Manage nodes in a swarmEstimated reading time: 7 minutes
As part of the swarm management lifecycle, you may need to view or update a node as follows:
To view a list of nodes in the swarm run
docker node ls from a manager node:
$ docker node ls ID HOSTNAME STATUS AVAILABILITY MANAGER STATUS 46aqrk4e473hjbt745z53cr3t node-5 Ready Active Reachable 61pi3d91s0w3b90ijw3deeb2q node-4 Ready Active Reachable a5b2m3oghd48m8eu391pefq5u node-3 Ready Active e7p8btxeu3ioshyuj6lxiv6g0 node-2 Ready Active ehkv3bcimagdese79dn78otj5 * node-1 Ready Active Leader
AVAILABILITY column shows whether or not the scheduler can assign tasks to
Activemeans that the scheduler can assign tasks to the node.
Pausemeans the scheduler doesn’t assign new tasks to the node, but existing tasks remain running.
Drainmeans the scheduler doesn’t assign new tasks to the node. The scheduler shuts down any existing tasks and schedules them on an available node.
MANAGER STATUS column shows node participation in the Raft consensus:
- No value indicates a worker node that does not participate in swarm management.
Leadermeans the node is the primary manager node that makes all swarm management and orchestration decisions for the swarm.
Reachablemeans the node is a manager node participating in the Raft consensus quorum. If the leader node becomes unavailable, the node is eligible for election as the new leader.
Unavailablemeans the node is a manager that can’t communicate with other managers. If a manager node becomes unavailable, you should either join a new manager node to the swarm or promote a worker node to be a manager.
For more information on swarm administration refer to the Swarm administration guide.
Inspect an individual node
You can run
docker node inspect <NODE-ID> on a manager node to view the
details for an individual node. The output defaults to JSON format, but you can
--pretty flag to print the results in human-readable format. For example:
$ docker node inspect self --pretty ID: ehkv3bcimagdese79dn78otj5 Hostname: node-1 Joined at: 2016-06-16 22:52:44.9910662 +0000 utc Status: State: Ready Availability: Active Manager Status: Address: 172.17.0.2:2377 Raft Status: Reachable Leader: Yes Platform: Operating System: linux Architecture: x86_64 Resources: CPUs: 2 Memory: 1.954 GiB Plugins: Network: overlay, host, bridge, overlay, null Volume: local Engine Version: 1.12.0-dev
Update a node
You can modify node attributes as follows:
Change node availability
Changing node availability lets you:
- drain a manager node so that only performs swarm management tasks and is unavailable for task assignment.
- drain a node so you can take it down for maintenance.
- pause a node so it can’t receive new tasks.
- restore unavailable or paused nodes available status.
For example, to change a manager node to
$ docker node update --availability drain node-1 node-1
See list nodes for descriptions of the different availability options.
Add or remove label metadata
Node labels provide a flexible method of node organization. You can also use node labels in service constraints. Apply constraints when you create a service to limit the nodes where the scheduler assigns tasks for the service.
docker node update --label-add on a manager node to add label metadata to
a node. The
--label-add flag supports either a
<key> or a
--label-add flag once for each node label you want to add:
$ docker node update --label-add foo --label-add bar=baz node-1 node-1
The labels you set for nodes using docker node update apply only to the node entity within the swarm. Do not confuse them with the docker daemon labels for dockerd.
Therefore, node labels can be used to limit critical tasks to nodes that meet certain requirements. For example, schedule only on machines where special workloads should be run, such as machines that meet PCI-SS compliance.
A compromised worker could not compromise these special workloads because it cannot change node labels.
Engine labels, however, are still useful because some features that do not affect secure orchestration of containers might be better off set in a decentralized manner. For instance, an engine could have a label to indicate that it has a certain type of disk device, which may not be relevant to security directly. These labels are more easily “trusted” by the swarm orchestrator.
Refer to the
docker service create CLI reference
for more information about service constraints.
Promote or demote a node
You can promote a worker node to the manager role. This is useful when a manager node becomes unavailable or if you want to take a manager offline for maintenance. Similarly, you can demote a manager node to the worker role.
Note: Regardless of your reason to promote or demote a node, you must always maintain a quorum of manager nodes in the swarm. For more information refer to the Swarm administration guide.
To promote a node or set of nodes, run
docker node promote from a manager
$ docker node promote node-3 node-2 Node node-3 promoted to a manager in the swarm. Node node-2 promoted to a manager in the swarm.
To demote a node or set of nodes, run
docker node demote from a manager node:
$ docker node demote node-3 node-2 Manager node-3 demoted in the swarm. Manager node-2 demoted in the swarm.
docker node promote and
docker node demote are convenience commands for
docker node update --role manager and
docker node update --role worker
Install plugins on swarm nodes
If your swarm service relies on one or more
plugins, these plugins need to be available on
every node where the service could potentially be deployed. You can manually
install the plugin on each node or script the installation. In Docker 17.07 and
higher, you can also deploy the plugin in a similar way as a global service
using the Docker API, by specifying a
PluginSpec instead of a
Note: There is currently no way to deploy a plugin to a swarm using the Docker CLI or Docker Compose. In addition, it is not possible to install plugins from a private repository.
Leave the swarm
docker swarm leave command on a node to remove it from the swarm.
For example to leave the swarm on a worker node:
$ docker swarm leave Node left the swarm.
When a node leaves the swarm, the Docker Engine stops running in swarm mode. The orchestrator no longer schedules tasks to the node.
If the node is a manager node, you receive a warning about maintaining the
quorum. To override the warning, pass the
--force flag. If the last manager
node leaves the swarm, the swarm becomes unavailable requiring you to take
disaster recovery measures.
For information about maintaining a quorum and disaster recovery, refer to the Swarm administration guide.
After a node leaves the swarm, you can run the
docker node rm command on a
manager node to remove the node from the node list.
$ docker node rm node-2