Drain a node on the swarm

Estimated reading time: 3 minutes

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.

Important: Setting a node to DRAIN does not remove standalone containers from that node, such as those created with docker run, docker-compose up, or the Docker Engine API. A node’s status, including DRAIN, only affects the node’s ability to schedule swarm service workloads.

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.

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

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.

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
```

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.

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.

Run 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:
- 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

What’s next?

Learn how to use a swarm mode routing mesh.

tutorial, cluster management, swarm, service, drain