I have a docker swarm environment with three managers. They are named swarm1, swarm2 and swarm3. Due to different circumstances (e.g. network and resources), swarm1 was set as the leader and should stay as the leader. However, after a resource upgrade, swarm1 was rebooted. This led to that swarm2 is set as the leader and swarm1 has now the status reachable. How is it possible to set again swarm1 to the leader?
With swarm managers, it's bad practice to have a "special" node that needs to be the leader at all times. All of your manager nodes should be as identical as possible. But, to answer your question, there is no way to manually set the swarm leader. However, what you can do is docker node demote the leader (swarm2), and the other manager (swarm3). Once the managers are demoted to workers, swarm1 by default becomes the leader. Then all you have to do is docker node promote swarm2 and swarm3.
I'll attempt to write the whole scenario, so everything is in one place about changing the leader and reverting.
Scenario: Three nodes are swarm1, swarm2 and swarm3. First leader (that needs resource upgrade is swarm1)
Step 1
Make swarm2 as leader
docker node promote swarm2
docker node ls
Make sure that the swarm2 Manager status is Reachable (Not Down)
Step 2
Demote swarm1 now to a worker
docker node demote swarm1
docker node ls
Ensure Manager status for swarm1 is now blank.
You may remove the node from swarm.
From swarm1 node, issue:
docker swarm leave
Do the necessary upgrades to the swarm1 node and join it back to the swarm.
Step 3
Rejoin to the swarm and change lead.
From swarm2 issue
docker swarm join-token manager
From swarm1 node issue the join command
eg:
docker swarm join --token <token> <ip:port>
Step 4
Now re-elect the leader.
From swarm1 issue:
docker node demote swarm2
docker node ls
Now you should see your old configuration with swarm1 as leader.
Related
I wish to understand the difference between docker swarm node running as Leader and running as a Manager.
I also, understand that there can be several docker managers but can there be multiple docker swarm Leader nodes and the reasons for it.
Note: im aware of what a docker worker node is.
Docker swarm has following terminology.
Manager Node (Can be a leader or manager)
Worker Node
Now for simple docker swarm mode , there is a single manager and other are worker node. In This manager is a leader.
It is possible to have more then one manager node. Like 2 manager ( Mostly odd number prefer like 1,3,5). In such case to one is leader who is responsible to scheduler task on worker node. Also manager node will talk to each other to maintain the state. To make highly available environment when manager node which is a leader at this moment get down , it should not stop scheduling work. At that moment another manager will automatically promoted as a leader and take responsibility to schedule task (container) on worked node.
I am trying to join a worker node to a manager in another machine. The former one is Mac and later one is Windows. The worker host on Mac have a response:
Timeout was reached before node joined. The attempt to join the swarm will continue in the background. Use the "docker info" command to see the current swarm status of your node.
When I typed the Join-Token command again, I received response saying the
This node is already part of a swarm. Use "docker swarm leave" to leave this swarm and join another one.
When I typed the command in manager side:
docker node ls
it only show one node which is the manager node.
Am I doing something wrong here.
Do you use the same docker version on all hosts?
I have set up a three-node quorum network using docker and if my network crashes, and I only have information of one of the node. Do I want to recover that node using binary? also, the blocks in the new network should be in sync with others. Please guide how is it possible?
I assume you’re using docker swarm. The clustering facility within the swarm is maintained by the manager nodes. If for any reason the manager leader becomes unavailable and no enough remaining managers to reach quorum and elect a new manager leader, a quorum is lost and no manager node is able to control the swarm.
In this kind of situations, it may be necessary to re-initialize the swarm and force the creation of a new cluster using the command on the manager leader when it is brought online again:
# docker swarm init --force-new-cluster
This removes all managers except the manager the command was run from. The good thing is that worker nodes will continue to function normally and the other manager nodes should resume functionality once the swarm has been re-initialized.
Sometimes it might be necessary to remove manager nodes from the swarm and rejoin them to the swarm.
But note that when a node rejoins the swarm, it must join the swarm via a manager node.
You can always monitor the health of manager nodes by querying the docker nodes API in JSON format through the /nodes HTTP endpoint:
# docker node inspect manager1 --format "{{ .ManagerStatus.Reachability }}"
# docker node inspect manager1 --format "{{ .Status.State }}"
Also, make it a practice to perform automate the backup of docker swarm config directory /var/lib/docker/swarm to easily recover from disaster.
I am aware I can manually a constraint to a leader node. But, there may already be a built-in way to specify a leader node in a swarm.
Basically, I need to prevent containers from running on the leader node. They can run anywhere else except the leader. Is there a built in way to specify a leader node in the constraint?
To prevent containers from running on a node, you can do this for all containers using:
docker node update --availability drain $your_node_name
To do this for a single service, you can add a constraint on the node type:
docker service create --constraint 'node.role==worker' --name $your_service $image_name
I don't think there's any way to do this on only the leader with a group of managers, it's all or none. You may be able to script something external that checks the current leader and updates node labels.
In docker swarm mode I can run docker node ls to list swarm nodes but it does not work on worker nodes. I need a similar function. I know worker nodes does not have a strong consistent view of the cluster, but there should be a way to get current leader or reachable leader.
So is there a way to get current leader/manager on worker node on docker swarm mode 1.12.1?
You can get manager addresses by running docker info from a worker.
The docs and the error message from a worker node mention that you have to be on a manager node to execute swarm commands or view cluster state:
Error message from a worker node: "This node is not a swarm manager. Worker nodes can't be used to view or modify cluster state. Please run this command on a manager node or promote the current node to a manager."
After further thought:
One way you could crack this nut is to use an external key/value store like etcd or any other key/value store that Swarm supports and store the elected node there so that it can be queried by all the nodes. You can see examples of that in the Shipyard Docker management / UI project: http://shipyard-project.com/
Another simple way would be to run a redis service on the cluster and another service to announce the elected leader. This announcement service would have a constraint to only run on the manager node(s): --constraint node.role == manager