[DPE-5714] Filter out degraded read only endpoints

[dragomirp]

Currently, the charm sets all replica units as readonly endpoints to clients, no matter if the unit is functioning or not.

The PR adds filtering for the read only endpoints based on Patroni's cluster endpoint.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.49%. Comparing base (f50d373) to head (0efacb1).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #679      +/-   ##
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+ Coverage   71.40%   71.49%   +0.08%     
==========================================
  Files          13       13              
  Lines        3189     3199      +10     
  Branches      475      476       +1     
==========================================
+ Hits         2277     2287      +10     
  Misses        797      797              
  Partials      115      115              

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[dragomirp]

I think that if the majority of units goes down, the cluster endpoint will stop being updated. Best to address this after the RAFT reinit PRs land.

[dragomirp]

We are getting the primary endpoint just above, so we should be able to get the cluster endpoint as well, but if we can't, no filtering will be done.

[dragomirp]

There are two units in the cluster, so there will be no readonly endpoints if the down unit is filtered out.

[marceloneppel]

LGTM!

[taurus-forever]

LGTM, but two points:

• Q: will we trigger DPlib events, so client app will react (restart) every time we add RO unit?
• BTW, did we agree on consensus with @delgod and @7annaba3l about adding RW as RO endpoint if the system has 1 unit only?

[dragomirp]

• Q: will we trigger DPlib events, so client app will react (restart) every time we add RO unit?

DPL should fire a separate event (read_only_endpoints_changed) for read only changes so it would be up to the client charm to decide.

• BTW, did we agree on consensus with @delgod and @7annaba3l about adding RW as RO endpoint if the system has 1 unit only?

I don't think so, we can add it in a follow up PR.