cache raft membership information

Summary: This allows us to cache membership information similarly to how we cache state and current term to `shardmd` can observe it.

Differential Revision: D53683966

fbshipit-source-id: 5a3e9bc00b93c2d409906b522c6cdf0bb29029ad

src/wa_raft_info.erl

@@ -12,20 +12,22 @@
 
 %% Public API
 -export([
-    get_state/2,
     get_current_term/2,
     get_leader/2,
-    get_stale/2
+    get_membership/2,
+    get_stale/2,
+    get_state/2
 ]).
 
 %% Internal API
 -export([
     init_tables/0,
-    set_state/3,
     delete_state/2,
     set_current_term/3,
     set_leader/3,
-    set_stale/3
+    set_membership/3,
+    set_stale/3,
+    set_state/3
 ]).
 
 %% Local RAFT server's current FSM state
@@ -36,6 +38,8 @@
 -define(RAFT_CURRENT_LEADER_KEY(Table, Partition), {leader, Table, Partition}).
 %% Local RAFT server's current stale flag - indicates if the server thinks its data is stale
 -define(RAFT_STALE_KEY(Table, Partition), {stale, Table, Partition}).
+%% Local RAFT server's most recently known membership
+-define(RAFT_MEMBERSHIP_KEY(Table, Partition), {membership, Table, Partition}).
 
 %%-------------------------------------------------------------------
 %% RAFT Info - Public API
@@ -65,6 +69,10 @@ get_state(Table, Partition) ->
 get_stale(Table, Partition) ->
     get(?RAFT_STALE_KEY(Table, Partition), true).
 
+-spec get_membership(wa_raft:table(), wa_raft:partition()) -> wa_raft_server:membership() | undefined.
+get_membership(Table, Partition) ->
+    get(?RAFT_MEMBERSHIP_KEY(Table, Partition), undefined).
+
 %%-------------------------------------------------------------------
 %% RAFT Info - Internal API
 %%-------------------------------------------------------------------
@@ -101,3 +109,7 @@ delete_state(Table, Partition) ->
 -spec set_stale(wa_raft:table(), wa_raft:partition(), boolean()) -> true.
 set_stale(Table, Partition, Stale) ->
     set(?RAFT_STALE_KEY(Table, Partition), Stale).
+
+-spec set_membership(wa_raft:table(), wa_raft:partition(), wa_raft_server:membership()) -> true.
+set_membership(Table, Partition, Membership) ->
+    set(?RAFT_MEMBERSHIP_KEY(Table, Partition), Membership).

src/wa_raft_server.erl

@@ -1820,9 +1820,10 @@ config_index(#raft_state{cached_config = undefined} = State) ->
 %% available in the RAFT log and so needs to be kept in sync with what
 %% the RAFT server expects is in storage.
 -spec load_config(State :: #raft_state{}) -> NewState :: #raft_state{}.
-load_config(#raft_state{storage = Storage} = State) ->
+load_config(#raft_state{storage = Storage, table = Table, partition = Partition} = State) ->
     case wa_raft_storage:read_metadata(Storage, config) of
         {ok, #raft_log_pos{index = ConfigIndex}, Config} ->
+            wa_raft_info:set_membership(Table, Partition, maps:get(membership, Config, [])),
             State#raft_state{cached_config = {ConfigIndex, maybe_upgrade_config(Config)}};
         undefined ->
             State#raft_state{cached_config = undefined};
@@ -1840,7 +1841,8 @@ maybe_upgrade_config(#{version := ?RAFT_CONFIG_CURRENT_VERSION} = Config) ->
 %% being applied. If it is, then update the cached configuration.
 -spec maybe_update_config(Index :: wa_raft_log:log_index(), Term :: wa_raft_log:log_term(),
                           Op :: wa_raft_acceptor:op() | [] | undefined, State :: #raft_state{}) -> NewState :: #raft_state{}.
-maybe_update_config(Index, _Term, {_Ref, {config, Config}}, State) ->
+maybe_update_config(Index, _Term, {_Ref, {config, Config}}, #raft_state{table = Table, partition = Partition} = State) ->
+    wa_raft_info:set_membership(Table, Partition, maps:get(membership, Config, [])),
     State#raft_state{cached_config = {Index, Config}};
 maybe_update_config(_Index, _Term, _Op, State) ->
     State.