Docker swarm — How to use node labels
Node labels can be a powerful tool, a label is nothing more than a metadata of the node, imagine that we have two swarm nodes, one in a…
Node labels can be a powerful tool, a label is nothing more than a metadata of the node, imagine that we have two swarm nodes, one in a data-center named east and one in a data-center named west, lets label them based on the data-center
On the swarm manager enter the following for the node on Data-center east$ docker node update --label-add DC=east worker_node2
Add a label for the node on data center west also$ docker node update --label-add DC=west worker_node3
Suppose that the one Data center, named east is our production data center and the west is the failover data center, so lets start our services on data center east$ docker service create --name nginx-east --constraint node.labels.DC==east --replicas 3 nginx
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overall progress: 3 out of 3 tasks
1/3: running [==================================================>]
2/3: running [==================================================>]
3/3: running [==================================================>]
verify: Service converged
Lets do the same for west$ docker service create --name nginx-west --constraint node.labels.DC==west --replicas 3 nginx
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overall progress: 3 out of 3 tasks
1/3: running [==================================================>]
2/3: running [==================================================>]
3/3: running [==================================================>]
verify: Service converged
verify that both services have started$ docker service list
ID NAME MODE REPLICAS IMAGE PORTS
jle0zw7b51sg nginx-east replicated 3/3 nginx:latest
od3fmka6nvag nginx-west replicated 3/3 nginx:latest
Verify that node constraints work as it should$ docker service ps nginx-west
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
yvjjwdgfbl10 nginx-west.1 nginx:latest worker_node3 Running Running 3 minutes ago
8tgqmjaqo1nj nginx-west.2 nginx:latest worker_node3 Running Running 3 minutes ago
yzu2cl5mr36b nginx-west.3 nginx:latest worker_node3 Running Running 3 minutes ago
And$ docker service ps nginx-east
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
cspu9krofwti nginx-east.1 nginx:latest worker_node2 Running Running about a minute ago
prk2a3eimdjy nginx-east.2 nginx:latest worker_node2 Running Running about a minute ago
o41geveazrdj nginx-east.3 nginx:latest worker_node2 Running Running about a minute ago
Now, lets delete the services$ docker service rm nginx-east
$ docker service rm nginx-west
A common scenario is to start services evenly on all nodes, for this purpose there is the placement-pref parameter which allows to spread running services across all nodes of the swarm based on label values$ docker service create --name nginx-spread --placement-pref spread=node.labels.DC --replicas 6 nginx
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overall progress: 6 out of 6 tasks
1/6: running [==================================================>]
2/6: running [==================================================>]
3/6: running [==================================================>]
4/6: running [==================================================>]
5/6: running [==================================================>]
6/6: running [==================================================>]
verify: Service converged
Doing this since the DC label has two values the services should be spread with services_number / 2, to verify this$ docker service ps nginx-spread
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
lljbufq3cf45 nginx-spread.1 nginx:latest worker_node2 Running Running 2 minutes ago
orwrewax6tib nginx-spread.2 nginx:latest worker_node3 Running Running 2 minutes ago
ytbdrx7b7h32 nginx-spread.3 nginx:latest worker_node2 Running Running 2 minutes ago
na6qmbcskum0 nginx-spread.4 nginx:latest worker_node3 Running Running 2 minutes ago
w1ls5ail8tnm nginx-spread.5 nginx:latest worker_node2 Running Running 2 minutes ago
rur1h80p6eo7 nginx-spread.6 nginx:latest worker_node3 Running Running 2 minutes ago
I hope you found the article interesting! :)
