[Hawkular-dev] Rolling upgrades (on Kubernetes)

[Heiko W.Rupp]

Hey,

on Kubernetes a common use case (that has been demonstrated by Thomas
and Juca recently) are rolling upgrades (blue/green, canary, ...) where a
new version of a service comes into play, gets scaled up and when this
is scaled up, the old version of the service gets scaled down and eventually
disappears.
It can also happen though that the new version is buggy and the previous
version gets scaled up again to take the full load until a fix is available.

We need to make sure to support those rolling upgrades and also the
rollback to the previous version of a service.

E.g. if a (No)Sql schema update happens in version N+1, a still
running instance of version N must not break by it, when N+1
is rolling out while N is still active. Similarly when N+1 has upgraded
the schema and the application gets rolled back to version N, it must
still be able to cope with the upgraded schema version.

Same applies to APIs, but this is something we have discussed and
mastered in the past.

We should perhaps investigate how we can automatically test
this scenario in our Travis testing.