add comment for memberlist config auto-apply, update changelog

Signed-off-by: Trevor Whitney <trevorjwhitney@gmail.com>

CHANGELOG.md

@@ -1,4 +1,5 @@
 ## Main
+* [4400](https://github.com/grafana/loki/pull/4400) **trevorwhitney**: Config: automatically apply memberlist config too all rings when provided
 
 # 2.3.0 (2021/08/06)
 

pkg/loki/config_wrapper.go

@@ -71,12 +71,21 @@ func (c *ConfigWrapper) ApplyDynamicConfig() cfg.Source {
 			}
 		}
 
-		if len(r.MemberlistKV.JoinMembers) > 0 {
-			r.Ingester.LifecyclerConfig.RingConfig.KVStore.Store = memberlistStr
-			r.Distributor.DistributorRing.KVStore.Store = memberlistStr
-			r.Ruler.Ring.KVStore.Store = memberlistStr
-		}
+		applyMemberlistConfig(r)
 
 		return nil
 	}
 }
+
+// applyMemberlistConfig will change the default ingester, distributor, and ruler ring configurations to use memberlist
+// if the -memberlist.join_members config is provided. The idea here is that if a user explicitly configured the
+// memberlist configuration section, they probably want to be using memberlist for all their ring configurations.
+// Since a user can still explicitly override a specific ring configuration (for example, use consul for the distributor),
+// it seems harmless to take a guess at better defaults here.
+func applyMemberlistConfig(r *ConfigWrapper) {
+	if len(r.MemberlistKV.JoinMembers) > 0 {
+		r.Ingester.LifecyclerConfig.RingConfig.KVStore.Store = memberlistStr
+		r.Distributor.DistributorRing.KVStore.Store = memberlistStr
+		r.Ruler.Ring.KVStore.Store = memberlistStr
+	}
+}