Docker Swarms

The components of distributed applications are called services. In docker, services are simply “containers in production”, but in practice these could be the frontend, database server, etc. A service runs one image and codifies what that image does (how many replicas of that image are running, etc.).

docker-compose

The docker-compose.yaml file:

version: "3"
services:
  web:
    # replace username/repo:tag with your name and image details
    image: username/repo:tag
    deploy:
      replicas: 5
      resources:
        limits:
          cpus: "0.1"
          memory: 50M
      restart_policy:
        condition: on-failure
    ports:
      - "80:80"
    networks:
      - webnet
networks:
  webnet:

This file tells docker the following:

• Pull image from online repo
• Create a service called web

• Set up 5 replicas of this image

  • Each gets 10% of CPU, 50M of RAM
  • Each shares port 80 on a load-balanced network called webnet

• Defines the new network webnet

Creating a Service

To create the desired service, we must first:

$ docker swarm init

We can now run the app:

$ docker stack deploy -c docker-compose.yaml getstartedlab

Now we can run docker service ls to see the running service

$ docker service ls
ID                  NAME                MODE                REPLICAS            IMAGE                    PORTS
4mj7miodkwgj        getstartedlab_web   replicated          5/5                 username/repo:tag        *:80->80/tcp

Note that the service “web” which we created has been prepended with the name “getstartedlab” which was given. We can also see each container (called a task) which is running from within the service:

$ docker service ps getstartedlab_web

ID                  NAME                  IMAGE                    NODE                DESIRED STATE       CURRENT STATE        ERROR               PORTS
a8cio0wt6afy        getstartedlab_web.1   username/repo:tag        default             Running             Running 3 days ago
ez226v9dcedk        getstartedlab_web.2   username/repo:tag        default             Running             Running 3 days ago
s8tr7489s9cn        getstartedlab_web.3   username/repo:tag        default             Running             Running 3 days ago
me7vkve7uqh7        getstartedlab_web.4   username/repo:tag        default             Running             Running 3 days ago
yxeadks6jrh2        getstartedlab_web.5   username/repo:tag        default             Running             Running 3 days ago

Updating/Changing a Service

Services can be updated in-place by changing the docker-compose.yaml file and simply running docker stack deploy with the same arguments as before. Containers do not need to be stopped and/or started again to complete this process.

Shutting Down a Service

1. .. code-block:: shell

docker stack rm getstartedlab

2. .. code-block:: shell

docker swarm leave –force