fix: run observer_web as hidden node to stop OTP global splitting IFT Mnesia cluster.

Without --hidden, observer joins the visible mesh and prevent_overlapping_partitions disconnects eventhub-node1/2 (running_partitioned_network → admin verification-token 404 in e2e).

docker/ObserverWeb.Dockerfile

@@ -39,4 +39,7 @@ COPY docker/observer_web/dev.exs ./dev.exs
 EXPOSE 4000
 
 ENV RELEASE_COOKIE=eventhub_cookie
-CMD elixir --sname observer_web@observer_web --cookie "${RELEASE_COOKIE}" -S mix run --no-halt dev.exs
+# --hidden: observer must not join the visible global mesh. Without it, OTP
+# prevent_overlapping_partitions disconnects eventhub-node1 <-> eventhub-node2
+# when observer connects to both (Mnesia running_partitioned_network → e2e 404).
+CMD elixir --hidden --sname observer_web@observer_web --cookie "${RELEASE_COOKIE}" -S mix run --no-halt dev.exs

src/infra/cluster_discovery.erl

@@ -16,14 +16,21 @@ discover_loop() ->
         io:format("Checking epmd on ~s...~n", [IPStr]),
         case erl_epmd:names(IP) of
           {ok, List} ->
+            %% Only eventhub-node* — connecting to observer_web (same cookie)
+            %% triggers OTP prevent_overlapping_partitions and splits the mesh.
             lists:foreach(fun({Name, _Port}) ->
+              case lists:prefix("eventhub-node", Name) of
+                false ->
+                  ok;
+                true ->
                   Node = list_to_atom(Name ++ "@" ++ Name),
                   io:format("    Trying net_kernel:connect_node(~s)...~n", [Node]),
                   case net_kernel:connect_node(Node) of
-                true  -> io:format("    *** Connected to ~s ***~n", [Node]), join_and_replicate(Node); %io:format("    *** Connected to ~s ***~n", [Node]);
+                    true  -> io:format("    *** Connected to ~s ***~n", [Node]), join_and_replicate(Node);
                     false -> io:format("    *** Failed to connect to ~s ***~n", [Node]);
                     ignored -> ok
                   end
+              end
             end, List);
           {error, Reason} ->
             io:format("  epmd error on ~s: ~p~n", [IPStr, Reason])