I need to upgrade old kubernetes cluster.
Current kubernetes cluster consists of nodes with flannel-0.5.3(CentOS Atomic Host 7.20160707).
What will happen if I add nodes with different flannel version, for example flannel-0.7.1(CentOS Atomic Host 7.1803).
So, that during transition period some node will be running with 0.5.3 version of flannel and new ones will use 0.7.1. I need to perform that online without any downtime.
In discussion, let's skip any details except flannel version. The only open question is: if flannel backward compatible?
There is no information on the official Flannel Project website or Releases document about backward compatibility between different versions. However, for the versions v0.9.0-rc1 and v0.9.0 they announced full compatibility with previous builds allowing easy upgrade.
Fully compatible with previous versions of flannel to allow for easy upgrade
Therefore, a discrepancy between various Flannel releases doesn't guarantee stability and fewer service issues in Kubernetes cluster.
It is strongly recommended to keep Kubernetes components within the same version on each Node, as the latest builds fixed common bugs and brought some new features, which might not be supported by old versions.