Placement constraint

This guide allows you to configure Docker services placement constraint.
It allows to specify required property the platform's node must fulfill to run a service container on it.

Configure node label

When you configure your inventory host file, you can define property on VMs using the labels property:

The demo-performance inventory hosts file defines 3 types of node property = cots, log and common

regards-master-1                             ansible_host=server01.regards.fr ansible_connection=local
regards-master-2                             ansible_host=server02.regards.fr
regards-master-3                             ansible_host=server03.regards.fr
regards-slave-1  labels='{"type": "cots"}'   ansible_host=server04.regards.fr
regards-slave-2  labels='{"type": "cots"}'   ansible_host=server05.regards.fr
regards-slave-3  labels='{"type": "cots"}'   ansible_host=server06.regards.fr
regards-slave-4  labels='{"type": "log"}'    ansible_host=server07.regards.fr
regards-slave-5  labels='{"type": "common"}' ansible_host=server08.regards.fr
regards-slave-6  labels='{"type": "common"}' ansible_host=server09.regards.fr
regards-slave-7  labels='{"type": "common"}' ansible_host=server10.regards.fr

When you install Docker Swarm using the playbook setup-vm.yml, it will add the label to the node:

$ docker node inspect server04.regards.fr
    [..]
    "Spec": {
        "Labels": {
            "type": "cots"
        },
        "Role": "worker",
        "Availability": "active"
    },

Configure service constraint

Every service (REGARDS service, Elasticsearch, RabbitMQ, Prometheus, Workers...) that you deploy thanks to the REGARDS Ansible inventory can define the node_label_placement_constraint property:

# WORKERS
group_docker_workers:
  - name: some-worker
    node_label_placement_constraint:
      key: type
      value: common

# COTS
group_docker_cots:
  elasticsearch:
    tag: "{{ group_docker_tags.cots }}"
    global_service: true
    node_label_placement_constraint:
      key: type
      value: cots

This example creates two services:

• a worker that can run on any node having labels='{"type": "common"}'
• an elasticsearch cluster that will run on all node having labels='{"type": "cots"}'

Global services

Global services are replicated on any node that accept it. global_service can be combined with node_label_placement_constraint to create a cluster on specific nodes.