feat(api): add /sessions_count api to count sessions

apps/emqx/src/emqx_cm_registry.erl

@@ -283,4 +283,4 @@ fold_hist(F, List) ->
 %% Return the session registration history retain duration.
 -spec retain_duration() -> non_neg_integer().
 retain_duration() ->
-    emqx:get_config([broker, session_registration_history_retain]).
+    emqx:get_config([broker, session_history_retain]).

apps/emqx/src/emqx_cm_registry_cleaner.erl

@@ -15,10 +15,13 @@
 %%--------------------------------------------------------------------
 
 %% @doc This module implements the global session registry history cleaner.
--module(emqx_cm_registry_cleaner).
+-module(emqx_cm_registry_keeper).
 -behaviour(gen_server).
 
--export([start_link/0]).
+-export([
+    start_link/0,
+    count/1
+]).
 
 %% gen_server callbacks
 -export([
@@ -30,8 +33,15 @@
     code_change/3
 ]).
 
+-include_lib("stdlib/include/ms_transform.hrl").
 -include("emqx_cm.hrl").
 
+-define(CACHE_COUNT_THRESHOLD, 1000).
+-define(MIN_COUNT_INTERVAL_SECONDS, 5).
+-define(CLEANUP_CHUNK_SIZE, 10000).
+
+-define(IS_HIST_ENABLED(RETAIN), (RETAIN > 0)).
+
 start_link() ->
     gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
 
@@ -44,6 +54,45 @@ init(_) ->
             ignore
     end.
 
+%% @doc Count the number of sessions.
+%% Include sessions which are expired since the given timestamp if `since' is greater than 0.
+-spec count(non_neg_integer()) -> non_neg_integer().
+count(Since) ->
+    Retain = retain_duration(),
+    Now = now_ts(),
+    %% Get table size if hist is not enabled or
+    %% Since is before the earliest possible retention time.
+    IsCountAll = (not ?IS_HIST_ENABLED(Retain) orelse (Now - Retain >= Since)),
+    case IsCountAll of
+        true ->
+            mnesia:table_info(?CHAN_REG_TAB, size);
+        false ->
+            %% make a gen call to avoid many callers doing the same concurrently
+            gen_server:call(?MODULE, {count, Since}, infinity)
+    end.
+
+handle_call({count, Since}, _From, State) ->
+    {LastCountTime, LastCount} =
+        case State of
+            #{last_count_time := T, last_count := C} ->
+                {T, C};
+            _ ->
+                {0, 0}
+        end,
+    Now = now_ts(),
+    Total = mnesia:table_info(?CHAN_REG_TAB, size),
+    %% Always count if the table is small enough
+    %% or when the last count is too old
+    IsTableSmall = (Total < ?CACHE_COUNT_THRESHOLD),
+    IsLastCountOld = (Now - LastCountTime > ?MIN_COUNT_INTERVAL_SECONDS),
+    case IsTableSmall orelse IsLastCountOld of
+        true ->
+            Count = do_count(Since),
+            CountFinishedAt = now_ts(),
+            {reply, Count, State#{last_count_time => CountFinishedAt, last_count => Count}};
+        false ->
+            {reply, LastCount, State}
+    end;
 handle_call(_Request, _From, State) ->
     {reply, ok, State}.
 
@@ -84,7 +133,7 @@ cleanup_one_chunk(NextClientId) ->
     IsExpired = fun(#channel{pid = Ts}) ->
         is_integer(Ts) andalso (Ts < Now - Retain)
     end,
-    cleanup_loop(NextClientId, 10000, IsExpired).
+    cleanup_loop(NextClientId, ?CLEANUP_CHUNK_SIZE, IsExpired).
 
 cleanup_loop(ClientId, 0, _IsExpired) ->
     ClientId;
@@ -114,7 +163,7 @@ is_hist_enabled() ->
 %% Return the session registration history retain duration in seconds.
 -spec retain_duration() -> non_neg_integer().
 retain_duration() ->
-    emqx:get_config([broker, session_registration_history_retain]).
+    emqx:get_config([broker, session_history_retain]).
 
 cleanup_delay() ->
     Default = timer:minutes(2),
@@ -137,3 +186,7 @@ send_delay_start(Delay) ->
 
 now_ts() ->
     erlang:system_time(seconds).
+
+do_count(Since) ->
+    Ms = ets:fun2ms(fun(#channel{pid = V}) -> is_pid(V) orelse (is_integer(V) andalso (V >= Since)) end),
+    ets:select_count(?CHAN_REG_TAB, Ms).

apps/emqx/src/emqx_cm_sup.erl

@@ -49,7 +49,7 @@ init([]) ->
     Locker = child_spec(emqx_cm_locker, 5000, worker),
     CmPool = emqx_pool_sup:spec(emqx_cm_pool_sup, [?CM_POOL, random, {emqx_pool, start_link, []}]),
     Registry = child_spec(emqx_cm_registry, 5000, worker),
-    RegistryCleaner = child_spec(emqx_cm_registry_cleaner, 5000, worker),
+    RegistryKeeper = child_spec(emqx_cm_registry_keeper, 5000, worker),
     Manager = child_spec(emqx_cm, 5000, worker),
     DSSessionGCSup = child_spec(emqx_persistent_session_ds_sup, infinity, supervisor),
     Children =
@@ -59,7 +59,7 @@ init([]) ->
             Locker,
             CmPool,
             Registry,
-            RegistryCleaner,
+            RegistryKeeper,
             Manager,
             DSSessionGCSup
         ],

apps/emqx/src/emqx_schema.erl

@@ -1356,13 +1356,13 @@ fields("broker") ->
                     desc => ?DESC(broker_enable_session_registry)
                 }
             )},
-        {session_registration_history_retain,
+        {session_history_retain,
             sc(
                 duration_s(),
                 #{
                     default => <<"0s">>,
                     importance => ?IMPORTANCE_LOW,
-                    desc => ?DESC("broker_session_registration_history_retain")
+                    desc => ?DESC("broker_session_history_retain")
                 }
             )},
         {session_locking_strategy,

apps/emqx_management/src/emqx_mgmt_api_clients.erl

@@ -45,7 +45,8 @@
     subscribe_batch/2,
     unsubscribe/2,
     unsubscribe_batch/2,
-    set_keepalive/2
+    set_keepalive/2,
+    sessions_count/2
 ]).
 
 -export([
@@ -96,7 +97,8 @@ paths() ->
         "/clients/:clientid/subscribe/bulk",
         "/clients/:clientid/unsubscribe",
         "/clients/:clientid/unsubscribe/bulk",
-        "/clients/:clientid/keepalive"
+        "/clients/:clientid/keepalive",
+        "/sessions_count"
     ].
 
 schema("/clients") ->
@@ -385,6 +387,30 @@ schema("/clients/:clientid/keepalive") ->
                 )
             }
         }
+    };
+schema("/sessions_count") ->
+    #{
+        'operationId' => sessions_count,
+        get => #{
+            description => ?DESC(get_sessions_count),
+            tags => ?TAGS,
+            parameters => [
+                {since,
+                    hoconsc:mk(non_neg_integer(), #{
+                        in => query,
+                        required => false,
+                        default => 0,
+                        desc =>
+                            <<"Include sessions expired after this time (UNIX Epoch in seconds precesion)">>,
+                        example => 1705391625
+                    })}
+            ],
+            responses => #{
+                200 => hoconsc:mk(binary(), #{
+                    desc => <<"Number of sessions">>
+                })
+            }
+        }
     }.
 
 fields(clients) ->
@@ -1059,3 +1085,8 @@ client_example() ->
         <<"recv_cnt">> => 4,
         <<"recv_msg.qos0">> => 0
     }.
+
+sessions_count(get, #{query_string := QString}) ->
+    Since = maps:get(<<"since">>, QString, undefined),
+    Count = emqx_cm_registry_keeper:count(Since),
+    {200, integer_to_binary(Count)}.

rel/i18n/emqx_mgmt_api_clients.hocon

@@ -60,4 +60,11 @@ set_keepalive_seconds.desc:
 set_keepalive_seconds.label:
 """Set the online client keepalive by seconds"""
 
+get_sessions_count.desc:
+"""Get the number of sessions. By default it returns the number of non-expired sessions.
+if `broker.session_history_retain` is set to a duration greater than `0s`,
+this API can also count expired sessions by providing the `since` parameter."""
+get_sessions_count.label:
+"""Count number of sessions"""
+
 }

rel/i18n/emqx_schema.hocon

@@ -1429,7 +1429,7 @@ Advantages of Disabling<br/>
 - Reduced Memory Usage: Turning off the session registry can lower the overall memory footprint of the system.
 - Improved Performance: Without the overhead of maintaining a global registry, the node can process client connections faster."""
 
-broker_session_registration_history_retain.desc:
+broker_session_history_retain.desc:
 """The duration to retain the session registration history. Setting this to a value greater than `0s` will increase memory usage and impact peformance.
 This retained history can be used to monitor how many sessions were registered in the past configured duration.
 Note: This config has no effect if `enable_session_registry` is set to `false`.<br/>