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In earlier steps of the tutorial, all the nodes have been running with
availability. The swarm manager can assign tasks to any
ACTIVE node, so up to
now all nodes have been available to receive tasks.
Sometimes, such as planned maintenance times, you need to set a node to
DRAIN availability prevents a node from receiving new tasks
from the swarm manager. It also means the manager stops tasks running on the
node and launches replica tasks on a node with
If you haven’t already, open a terminal and ssh into the machine where you
run your manager node. For example, the tutorial uses a machine named
Verify that all your nodes are actively available.
$ docker node ls ID HOSTNAME STATUS AVAILABILITY MANAGER STATUS 1bcef6utixb0l0ca7gxuivsj0 worker2 Ready Active 38ciaotwjuritcdtn9npbnkuz worker1 Ready Active e216jshn25ckzbvmwlnh5jr3g * manager1 Ready Active Leader
If you aren’t still running the
redis service from the rolling
update tutorial, start it now:
$ docker service create --replicas 3 --name redis --update-delay 10s redis:3.0.6 c5uo6kdmzpon37mgj9mwglcfw
docker service ps redis to see how the Swarm manager assigned the
tasks to different nodes:
$ docker service ps redis ID NAME SERVICE IMAGE LAST STATE DESIRED STATE NODE 7q92v0nr1hcgts2amcjyqg3pq redis.1 redis redis:3.0.6 Running 26 seconds Running manager1 7h2l8h3q3wqy5f66hlv9ddmi6 redis.2 redis redis:3.0.6 Running 26 seconds Running worker1 9bg7cezvedmkgg6c8yzvbhwsd redis.3 redis redis:3.0.6 Running 26 seconds Running worker2
In this case the swarm manager distributed one task to each node. You may see the tasks distributed differently among the nodes in your environment.
docker node update --availability drain <NODE-ID> to drain a node that
had a task assigned to it:
docker node update --availability drain worker1 worker1
Inspect the node to check its availability:
$ docker node inspect --pretty worker1 ID: 38ciaotwjuritcdtn9npbnkuz Hostname: worker1 Status: State: Ready Availability: Drain ...snip...
The drained node shows
docker service ps redis to see how the Swarm manager updated the
task assignments for the
$ docker service ps redis ID NAME SERVICE IMAGE LAST STATE DESIRED STATE NODE 7q92v0nr1hcgts2amcjyqg3pq redis.1 redis redis:3.0.6 Running 4 minutes Running manager1 b4hovzed7id8irg1to42egue8 redis.2 redis redis:3.0.6 Running About a minute Running worker2 9bg7cezvedmkgg6c8yzvbhwsd redis.3 redis redis:3.0.6 Running 4 minutes Running worker2
The Swarm manager maintains the desired state by ending the task on a node
Drain availability and creating a new task on a node with
docker node update --availability active <NODE-ID> to return the
drained node to an active state:
$ docker node update --availability active worker1 worker1
Inspect the node to see the updated state:
$ docker node inspect --pretty worker1 ID: 38ciaotwjuritcdtn9npbnkuz Hostname: worker1 Status: State: Ready Availability: Active ...snip...
When you set the node back to
Active availability, it can receive new tasks: