add API to list voters

Author: Alex Valiushko

src/ra.erl

@@ -23,6 +23,8 @@
          %% queries
          members/1,
          members/2,
+         voters/1,
+         voters/2,
          initial_members/1,
          initial_members/2,
          local_query/2,
@@ -1037,6 +1039,44 @@ members({local, ServerId}, Timeout) ->
 members(ServerId, Timeout) ->
     ra_server_proc:state_query(ServerId, members, Timeout).
 
+%% @doc Returns a list of cluster voters
+%%
+%% Except if `{local, ServerId}' is passed, the query is sent to the specified
+%% server which may redirect it to the leader if it is a follower. It may
+%% timeout if there is currently no leader (i.e. an election is in progress).
+%%
+%% With `{local, ServerId}', the query is always handled by the specified
+%% server. It means the returned list might be out-of-date compared to what the
+%% leader would have returned.
+%%
+%% @param ServerId the Ra server(s) to send the query to
+%% @end
+-spec voters(ra_server_id() | [ra_server_id()] | {local, ra_server_id()}) ->
+    ra_server_proc:ra_leader_call_ret([ra_server_id()]).
+voters(ServerId) ->
+    voters(ServerId, ?DEFAULT_TIMEOUT).
+
+%% @doc Returns a list of cluster voters
+%%
+%% Except if `{local, ServerId}' is passed, the query is sent to the specified
+%% server which may redirect it to the leader if it is a follower. It may
+%% timeout if there is currently no leader (i.e. an election is in progress).
+%%
+%% With `{local, ServerId}', the query is always handled by the specified
+%% server. It means the returned list might be out-of-date compared to what the
+%% leader would have returned.
+%%
+%% @param ServerId the Ra server(s) to send the query to
+%% @param Timeout the timeout to use
+%% @end
+-spec voters(ra_server_id() | [ra_server_id()] | {local, ra_server_id()},
+              timeout()) ->
+    ra_server_proc:ra_leader_call_ret([ra_server_id()]).
+voters({local, ServerId}, Timeout) ->
+    ra_server_proc:local_state_query(ServerId, voters, Timeout);
+voters(ServerId, Timeout) ->
+    ra_server_proc:state_query(ServerId, voters, Timeout).
+
 %% @doc Returns a list of initial (seed) cluster members.
 %%
 %% This allows Ra-based systems with dynamic cluster membership

src/ra_server_proc.erl

@@ -1552,7 +1552,7 @@ do_state_query(voters, #{cluster := Cluster}) ->
                                     end
                            end
               end, [], Cluster),
-    Vs;
+    lists:sort(Vs);
 do_state_query(members, #{cluster := Cluster}) ->
     maps:keys(Cluster);
 do_state_query(initial_members, #{log := Log}) ->

test/ra_SUITE.erl

@@ -44,6 +44,8 @@ all_tests() ->
      local_query_boom,
      local_query_stale,
      members,
+     voters_promotable,
+     voters_non_voter,
      consistent_query,
      consistent_query_after_restart,
      consistent_query_minority,
@@ -537,6 +539,29 @@ members(Config) ->
     {ok, Cluster, Leader} = ra:members(Leader),
     terminate_cluster(Cluster).
 
+voters_promotable(Config) ->
+    Name = ?config(test_name, Config),
+    [A, B] = Cluster = start_local_cluster(2, Name, add_machine()),
+    {ok, _, Leader} = ra:process_command(A, 9),
+    C = {ra_server:name(Name, "3"), node()},
+    ok = ra:start_server(default, Name, C, add_machine(), Cluster),
+    {ok, _, _} = ra:add_member(Leader, #{id => C, uid => <<"4">>, membership => promotable}),
+    {ok, [A, B, C], Leader} = ra:members(Leader),
+    {ok, [A, B], Leader} = ra:voters(Leader),
+    terminate_cluster([C | Cluster]).
+
+voters_non_voter(Config) ->
+    Name = ?config(test_name, Config),
+    [A, B] = Cluster = start_local_cluster(2, Name, add_machine()),
+    {ok, _, Leader} = ra:process_command(A, 9),
+        C = {ra_server:name(Name, "3"), node()},
+    ok = ra:start_server(default, Name, C, add_machine(), Cluster),
+    {ok, _, _} = ra:add_member(Leader, #{id => C, uid => <<"4">>, membership => non_voter}),
+    {ok, [A, B, C], Leader} = ra:members(Leader),
+    {ok, [A, B], Leader} = ra:voters(Leader),
+    terminate_cluster([C | Cluster]).
+
+
 consistent_query(Config) ->
     [A, _, _]  = Cluster = start_local_cluster(3, ?config(test_name, Config),
                                                add_machine()),
@@ -1073,7 +1098,7 @@ voter_gets_promoted_consistent_leader(Config) ->
     timer:sleep(100),
     All = [N1, N2, N3],
     % in server state
-    lists:map(fun(O) -> ?assertEqual(All, voters(O)) end, overviews(N1)),
+    lists:map(fun(O) -> ?assertEqual(All, filter_voters(O)) end, overviews(N1)),
     % in ets
     #{servers := Servers} = ra:overview(?SYS),
     lists:map(fun({Name, _}) -> #{Name := #{membership := voter}} = Servers end, All),
@@ -1098,7 +1123,7 @@ voter_gets_promoted_new_leader(Config) ->
     timer:sleep(100),
     All = [N1, N2, N3],
     % in server state
-    lists:map(fun(O) -> ?assertEqual(All, voters(O)) end, overviews(N1)),
+    lists:map(fun(O) -> ?assertEqual(All, filter_voters(O)) end, overviews(N1)),
     % in ets
     #{servers := Servers} = ra:overview(?SYS),
     lists:map(fun({Name, _}) -> #{Name := #{membership := voter}} = Servers end, All),
@@ -1217,7 +1242,7 @@ overviews(Node) ->
     {ok, Members, _From} = ra:members(Node),
     [ra:member_overview(P) || {_, _} = P <- Members].
 
-voters({ok, #{cluster := Peers}, _} = _Overview) ->
+filter_voters({ok, #{cluster := Peers}, _} = _Overview) ->
     [Id || {Id, Status} <- maps:to_list(Peers), maps:get(membership, Status, voter) == voter].
 
 %% machine impl