Document DB load balancing SRV records handling

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/14048
Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/14049

doc/administration/database_load_balancing.md

@@ -146,7 +146,11 @@ The following options can be set:
 | `use_tcp`            | Lookup DNS resources using TCP instead of UDP                                                     | false     |
 
 If `record_type` is set to `SRV`, GitLab will continue to use a round-robin algorithm
-and will ignore the `weight` and `priority` in the record.
+and will ignore the `weight` and `priority` in the record. Since SRV records usually
+return hostnames instead of IPs, GitLab will look for the IPs of returned hostnames
+in the additional section of the SRV response. If no IP is found for a hostname, Gitlab
+will query the configured `nameserver` for ANY record for each such hostname looking for A or AAAA records, eventually
+dropping this hostname from rotation if it can't resolve its IP.
 
 The `interval` value specifies the _minimum_ time between checks. If the A
 record has a TTL greater than this value, then service discovery will honor said

ee/changelogs/unreleased/add-docs-for-srv-handling.yml

+---
+title: Document SRV handling for DB load balancing
+merge_request: 16000
+author:
+type: other

ee/lib/gitlab/database/load_balancing/srv_resolver.rb

@@ -3,6 +3,12 @@
 module Gitlab
   module Database
     module LoadBalancing
+      # Hostnames returned in SRV records cannot sometimes be resolved by a local
+      # resolver, however, there's a possibility that their A/AAAA records are
+      # returned as part of the SRV query in the additional section, so we try
+      # to extract the IPs from there first, failing back to querying the
+      # hostnames A/AAAA records one by one, using the same resolver that
+      # queried the SRV record.
       class SrvResolver
         include Gitlab::Utils::StrongMemoize