Re: Advice on cluster architecture for two related, but distinct, use cases

Some of those requirements are vague, but yes, Patroni should probably be
the first approach you look at. If the second datacenter is just for
redundancy, then a simple setup would be:

DCA (data center A):
Postgres server 1
Postgres server 2

DCB:
Postgres server 3 (set no_failover: true)

You will also need a DCS system of some sort (e.g. etcd on all three
nodes), as well as a backup system (e.g. pgBackRest). Will also need to
decide how automated you want things to be (for example, cross datacenter
failover in the above would be manually done). It should definitely be able
to handle your RPO/RTO requirements easily enough.

[Patroni] However it seems relatively complex to set up and operate

Setting things up can be a little complex, yes, but once done it just
works, so very little operation resources are needed.

We can not assume eg a load balancer.

Possible via the application: see
https://www.postgresql.org/docs/current/libpq-connect.html (esp.
target_session_attrs)

Cheers,
Greg