Docker Swarm Creating a Complete Stack File

Docker Swarm is a clustering and planning tool for Docker compartments. With Swarm, IT administrators and designers can set up and deal with a bunch of Docker nodes as one virtual system. Clustering is a vital component for container technology since it makes an agreeable gathering of frameworks that can give repetition, empowering Docker Swarm failover in the event that at least one nodes encounter an outage. A Docker Swarm cluster additionally gives administrators and engineers the capacity to include or subtract container iterations as computing requests change.

And to unleash the beast provided by swarm mode and use it on full capacity, the must thing one needs is to make a proper stack file for that. There are multiple components present which can assist the containers to remove any downtime issue and give it a healing capability.

Here is a full-fledged stack file for swarm mode.

web:
   image: ubuntu:16.04
   deploy:
     mode: global
     replicas: 1
     resources:
       limits:
         cpus: '0.5'
         memory: 500M
       reservations:
         cpus: '0.2'
         memory: 200M
     restart_policy:
       condition: on-failure
       deplay: 3s
       max_attempts: 5
     update_config:
       parallelism: 2
       delay: 5s
       failure_action: rollback
       monitor: 200s
       order: start_first
       max_failure_ratio: 0.2
     placement:
       constraints: [node.role == manager]
   networks:
     - webmaster

There parameters depend on the version of compose you are using. But there are many parameters added after the version 3.

Deploy Key

The deploy key enables you to indicate design related to the deployment and running of services. The deploy key can likewise be utilized to constrain a few services to run just on the manager node or only on the worker nodes.

deploy:
  mode: replicated
  replicas: 5
  update_config:
    parallelism: 2
    delay: 10s
    order: start_first
  restart_policy:
    condition: on-failure
  resources:
    reservations:
      cpus: '0.3'
      memory: 200M
  rollback_config:
    parallelism: 1
    delay: 80s
    failure_action: continue
    monitor: 180s
    max_failure_ratio: 0.1
    order: start-first

Labels:

Specify labels on any docker service. Labels are specified only on the services and not on container.

deploy:
  labels: com.eg.description: "The label here will appear on web services"

Mode:

Modes can be of two types,

Replicated and Global. Global: Exactly one container per swarm mode and Replicated: a specified no of containers in total.

deploy:
  mode: global

Placement:

To specify the placement constraints and other preferences if the service.

deploy:
      placement:
        constraints:
          - node.role == manager
        preferences:
          - spread: node.labels.zone

Replicas:

If the mode of service is replicated (or default), this parameter specifies the no of containers to run.

deploy:
      mode: replicated
      replicas: 6

Update_Config:

This key provides the conditions how the service should be updated.

deploy:
      replicas: 6
      update_config:
        parallelism: 2
        delay: 20s
        order: stop-first

Similarily there are many more keys provided by docker in their 3 and above versions. These keys are very useful for a proper self sustained environment creation and is provides a smooth continuous updates and rollbacks.

Some other important keys are:

- Rollback_Config

- Resources

- depends_on

- entrypoint

- env_file

We will explain all these keys and their usage in the next part of this. If you want to add some environment variables, or to define the conditions for rollbalk in case of failure, or the amount of resources the container will consume, and many more keys and their usage, please refer to the part 2 of this blog.