Controlling Container Placements

* By default a swarm service spreads its tasks out over Nodes.Try's to use the least-used Node for a task.
* Swarm has multiple ways to control which node a container runs on.
* Can be used togther for complex requirements.
1. Node Labels plus service contraints ( <key>=<value> )
2. Service Modes (replicated|global)
3. (new in 17.04+) Placement Preferences (spread)
4. Node Availability (active|pause|drain)
5. Resource Requirements (cpu|memory)

Service Constraints:

-bash-4.2$ docker service create --name app1 --constraint node.role==worker nginx
i342kxxhjqvp3js26v7a3ha4j
overall progress: 1 out of 1 tasks
1/1: running   [==================================================>]
verify: Service converged

-bash-4.2$ docker service ps app1
ID                  NAME                IMAGE               NODE                DESIRED STATE       CURRENT STATE            ERROR               PORTS
29jycn7bjrib        app1.1              nginx:latest        worker2             Running             Running 38 seconds ago


-bash-4.2$ 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
-bash-4.2$ docker service ps app1
ID                  NAME                IMAGE               NODE                DESIRED STATE       CURRENT STATE             ERROR               PORTS
r9auvuykv2j2        app1.1              nginx:latest        manager             Running             Running 19 seconds ago
29jycn7bjrib         \_ app1.1          nginx:latest        worker2             Shutdown            Shutdown 20 seconds ago

-bash-4.2$ docker node update --label-add dmz=true worker2
worker2

-bash-4.2$ docker inspect worker2
[
    {
        "ID": "p5k4qztbwe92rmq1ygdu9vqkk",
        "Version": {
            "Index": 622
        },
        "CreatedAt": "2019-12-02T06:35:12.002886593Z",
        "UpdatedAt": "2019-12-09T04:02:06.979884066Z",
        "Spec": {
            "Labels": {
                "dmz": "true"
            },



-bash-4.2$ docker service create --name dmz-nginx --constraint node.labels.dmz==true --replicas 2 nginx
jkhos0jc8z6t55v4zzhik1fyq
overall progress: 2 out of 2 tasks
1/2: running   [==================================================>]
2/2: running   [==================================================>]
verify: Service converged

-bash-4.2$ docker service ps dmz-nginx
ID                  NAME                IMAGE               NODE                DESIRED STATE       CURRENT STATE            ERROR               PORTS
6bjm1tkdg5pl        dmz-nginx.1         nginx:latest        worker2             Running             Running 25 seconds ago
x8ljpqalykol        dmz-nginx.2         nginx:latest        worker2             Running             Running 25 seconds ago
-bash-4.2$

Built-in labels:

node.id (listed in docker node ls)
node.hostname (listed in docker node ls)
node.ip (eg. 172.19.17.0/24)
node.role (worker | manager)
node.platform.os (linux|windows|etc)
node.platform.arch (x86_64|arm64|386|etc)
node.labels (empty by default)

Cleaning up:

-bash-4.2$ docker service rm app1 dmz-nginx
app1
dmz-nginx
-bash-4.2$ docker service ls
ID                  NAME                 MODE                REPLICAS            IMAGE                             PORTS
j2nbudo31jhd        voteapp_visualizer   replicated          1/1                 dockersamples/visualizer:stable   *:8080->8080/tcp


-bash-4.2$ docker node update --label-rm dmz worker2
worker2
-bash-4.2$