Question: I'm still wrapping my head around Kubernetes and how that's supposed to work. Currently, I'm struggling to understand how to model something like a PostgreSQL cluster with streaming replication, scaling out and automatic failover/failback (`pgpool-II`, `repmgr`, pick your poison).

My main problem with the approach is the dual nature of a PostgreSQL instance, configuration-wise -- it's either a master or a cold/warm/hot standby. If I increase the number of replicas, I'd expect them all to come up as standbys, so I'd imagine creating a `postgresql-standby` replication controller separately from a `postgresql-master` pod. However I'd also expect one of those standbys to become a master in case current master is down, so it's a common `postgresql` replication controller after all.

The only idea I've had so far is to put the replication configuration on an external volume and manage the state and state changes outside the containers.

*(in case of PostgreSQL the configuration would probably already be on a volume inside its `data` directory, which itself is obviously something I'd want on a volume, but that's beside the point)*

Is that the correct approaach, or is there any other cleaner way?

Answer: 
There's an example in OpenShift: <https://github.com/openshift/postgresql/tree/master/examples/replica> The principle is the same in pure Kube (it's not using anything truly OpenShift specific, and you can use the images in plain docker)
Kubernetes's statefulset is a good base for setting up the stateful service. You will still need some work to configure the correct membership among PostgreSQL replicas. 

Kubernetes has one example for it. <http://blog.kubernetes.io/2017/02/postgresql-clusters-kubernetes-statefulsets.html>