Docker deploy swarm instance on specific node matching instance index

Question

Using docker swarm, I am trying to deploy N instances of my app on N nodes in a way that each app is deployed on the node with the corresponding index. E.g.: app1 must be deployed on node1, app2 on node2, ...

The bellow is not working as it complains Error response from daemon: rpc error: code = Unknown desc = value 'node{{.Task.Slot}}' is invalid.

Any suggestion how to achieve this ?

I also have the impression, in a long shot, to use something with labels but I cannot wrap my head over it yet. Anyhow please advise.

version: "3.8"

services:

  app:
    image: app:latest
    hostname: "app{{.Task.Slot}}"
    networks:
      - app-net
    volumes:
      - "/data/shared/app{{.Task.Slot}}/config:/app/config"
    deploy:
      replicas: 5
      update_config:
        parallelism: 1
        delay: 10s
      restart_policy:
        condition: any
      placement:
        constraints:
          - "node.hostname==node{{.Task.Slot}}"   <========= ERROR

Answer 1

Service template parameters are documented as only resolving in:

• the hostname: directive
• for volume definitions
• in labels.
• environment variables.

Placement preference / constraints is not supported, but would be brilliant as it would allow simple deployments of Minio, etcd, consul and other clustered services where you need to pin replicas to nodes.