I'd like to upgrade the Docker engine on my Docker Swarm managed nodes (both manager and worker nodes) from 18.06 to 19.03, without causing any downtime.
I see there are many tutorials online for rolling update of a Dockerized application without downtime, but nothing related to upgrading the Docker engine on all Docker Swarm managed nodes.
Is it really not possible to upgrade the Docker daemon on Docker Swarm managed nodes without a downtime? If true, that would indeed be a pity.
Thanks in advance to the wonderful community at SO!
You can upgrade managers, in place, one at a time. During this upgrade process, you would drain the node with docker node update, and run the upgrade to the docker engine with the normal OS commands, and then return the node to active. What will not work is to add or remove nodes to the cluster while the managers have mixed versions. This means you cannot completely replace nodes with an install from scratch at the same time you upgrade the versions. All managers need to be the same version (upgraded) and then you can look at rebuilding/replacing the hosts. What I've seen in the past is that nodes do not fully join the manager quorum, and after losing enough managers you eventually lose quorum.
Once all managers are upgraded, then you can upgrade the workers, either with in place upgrades or replacing the nodes. Until the workers have all been upgraded, do not use any new features.