FR: backup servers in the server list

[vthriller]

Is your feature request related to a problem? Please describe.

I'd like to contain traffic within each data center, and only use other servers if all local peers are banned via health check.

Describe the solution you'd like
Describe alternatives you've considered

I'd like to see an option that would mark server as backup, similar to server ... backup in nginx/haproxy. Such server would not receive traffic unless all other servers are banned. It might be an extra option added to a servers item, it might be separate array with similar structure.

Additional context

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[is-jonreeves]

We started evaluating pgcat recently and came looking for this feature too. Similarly, we have various out-of-region replicas that we would like to include as servers but not be part of the usual rotation (for latency sake), unless absolutely necessary.

There is also the potential for standby primary instances that would be good to fallback to.

Right now, it looks like we'd have to layer HAProxys (for their backup server capabilities) with our pgcats, but it would be much simpler if we didn't have to. As such, we would also love to see this feature.