Add configuration for StatefulSet updateStrategy

[lawliet89]

If Auto Unseal is configured, it should be safe to allow a rolling update strategy. This should be an option for a less involved upgrade process.

I have been using the "incubator/vault" chart which deploys Vault as a Deployment and I've not had issues performing upgrades.

[jasonodonnell]

Hi @lawliet89, thanks for the contribution.

The update strategy is this way to avoid upgrading the primary Vault server before replicas in HA clusters. I think would probably make sense if you aren't running in HA mode, but this isn't helpful for HA clusters. Incubator can get away with rolling updates because it's a single deployment, not a statefulset.

[lawliet89]

Could you explain to me how a rolling update for a StatefulSet would differ from a Deployment please? I think it would be nice to have an option (that is not default) to have a "hands-off" upgrade approach. Otherwise, it's quite laborious to upgrade Vault.

Also, the zeroth replica might not always be the leader too.

[jasonodonnell]

@lawliet89 Actually, I was wrong, there wouldn't be a difference between the two. I'm going to merge this as there are use cases for non-HA deployments.

[fischerman]

@lawliet89 Beware that this does not free you from recreating the pods if the config file changes