Drain a node on the swarm

In earlier steps of the tutorial, all the nodes have been running with ACTIVE 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. 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 ACTIVE availability.

1. 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 manager1.

2. 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
   

3. 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
   

4. Run docker service ps redis to see how the swarm manager assigned the tasks to different nodes:

   $ docker service ps redis
   
   NAME                               IMAGE        NODE     DESIRED STATE  CURRENT STATE
   redis.1.7q92v0nr1hcgts2amcjyqg3pq  redis:3.0.6  manager1 Running        Running 26 seconds
   redis.2.7h2l8h3q3wqy5f66hlv9ddmi6  redis:3.0.6  worker1  Running        Running 26 seconds
   redis.3.9bg7cezvedmkgg6c8yzvbhwsd  redis:3.0.6  worker2  Running        Running 26 seconds
   

   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.

5. Run 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
   

6. 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 Drain for AVAILABILITY.

7. Run docker service ps redis to see how the swarm manager updated the task assignments for the redis service:

   $ docker service ps redis
   
   NAME                                    IMAGE        NODE      DESIRED STATE  CURRENT STATE           ERROR
   redis.1.7q92v0nr1hcgts2amcjyqg3pq       redis:3.0.6  manager1  Running        Running 4 minutes
   redis.2.b4hovzed7id8irg1to42egue8       redis:3.0.6  worker2   Running        Running About a minute
    \_ redis.2.7h2l8h3q3wqy5f66hlv9ddmi6   redis:3.0.6  worker1   Shutdown       Shutdown 2 minutes ago
   redis.3.9bg7cezvedmkgg6c8yzvbhwsd       redis:3.0.6  worker2   Running        Running 4 minutes
   

   The swarm manager maintains the desired state by ending the task on a node with Drain availability and creating a new task on a node with Active availability.

8. Run docker node update --availability active <NODE-ID> to return the drained node to an active state:

   $ docker node update --availability active worker1
   
   worker1
   

9. 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:

   • during a service update to scale up
   • during a rolling update
   • when you set another node to Drain availability
   • when a task fails on another active node

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