Make it possible to opt out of peer discovery registration

for the backends that support it in the first place.

When forming a cluster, registration of the node
joining the cluster might be left to (container)
orchestration tools like Nomad or Kubernetes.

This PR add a new configuration option,
'cluster_formation.registration.enable',
which defaults to true.
When set to false node registration will be skipped.

There is at least one important advantage using a
tool such as Nomad (plus Consul) over the application
(RabbitMQ) doing the registration.

When the application is not stopped gracefully for
any reason, e.g. its OOM killed,
it cannot deregister the service/node.

This leaves behind an unlinked service entry in the registry.
This problem is fundamentally avoided by allowing
Nomad (or similar tools) to register the
node'service.

See #11233  #11045 for prior discussions.

Co-authored-by: Frederik Bosch <[email protected]>
(cherry picked from commit 269685d)

Author: michaelklishin

deps/rabbit/priv/schema/rabbit.schema

@@ -1373,7 +1373,7 @@ end}.
 %% Register node during cluster formation when backend supports registration.
 %%
 
-{mapping, "cluster_formation.registration", "rabbit.cluster_formation.register", [
+{mapping, "cluster_formation.registration.enabled", "rabbit.cluster_formation.perform_registration", [
     {datatype, {enum, [true, false]}}
 ]}.
 

deps/rabbit/src/rabbit_peer_discovery.erl

@@ -46,8 +46,8 @@
 %% a new cluster as a virgin node
 -define(DEFAULT_NODE_TYPE, disc).
 
-%% register node by default
--define(DEFAULT_REGISTRATION, true).
+%% Register node by default (with the backends that support registration)
+-define(PERFORM_REGISTRATION_BY_DEFAULT, true).
 
 %% default node prefix to attach to discovered hostnames
 -define(DEFAULT_PREFIX, "rabbit").
@@ -85,14 +85,14 @@ node_type() ->
             ?DEFAULT_NODE_TYPE
     end.
 
--spec registration() -> true | false.
+-spec should_perform_registration() -> true | false.
 
-registration() ->
+should_perform_registration() ->
     case application:get_env(rabbit, cluster_formation) of
         {ok, Proplist} ->
-            proplists:get_value(registration, Proplist, ?DEFAULT_REGISTRATION);
+            proplists:get_value(perform_registration, Proplist, ?PERFORM_REGISTRATION_BY_DEFAULT);
         undefined ->
-            ?DEFAULT_REGISTRATION
+            ?PERFORM_REGISTRATION_BY_DEFAULT
     end.
 
 -spec lock_acquisition_failure_mode() -> ignore | fail.
@@ -981,7 +981,7 @@ error_description({invalid_cluster_node_type, BadType}) ->
 -spec maybe_register() -> ok.
 
 maybe_register() ->
-    case registration() of
+    case should_perform_registration() of
         true ->
             Backend = persistent_term:get(?PT_PEER_DISC_BACKEND, backend()),
             case Backend:supports_registration() of
@@ -994,21 +994,21 @@ maybe_register() ->
                     ok;
                 false ->
                     ?LOG_DEBUG(
-                       "Peer discovery: registration unsupported, skipping register",
+                       "Peer discovery: registration is not supported, skipping it",
                        #{domain => ?RMQLOG_DOMAIN_PEER_DISC}),
                     ok
-            end.
+            end;
         false ->
             ?LOG_DEBUG(
-               "Peer discovery: registration disabled, skipping register",
+               "Peer discovery: registration is disabled via configuration, skipping it",
                #{domain => ?RMQLOG_DOMAIN_PEER_DISC}),
             ok
     end.
 
 -spec maybe_unregister() -> ok.
 
 maybe_unregister() ->
-    case registration() of
+    case should_perform_registration() of
         true ->
             Backend = persistent_term:get(?PT_PEER_DISC_BACKEND),
             case Backend:supports_registration() of
@@ -1019,13 +1019,13 @@ maybe_unregister() ->
                     unregister(Backend);
                 false ->
                     ?LOG_DEBUG(
-                       "Peer discovery: registration unsupported, skipping unregister",
+                       "Peer discovery: registration is not supported, skipping unregistration",
                        #{domain => ?RMQLOG_DOMAIN_PEER_DISC}),
                     ok
-            end.
+            end;
         false ->
             ?LOG_DEBUG(
-               "Peer discovery: registration disabled, skipping unregister",
+               "Peer discovery: registration is disabled via configuration, skipping unregistration",
                #{domain => ?RMQLOG_DOMAIN_PEER_DISC}),
             ok
     end.

deps/rabbit/test/config_schema_SUITE_data/rabbit.snippets

@@ -310,6 +310,25 @@ cluster_formation.dns.hostname = discovery.eng.example.local",
    ]}],
  []},
 
+ %% registration is enabled by default for the backends that support it
+ {cluster_formation_explicitly_enable_of_registration,
+   "cluster_formation.registration.enabled = true",
+   [{rabbit,
+        [{cluster_formation, [
+          {perform_registration, true}
+         ]}]
+   }],
+   []},
+
+  {cluster_formation_opt_out_of_registration,
+   "cluster_formation.registration.enabled = false",
+   [{rabbit,
+        [{cluster_formation, [
+          {perform_registration, false}
+         ]}]
+   }],
+   []},
+
  {tcp_listen_options,
   "tcp_listen_options.backlog = 128
 tcp_listen_options.nodelay = true