Service Constraints
- Can filter task placement based on built-in or custom labels
- Can be added at create time, or add/remove at update time
- Creates a hard requirement, placement fails if not matched
- Supports multiple constraints
- Supports either a <key> or a <key>=<value> pair
- Can match with == or !=
- Labels can be node.labels or engine.labels
- node.labels only added via manager to raft rol
- engine.labels added in
daemon.json to any node {"labels":["dmz=true"]}
- Default to using node.labels, use engine.labels for autoscaling hardware, os
Examples
- place only on a manager(2 options for same result)
docker service create --constraints=node.role==manager nginx
docker service create --constraint=node.role!=worker nginx
- Add label to node2 for dmz=true, and constrain to it
docker node update --label-add=dmz=true node2
docker service create --constraint=node.labels.dmz==true nginx
Hands On
[node1] ~> docker service create --name app1 --constraint node.role==worker nginx
d4dvqu89nuh7jt0y302fkm4o1
overall progress: 1 out of 1 tasks
1/1: running [==================================================>]
verify: Service converged
[node1] ~> docker service update --constraint-rm node.role==worker --constraint-add node.role==manager app1
app1
overall progress: 1 out of 1 tasks
1/1: running [==================================================>]
verify: Service converged
[node1] ~> docker node update --label-add dmz=true manager2
manager2
[node1] ~> docker service create --name dmz-nginx --constraint node.labels.dmz==true --replicas 2 nginx
5khmygm7kee8mlo7ncjfj0wj0
overall progress: 2 out of 2 tasks
1/2: running [==================================================>]
2/2: running [==================================================>]
verify: Service converged
Service Constraints in Stack Files
version: "3.1"
services:
db:
image: mysql:5.7
deploy:
placement:
constraints:
- node.labels.disk == ssd
Built-in Labels
- node.id (listed in docker node ls)
- node.hostname (listed in docker node ls)
- node.ip (e.g. 172.19.17.0/24)
- node.role (manager|worker)
- node.platform.os (linux|windows|etc.)
- node.platform.arch (x86_64|arm64|386|etc.)
- node.labels (empty by default)
- Might be others out there, but not well documented
External links