fix(client mgmt api): cache disconnected durable session count for `/clients` api

Fixes https://emqx.atlassian.net/browse/EMQX-12396

apps/emqx/src/emqx_persistent_session_bookkeeper.erl

@@ -21,7 +21,8 @@
 %% API
 -export([
     start_link/0,
-    get_subscription_count/0
+    get_subscription_count/0,
+    get_disconnected_session_count/0
 ]).
 
 %% `gen_server' API
@@ -39,7 +40,9 @@
 
 %% call/cast/info events
 -record(tally_subs, {}).
+-record(tally_disconnected_sessions, {}).
 -record(get_subscription_count, {}).
+-record(get_disconnected_session_count, {}).
 
 %%------------------------------------------------------------------------------
 %% API
@@ -59,6 +62,16 @@ get_subscription_count() ->
             0
     end.
 
+%% @doc Gets a cached view of the cluster-global count of disconnected persistent sessions.
+-spec get_disconnected_session_count() -> non_neg_integer().
+get_disconnected_session_count() ->
+    case emqx_persistent_message:is_persistence_enabled() of
+        true ->
+            gen_server:call(?MODULE, #get_disconnected_session_count{}, infinity);
+        false ->
+            0
+    end.
+
 %%------------------------------------------------------------------------------
 %% `gen_server' API
 %%------------------------------------------------------------------------------
@@ -66,7 +79,10 @@ get_subscription_count() ->
 init(_Opts) ->
     case emqx_persistent_message:is_persistence_enabled() of
         true ->
-            State = #{subs_count => 0},
+            State = #{
+                subs_count => 0,
+                disconnected_session_count => 0
+            },
             {ok, State, {continue, #tally_subs{}}};
         false ->
             ignore
@@ -75,11 +91,18 @@ init(_Opts) ->
 handle_continue(#tally_subs{}, State0) ->
     State = tally_persistent_subscriptions(State0),
     ensure_subs_tally_timer(),
+    {noreply, State, {continue, #tally_disconnected_sessions{}}};
+handle_continue(#tally_disconnected_sessions{}, State0) ->
+    State = tally_disconnected_persistent_sessions(State0),
+    ensure_disconnected_sessions_tally_timer(),
     {noreply, State}.
 
 handle_call(#get_subscription_count{}, _From, State) ->
     #{subs_count := N} = State,
     {reply, N, State};
+handle_call(#get_disconnected_session_count{}, _From, State) ->
+    #{disconnected_session_count := N} = State,
+    {reply, N, State};
 handle_call(_Call, _From, State) ->
     {reply, {error, bad_call}, State}.
 
@@ -90,6 +113,10 @@ handle_info(#tally_subs{}, State0) ->
     State = tally_persistent_subscriptions(State0),
     ensure_subs_tally_timer(),
     {noreply, State};
+handle_info(#tally_disconnected_sessions{}, State0) ->
+    State = tally_disconnected_persistent_sessions(State0),
+    ensure_disconnected_sessions_tally_timer(),
+    {noreply, State};
 handle_info(_Info, State) ->
     {noreply, State}.
 
@@ -101,7 +128,38 @@ tally_persistent_subscriptions(State0) ->
     N = emqx_persistent_session_ds_state:total_subscription_count(),
     State0#{subs_count := N}.
 
+tally_disconnected_persistent_sessions(State0) ->
+    N = do_tally_disconnected_persistent_sessions(),
+    State0#{disconnected_session_count := N}.
+
 ensure_subs_tally_timer() ->
     Timeout = emqx_config:get([durable_sessions, subscription_count_refresh_interval]),
     _ = erlang:send_after(Timeout, self(), #tally_subs{}),
     ok.
+
+ensure_disconnected_sessions_tally_timer() ->
+    Timeout = emqx_config:get([durable_sessions, disconnected_session_count_refresh_interval]),
+    _ = erlang:send_after(Timeout, self(), #tally_disconnected_sessions{}),
+    ok.
+
+do_tally_disconnected_persistent_sessions() ->
+    Iter = emqx_persistent_session_ds_state:make_session_iterator(),
+    do_tally_disconnected_persistent_sessions(Iter, 0).
+
+do_tally_disconnected_persistent_sessions('$end_of_table', N) ->
+    N;
+do_tally_disconnected_persistent_sessions(Iter0, N) ->
+    case emqx_persistent_session_ds_state:session_iterator_next(Iter0, 1) of
+        {[], _} ->
+            N;
+        {[{Id, _Meta}], Iter} ->
+            case is_live_session(Id) of
+                true ->
+                    do_tally_disconnected_persistent_sessions(Iter, N);
+                false ->
+                    do_tally_disconnected_persistent_sessions(Iter, N + 1)
+            end
+    end.
+
+is_live_session(Id) ->
+    [] =/= emqx_cm_registry:lookup_channels(Id).

apps/emqx/src/emqx_schema.erl

@@ -1719,6 +1719,14 @@ fields("durable_sessions") ->
                     importance => ?IMPORTANCE_HIDDEN
                 }
             )},
+        {"disconnected_session_count_refresh_interval",
+            sc(
+                timeout_duration(),
+                #{
+                    default => <<"5s">>,
+                    importance => ?IMPORTANCE_HIDDEN
+                }
+            )},
         {"message_retention_period",
             sc(
                 timeout_duration(),

apps/emqx/test/emqx_config_SUITE.erl

@@ -476,6 +476,7 @@ zone_global_defaults() ->
                 renew_streams_interval => 5000,
                 session_gc_batch_size => 100,
                 session_gc_interval => 600000,
-                subscription_count_refresh_interval => 5000
+                subscription_count_refresh_interval => 5000,
+                disconnected_session_count_refresh_interval => 5000
             }
     }.

apps/emqx_management/src/emqx_mgmt_api_clients.erl

@@ -1383,7 +1383,6 @@ do_list_clients_cluster_query(
         {Rows, QueryState1 = #{complete := Complete0}} ->
             case emqx_mgmt_api:accumulate_query_rows(Node, Rows, QueryState1, ResultAcc) of
                 {enough, NResultAcc} ->
-                    %% TODO: add persistent session count?
                     %% TODO: this may return `{error, _, _}'...
                     QueryState2 = emqx_mgmt_api:maybe_collect_total_from_tail_nodes(
                         Tail, QueryState1
@@ -1427,8 +1426,9 @@ add_persistent_session_count(QueryState0 = #{total := Totals0}) ->
             %% to traverse the whole table), but also hard to deduplicate live connections
             %% from it...  So this count will possibly overshoot the true count of
             %% sessions.
-            SessionCount = persistent_session_count(),
-            Totals = Totals0#{undefined => SessionCount},
+            DisconnectedSessionCount =
+                emqx_persistent_session_bookkeeper:get_disconnected_session_count(),
+            Totals = Totals0#{undefined => DisconnectedSessionCount},
             QueryState0#{total := Totals};
         false ->
             QueryState0
@@ -1476,27 +1476,6 @@ no_persistent_sessions() ->
             true
     end.
 
--spec persistent_session_count() -> non_neg_integer().
-persistent_session_count() ->
-    %% N.B.: this is potentially costly.  Should not be called in hot paths.
-    %% `mnesia:table_info(_, size)' is always zero for rocksdb, so we need to traverse...
-    do_persistent_session_count(init_persistent_session_iterator(), 0).
-
-do_persistent_session_count('$end_of_table', N) ->
-    N;
-do_persistent_session_count(Cursor, N) ->
-    case emqx_persistent_session_ds_state:session_iterator_next(Cursor, 1) of
-        {[], _} ->
-            N;
-        {[{_Id, Meta}], NextCursor} ->
-            case is_expired(Meta) of
-                true ->
-                    do_persistent_session_count(NextCursor, N);
-                false ->
-                    do_persistent_session_count(NextCursor, N + 1)
-            end
-    end.
-
 is_expired(#{last_alive_at := LastAliveAt, expiry_interval := ExpiryInterval}) ->
     LastAliveAt + ExpiryInterval < erlang:system_time(millisecond).
 

apps/emqx_management/test/emqx_mgmt_api_clients_SUITE.erl

@@ -79,7 +79,9 @@ end_per_suite(Config) ->
 
 init_per_group(persistent_sessions, Config) ->
     AppSpecs = [
-        {emqx, "durable_sessions.enable = true"},
+        {emqx,
+            "durable_sessions.enable = true\n"
+            "durable_sessions.disconnected_session_count_refresh_interval = 100ms"},
         emqx_management
     ],
     Dashboard = emqx_mgmt_api_test_util:emqx_dashboard(
@@ -457,9 +459,7 @@ t_persistent_sessions5(Config) ->
                     {{_, 200, _}, _, #{
                         <<"data">> := [_, _, _],
                         <<"meta">> := #{
-                            %% TODO: if/when we fix the persistent session count, this
-                            %% should be 4.
-                            <<"count">> := 6,
+                            <<"count">> := 4,
                             <<"hasnext">> := true
                         }
                     }}},
@@ -470,9 +470,7 @@ t_persistent_sessions5(Config) ->
                     {{_, 200, _}, _, #{
                         <<"data">> := [_],
                         <<"meta">> := #{
-                            %% TODO: if/when we fix the persistent session count, this
-                            %% should be 4.
-                            <<"count">> := 6,
+                            <<"count">> := 4,
                             <<"hasnext">> := false
                         }
                     }}},
@@ -489,9 +487,7 @@ t_persistent_sessions5(Config) ->
                     {{_, 200, _}, _, #{
                         <<"data">> := [_, _],
                         <<"meta">> := #{
-                            %% TODO: if/when we fix the persistent session count, this
-                            %% should be 4.
-                            <<"count">> := 6,
+                            <<"count">> := 4,
                             <<"hasnext">> := true
                         }
                     }}},
@@ -1996,7 +1992,11 @@ assert_single_client(Opts) ->
         100,
         20,
         ?assertMatch(
-            {ok, {{_, 200, _}, _, #{<<"data">> := [#{<<"connected">> := IsConnected}]}}},
+            {ok,
+                {{_, 200, _}, _, #{
+                    <<"data">> := [#{<<"connected">> := IsConnected}],
+                    <<"meta">> := #{<<"count">> := 1}
+                }}},
             list_request(APIPort)
         )
     ),