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Scale Your Docker Containers with Docker Swarm
Extending the Hive
Promoting and Demoting Nodes
Docker allows you to promote and demote nodes, which comes in handy if a manager node becomes unavailable. For example, say your dockermanager needs to go down (for maintenance or some other reason), and you need to promote dockernode1 to a manager role. To do that, issue the command (on the manager):
docker node promote dockernode1
The command will inform you the node has been promoted (Figure 9).
If you want to demote a node (e.g., because you have the original dockermanager back up and functioning), you can demote dockernode1 back to regular node status (Figure 10) with the command (again, run from the manager):
docker node demote dockernode1
You can enter
docker node promote dockernode1 dockernode2 docker node demote dockernode1 dockernode2
to promote/demote more than one node at a time, as well.
Leaving the Swarm
If, for whatever reason you have a node you want to pull out of the swarm, Docker has you covered. Head over to the node you want removed from the swarm and issue the command:
docker swarm leave
Once the node has left the swarm, enter (on the manager)
docker node rm dockernode2 docker node ls
to remove the node from the node list and verify that dockernode2 has, in fact, been removed (Figure 11). You can rejoin that node to the swarm with the same command you used before,
docker swarm join --token TOKEN 192.168.1.71:2377
where TOKEN
is the token issued when the manager was originally initialized.
At some point, you might need to force a manager to leave the swarm. To do this, enter:
docker swarm leave --force
At this point, your swarm is no longer functioning. To get it started again, start from the beginning to recreate your swarm.
Conclusion
With a few quick commands run on your Linux servers, you can scale containers with Docker Swarm to match the needs of your project or your business. Make use of three, four, five, 10, 20, … however many nodes it takes to give your containers the necessary horsepower and failover you need. With the help of a cluster of containers, your business can become more agile and enjoy serious uptime for services and apps.
Infos
- Raft consensus algorithm: https://en.wikipedia.org/wiki/Raft_(computer_science)
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