cd ..
$ docker stack deploy demo \
  --compose-file docker-compose-for-stack.yml

Updating the configuration

If you make any change to your docker-compose.yml file you can simply execute the deploy-command again to update your stack.

How to restart a single service?

Something that was very handy while when working with docker-compose was that you could update/restart specific services using the up and down commands.

With Docker Stacks you’ll have to use the scaling feature of Docker Swarm. If we make a small change to the main.go of our backend service, we can now deploy that by setting the number of replicas of the backend first to 0, replace the binary, and then scale it back up to 1.

$ cd backend
$ make backend
$ docker service scale \
  demo_backend=0 \
  demo_backend=1 \
  --detach=false

Concluding

… I have to say that I really like the new workflow and the options that it provides. Sure, there are probably a few areas that still need some work, but for my use-cases docker stack already works quite well right now 😀