Why my docker container is running on swarm manager node instead of worker?

Question

This is my docker-stackk.yaml file.

# cat docker-stack.yml 
version: "3.4"
services:

  redis:
    image: redis:alpine
    ports:
      - "6379"
    networks:
      - frontend
    deploy:
      replicas: 1
      update_config:
        parallelism: 2
        delay: 10s
      restart_policy:
        condition: on-failure

networks:
  frontend:

volumes:
  db-data:

I did not specified any constraint.

# docker node ls
ID                            HOSTNAME   STATUS    AVAILABILITY   MANAGER STATUS   ENGINE VERSION
snbsjfcg8eu6w88oyinsaa2q0 *   infinity   Ready     Active         Reachable        20.10.8
bi2iblsfma9dno4thvcpetc29     rockpi     Ready     Active         Leader           20.10.8
d5nk1bgdpmo05u1pdlbtxppem     rockpix    Ready     Active                          19.03.8

if you see above rockpix is worker node. and infinity is manager.

but when I started the stack the container started on manager node instead of worker node.

# docker stack ps redis-stack
ID             NAME                  IMAGE          NODE       DESIRED STATE   CURRENT STATE           ERROR     PORTS
zvd9dyx10ro9   redis-stack_redis.1   redis:alpine   infinity   Running         Running 2 minutes ago  

Answer 1

A manager is able to run containers just as workers do. To prevent workloads from running on the manager, you need to include a constraint. This is different from the kubernetes model where the managers are tainted and you include a toleration to run on managers.

version: "3.4"
services:

  redis:
    image: redis:alpine
    ports:
      - "6379"
    networks:
      - frontend
    deploy:
      replicas: 1
      update_config:
        parallelism: 2
        delay: 10s
      restart_policy:
        condition: on-failure
      # add the below constraint
      placement:
        constraints:
          - "node.role==worker"

networks:
  frontend:

volumes:
  db-data:

For more on the placement policies and constraints, see:

• https://docs.docker.com/compose/compose-file/compose-file-v3/#placement
• https://docs.docker.com/engine/reference/commandline/service_create/#specify-service-constraints---constraint