Placement Preferences

• 17.04+ feature for a best effort "soft requirement"
• Only one strategy for now, "spread"
  • Spreads tasks among all values of a Label
  • Good to ensure distribution across availability zones, datacenters, racks, subnets, etc.
• Works on service create and update
• Can add multiple preferences for multi-layer placement control
• Won't move service tasks if labels change
• Use with Constraints if labels aren't on all nodes
  • "missing label is the same as having the label with a null value"

Examples

• Label all your AWS nodes with availability zone
  • docker node update --label-add=azone=1 nodeX
  • docker node update --label-add=azone=2 nodeY
  • docker node update --label-add=azone=3 nodeX
• Make sure your service is deployed to all availability zones
  • docker service create --placement-pref spread=node.labels.azone --replicas 3 nginx

Hands On

[node1] ~> docker node update --label-add azone=1 manager1
manager1

[node1] ~> docker node update --label-add azone=2 manager2
manager2

[node1] ~> docker node update --label-add azone=2 worker1
worker1

[node1] ~> docker service create --placement-pref spread=node.labels.azone --replicas 2 --name webapp1 nginx
mi0rgp8uylpweml8m1rbh347z
overall progress: 2 out of 2 tasks
1/2: running   [==================================================>]
2/2: running   [==================================================>]
verify: Service converged

[node1] ~> docker service create --placement-pref spread=node.labels.azone --replicas 2 --name webapp2 nginx
pvjulrsmouklw47lqlo4a5wg6
overall progress: 2 out of 2 tasks
1/2: running   [==================================================>]
2/2: running   [==================================================>]
verify: Service converged

[node1] ~> docker service update --placement-pref-rm spread=node.labels.azone webapp1
webapp1
overall progress: 2 out of 2 tasks
1/2: running   [==================================================>]
2/2: running   [==================================================>]
verify: Service converged
[node1] ~> docker service scale webapp2=8
webapp2 scaled to 8
overall progress: 8 out of 8 tasks
1/8: running   [==================================================>]
2/8: running   [==================================================>]
3/8: running   [==================================================>]
4/8: running   [==================================================>]
5/8: running   [==================================================>]
6/8: running   [==================================================>]
7/8: running   [==================================================>]
8/8: running   [==================================================>]
verify: Service converged

[node1] ~> docker service update --constraint-add node.role==worker webapp2
webapp2
overall progress: 8 out of 8 tasks
1/8: running   [==================================================>]
2/8: running   [==================================================>]
3/8: running   [==================================================>]
4/8: running   [==================================================>]
5/8: running   [==================================================>]
6/8: running   [==================================================>]
7/8: running   [==================================================>]
8/8: running   [==================================================>]
verify: Service converged

Placement Preference in Stack Files

version: "3.1"
services:
  web:
    image: nginx
    deploy:
      placement:
        preferences:
          spread: node.labels.azone

External links

• https://docs.docker.com/engine/swarm/services/#placement-preferences