feat(retainer): make additional implementations pluggable

Co-authored-by: Thales Macedo Garitezi <thalesmg@gmail.com>

apps/emqx_retainer/src/emqx_retainer.erl

@@ -32,8 +32,7 @@
 
 -export([
     delete_message/2,
-    store_retained/2,
-    get_backend_module/0
+    store_retained/2
 ]).
 
 -export([
@@ -45,7 +44,15 @@
     page_read/3,
     post_config_update/5,
     stats_fun/0,
-    retained_count/0
+    retained_count/0,
+    backend_module/0,
+    backend_module/1,
+    enabled/0
+]).
+
+%% For testing only
+-export([
+    context/0
 ]).
 
 %% gen_server callbacks
@@ -54,8 +61,7 @@
     handle_call/3,
     handle_cast/2,
     handle_info/2,
-    terminate/2,
-    code_change/3
+    terminate/2
 ]).
 
 -export_type([
@@ -71,29 +77,35 @@
     context := undefined | context(),
     clear_timer := undefined | reference()
 }.
--type context() :: term().
+
+-type backend_state() :: term().
+
+-type context() :: #{
+    module := module(),
+    state := backend_state()
+}.
 
 -type topic() :: emqx_types:topic().
 -type message() :: emqx_types:message().
 -type cursor() :: undefined | term().
--type backend() :: emqx_retainer_mnesia | module().
+-type has_next() :: boolean().
 
 -define(DEF_MAX_PAYLOAD_SIZE, (1024 * 1024)).
 -define(DEF_EXPIRY_INTERVAL, 0).
 -define(MAX_PAYLOAD_SIZE_CONFIG_PATH, [retainer, max_payload_size]).
 
--callback create(map()) -> context().
--callback update(context(), map()) -> ok | need_recreate.
--callback close(context()) -> ok.
--callback delete_message(context(), topic()) -> ok.
--callback store_retained(context(), message()) -> ok.
--callback read_message(context(), topic()) -> {ok, list()}.
--callback page_read(context(), topic(), non_neg_integer(), non_neg_integer()) ->
-    {ok, HasNext :: boolean(), list()}.
--callback match_messages(context(), topic(), cursor()) -> {ok, list(), cursor()}.
--callback clear_expired(context()) -> ok.
--callback clean(context()) -> ok.
--callback size(context()) -> non_neg_integer().
+-callback create(hocon:config()) -> backend_state().
+-callback update(backend_state(), hocon:config()) -> ok | need_recreate.
+-callback close(backend_state()) -> ok.
+-callback delete_message(backend_state(), topic()) -> ok.
+-callback store_retained(backend_state(), message()) -> ok.
+-callback read_message(backend_state(), topic()) -> {ok, list(message())}.
+-callback page_read(backend_state(), emqx_maybe:t(topic()), non_neg_integer(), non_neg_integer()) ->
+    {ok, has_next(), list(message())}.
+-callback match_messages(backend_state(), topic(), cursor()) -> {ok, list(message()), cursor()}.
+-callback clear_expired(backend_state()) -> ok.
+-callback clean(backend_state()) -> ok.
+-callback size(backend_state()) -> non_neg_integer().
 
 %%------------------------------------------------------------------------------
 %% Hook API
@@ -131,6 +143,13 @@ on_message_publish(Msg = #message{flags = #{retain := true}}, Context) ->
 on_message_publish(Msg, _) ->
     {ok, Msg}.
 
+%%------------------------------------------------------------------------------
+%% Config API
+%%------------------------------------------------------------------------------
+
+post_config_update(_, _UpdateReq, NewConf, OldConf, _AppEnvs) ->
+    call({update_config, NewConf, OldConf}).
+
 %%------------------------------------------------------------------------------
 %% APIs
 %%------------------------------------------------------------------------------
@@ -140,6 +159,7 @@ on_message_publish(Msg, _) ->
 start_link() ->
     gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
 
+-spec get_expiry_time(message()) -> non_neg_integer().
 get_expiry_time(#message{headers = #{properties := #{'Message-Expiry-Interval' := 0}}}) ->
     0;
 get_expiry_time(#message{
@@ -154,41 +174,46 @@ get_expiry_time(#message{timestamp = Ts}) ->
         _ -> Ts + Interval
     end.
 
-get_stop_publish_clear_msg() ->
-    emqx_conf:get([retainer, stop_publish_clear_msg], false).
-
 -spec update_config(hocon:config()) -> {ok, _} | {error, _}.
 update_config(Conf) ->
     emqx_conf:update([retainer], Conf, #{override_to => cluster}).
 
+-spec clean() -> ok.
 clean() ->
     call(?FUNCTION_NAME).
 
+-spec delete(topic()) -> ok.
 delete(Topic) ->
     call({?FUNCTION_NAME, Topic}).
 
+-spec retained_count() -> non_neg_integer().
 retained_count() ->
     call(?FUNCTION_NAME).
 
+-spec read_message(topic()) -> {ok, list(message())}.
 read_message(Topic) ->
     call({?FUNCTION_NAME, Topic}).
 
+-spec page_read(emqx_maybe:t(topic()), non_neg_integer(), non_neg_integer()) ->
+    {ok, has_next(), list(message())}.
 page_read(Topic, Page, Limit) ->
     call({?FUNCTION_NAME, Topic, Page, Limit}).
 
-post_config_update(_, _UpdateReq, NewConf, OldConf, _AppEnvs) ->
-    call({update_config, NewConf, OldConf}).
-
-call(Req) ->
-    gen_server:call(?MODULE, Req, infinity).
+-spec enabled() -> boolean().
+enabled() ->
+    call(?FUNCTION_NAME).
 
-stats_fun() ->
-    gen_server:cast(?MODULE, ?FUNCTION_NAME).
+-spec context() -> ok.
+context() ->
+    call(?FUNCTION_NAME).
 
 %%------------------------------------------------------------------------------
-%% APIs
+%% Internal APIs
 %%------------------------------------------------------------------------------
 
+stats_fun() ->
+    gen_server:cast(?MODULE, ?FUNCTION_NAME).
+
 -spec get_basic_usage_info() -> #{retained_messages => non_neg_integer()}.
 get_basic_usage_info() ->
     try
@@ -206,13 +231,14 @@ init([]) ->
     erlang:process_flag(trap_exit, true),
     emqx_conf:add_handler([retainer], ?MODULE),
     State = new_state(),
-    #{enable := Enable} = Cfg = emqx:get_config([retainer]),
+    RetainerConfig = emqx:get_config([retainer]),
     {ok,
-        case Enable of
+        case maps:get(enable, RetainerConfig) of
+            false ->
+                State;
             true ->
-                enable_retainer(State, Cfg);
-            _ ->
-                State
+                BackendConfig = enabled_backend_config(RetainerConfig),
+                enable_retainer(State, RetainerConfig, BackendConfig)
         end}.
 
 handle_call({update_config, NewConf, OldConf}, _, State) ->
@@ -220,39 +246,34 @@ handle_call({update_config, NewConf, OldConf}, _, State) ->
     emqx_retainer_dispatcher:refresh_limiter(NewConf),
     {reply, ok, State2};
 handle_call(clean, _, #{context := Context} = State) ->
-    clean(Context),
+    _ = clean(Context),
     {reply, ok, State};
 handle_call({delete, Topic}, _, #{context := Context} = State) ->
-    delete_message(Context, Topic),
+    _ = delete_message(Context, Topic),
     {reply, ok, State};
 handle_call({read_message, Topic}, _, #{context := Context} = State) ->
-    Mod = get_backend_module(),
-    Result = Mod:read_message(Context, Topic),
-    {reply, Result, State};
+    {reply, read_message(Context, Topic), State};
 handle_call({page_read, Topic, Page, Limit}, _, #{context := Context} = State) ->
-    Mod = get_backend_module(),
-    Result = Mod:page_read(Context, Topic, Page, Limit),
-    {reply, Result, State};
+    {reply, page_read(Context, Topic, Page, Limit), State};
 handle_call(retained_count, _From, State = #{context := Context}) ->
-    Mod = get_backend_module(),
-    RetainedCount = Mod:size(Context),
-    {reply, RetainedCount, State};
+    {reply, count(Context), State};
+handle_call(enabled, _From, State = #{enable := Enable}) ->
+    {reply, Enable, State};
+handle_call(context, _From, State = #{context := Context}) ->
+    {reply, Context, State};
 handle_call(Req, _From, State) ->
     ?SLOG(error, #{msg => "unexpected_call", call => Req}),
     {reply, ignored, State}.
 
 handle_cast(stats_fun, #{context := Context} = State) ->
-    Mod = get_backend_module(),
-    Size = Mod:size(Context),
-    emqx_stats:setstat('retained.count', 'retained.max', Size),
+    emqx_stats:setstat('retained.count', 'retained.max', count(Context)),
     {noreply, State};
 handle_cast(Msg, State) ->
     ?SLOG(error, #{msg => "unexpected_cast", cast => Msg}),
     {noreply, State}.
 
 handle_info(clear_expired, #{context := Context} = State) ->
-    Mod = get_backend_module(),
-    Mod:clear_expired(Context),
+    ok = clear_expired(Context),
     Interval = emqx_conf:get([retainer, msg_clear_interval], ?DEF_EXPIRY_INTERVAL),
     {noreply, State#{clear_timer := add_timer(Interval, clear_expired)}, hibernate};
 handle_info(Info, State) ->
@@ -264,12 +285,16 @@ terminate(_Reason, #{clear_timer := ClearTimer}) ->
     _ = stop_timer(ClearTimer),
     ok.
 
-code_change(_OldVsn, State, _Extra) ->
-    {ok, State}.
-
 %%------------------------------------------------------------------------------
 %% Internal functions
 %%------------------------------------------------------------------------------
+
+call(Req) ->
+    gen_server:call(?MODULE, Req, infinity).
+
+get_stop_publish_clear_msg() ->
+    emqx_conf:get([retainer, stop_publish_clear_msg], false).
+
 -spec new_state() -> state().
 new_state() ->
     #{
@@ -287,8 +312,34 @@ dispatch(Context, Topic) ->
 
 -spec delete_message(context(), topic()) -> ok.
 delete_message(Context, Topic) ->
-    Mod = get_backend_module(),
-    Mod:delete_message(Context, Topic).
+    Mod = backend_module(Context),
+    BackendState = backend_state(Context),
+    Mod:delete_message(BackendState, Topic).
+
+-spec read_message(context(), topic()) -> {ok, list(message())}.
+read_message(Context, Topic) ->
+    Mod = backend_module(Context),
+    BackendState = backend_state(Context),
+    Mod:read_message(BackendState, Topic).
+
+-spec page_read(context(), emqx_maybe:t(topic()), non_neg_integer(), non_neg_integer()) ->
+    {ok, has_next(), list(message())}.
+page_read(Context, Topic, Page, Limit) ->
+    Mod = backend_module(Context),
+    BackendState = backend_state(Context),
+    Mod:page_read(BackendState, Topic, Page, Limit).
+
+-spec count(context()) -> non_neg_integer().
+count(Context) ->
+    Mod = backend_module(Context),
+    BackendState = backend_state(Context),
+    Mod:size(BackendState).
+
+-spec clear_expired(context()) -> ok.
+clear_expired(Context) ->
+    Mod = backend_module(Context),
+    BackendState = backend_state(Context),
+    Mod:clear_expired(BackendState).
 
 -spec store_retained(context(), message()) -> ok.
 store_retained(Context, #message{topic = Topic, payload = Payload} = Msg) ->
@@ -303,52 +354,47 @@ store_retained(Context, #message{topic = Topic, payload = Payload} = Msg) ->
                 limit => Limit
             });
         _ ->
-            Mod = get_backend_module(),
-            Mod:store_retained(Context, Msg)
+            Mod = backend_module(Context),
+            BackendState = backend_state(Context),
+            Mod:store_retained(BackendState, Msg)
     end.
 
 -spec clean(context()) -> ok.
 clean(Context) ->
-    Mod = get_backend_module(),
-    Mod:clean(Context).
+    Mod = backend_module(Context),
+    BackendState = backend_state(Context),
+    Mod:clean(BackendState).
 
 -spec update_config(state(), hocon:config(), hocon:config()) -> state().
-update_config(State, Conf, OldConf) ->
+update_config(State, NewConfig, OldConfig) ->
     update_config(
-        maps:get(enable, Conf),
-        maps:get(enable, OldConf),
+        maps:get(enable, NewConfig),
+        maps:get(enable, OldConfig),
         State,
-        Conf,
-        OldConf
+        NewConfig,
+        OldConfig
     ).
 
 -spec update_config(boolean(), boolean(), state(), hocon:config(), hocon:config()) -> state().
 update_config(false, _, State, _, _) ->
     disable_retainer(State);
-update_config(true, false, State, NewConf, _) ->
-    enable_retainer(State, NewConf);
+update_config(true, false, State, NewConfig, _) ->
+    enable_retainer(State, NewConfig, enabled_backend_config(NewConfig));
 update_config(
     true,
     true,
     #{clear_timer := ClearTimer, context := Context} = State,
-    NewConf,
-    OldConf
+    NewConfig,
+    OldConfig
 ) ->
-    #{
-        backend := #{
-            type := BackendType
-        } = NewBackendConfig,
-        msg_clear_interval := ClearInterval
-    } = NewConf,
-
-    #{
-        backend := #{
-            type := OldBackendType
-        }
-    } = OldConf,
-    SameBackendType = BackendType =:= OldBackendType,
-    Mod = get_backend_module(),
-    case SameBackendType andalso ok =:= Mod:update(Context, NewBackendConfig) of
+    #{msg_clear_interval := ClearInterval} = NewConfig,
+    OldBackendConfig = enabled_backend_config(OldConfig),
+    NewBackendConfig = enabled_backend_config(NewConfig),
+    OldMod = config_backend_module(OldBackendConfig),
+    NewMod = config_backend_module(NewBackendConfig),
+
+    SameBackendType = NewMod =:= OldMod,
+    case SameBackendType andalso ok =:= OldMod:update(Context, NewBackendConfig) of
         true ->
             State#{
                 clear_timer := check_timer(
@@ -359,18 +405,16 @@ update_config(
             };
         false ->
             State2 = disable_retainer(State),
-            enable_retainer(State2, NewConf)
+            enable_retainer(State2, NewConfig, NewBackendConfig)
     end.
 
--spec enable_retainer(state(), hocon:config()) -> state().
+-spec enable_retainer(state(), hocon:config(), hocon:config()) -> state().
 enable_retainer(
     State,
-    #{
-        msg_clear_interval := ClearInterval,
-        backend := BackendCfg
-    }
+    #{msg_clear_interval := ClearInterval} = _RetainerConfig,
+    BackendConfig
 ) ->
-    Context = create(BackendCfg),
+    Context = create(BackendConfig),
     ok = load(Context),
     State#{
         enable := true,
@@ -415,20 +459,43 @@ check_timer(Timer, undefined, _) ->
 check_timer(Timer, _, _) ->
     Timer.
 
--spec get_backend_module() -> backend().
-get_backend_module() ->
-    case emqx:get_config([retainer, backend]) of
+-spec enabled_backend_config(hocon:config()) -> hocon:config() | no_return().
+enabled_backend_config(#{backend := Backend, external_backends := ExternalBackends} = Config) ->
+    AllBackends = [Backend | maps:values(ExternalBackends)],
+    case lists:search(fun(#{enable := Enable}) -> Enable end, AllBackends) of
+        {value, EnabledBackend} -> EnabledBackend;
+        false -> error({no_enabled_backend, Config})
+    end.
+
+-spec config_backend_module(hocon:config()) -> module().
+config_backend_module(Config) ->
+    case Config of
         #{type := built_in_database} -> emqx_retainer_mnesia;
-        #{type := Backend} -> Backend
+        #{module := Module} -> Module
     end.
 
-create(#{type := built_in_database} = Cfg) ->
-    Mod = get_backend_module(),
-    Mod:create(Cfg).
+-spec backend_module(context()) -> module().
+backend_module(#{module := Module}) -> Module.
+
+-spec backend_state(context()) -> backend_state().
+backend_state(#{state := State}) -> State.
+
+-spec backend_module() -> module() | no_return().
+backend_module() ->
+    Config = enabled_backend_config(emqx:get_config([retainer])),
+    config_backend_module(Config).
+
+-spec create(hocon:config()) -> context().
+create(Cfg) ->
+    Mod = config_backend_module(Cfg),
+    #{
+        module => Mod,
+        state => Mod:create(Cfg)
+    }.
 
 -spec close(context()) -> ok | {error, term()}.
 close(Context) ->
-    Mod = get_backend_module(),
+    Mod = backend_module(Context),
     Mod:close(Context).
 
 -spec load(context()) -> ok.

apps/emqx_retainer/src/emqx_retainer_api.erl

@@ -185,11 +185,11 @@ lookup_retained(get, #{query_string := Qs}) ->
     Page = maps:get(<<"page">>, Qs, 1),
     Limit = maps:get(<<"limit">>, Qs, emqx_mgmt:default_row_limit()),
     Topic = maps:get(<<"topic">>, Qs, undefined),
-    {ok, HasNext, Msgs} = emqx_retainer_mnesia:page_read(undefined, Topic, Page, Limit),
+    {ok, HasNext, Msgs} = emqx_retainer:page_read(Topic, Page, Limit),
     Meta =
         case Topic of
             undefined ->
-                #{count => emqx_retainer_mnesia:size(?TAB_MESSAGE)};
+                #{count => emqx_retainer:retained_count()};
             _ ->
                 #{}
         end,
@@ -205,7 +205,7 @@ lookup_retained(get, #{query_string := Qs}) ->
 
 with_topic(get, #{bindings := Bindings}) ->
     Topic = maps:get(topic, Bindings),
-    {ok, _, Msgs} = emqx_retainer_mnesia:page_read(undefined, Topic, 1, 1),
+    {ok, _, Msgs} = emqx_retainer:page_read(Topic, 1, 1),
     case Msgs of
         [H | _] ->
             {200, format_detail_message(H)};
@@ -217,14 +217,14 @@ with_topic(get, #{bindings := Bindings}) ->
     end;
 with_topic(delete, #{bindings := Bindings}) ->
     Topic = maps:get(topic, Bindings),
-    case emqx_retainer_mnesia:page_read(undefined, Topic, 1, 1) of
+    case emqx_retainer:page_read(Topic, 1, 1) of
         {ok, _, []} ->
             {404, #{
                 code => <<"NOT_FOUND">>,
                 message => <<"Viewed message doesn't exist">>
             }};
         {ok, _, _} ->
-            emqx_retainer_mnesia:delete_message(undefined, Topic),
+            emqx_retainer:delete(Topic),
             {204}
     end.
 
@@ -265,8 +265,8 @@ to_bin_string(Data) ->
     list_to_binary(io_lib:format("~p", [Data])).
 
 check_backend(Type, Params, Cont) ->
-    case emqx:get_config([retainer, backend, type]) of
-        built_in_database ->
+    case emqx_retainer:backend_module() of
+        emqx_retainer_mnesia ->
             Cont(Type, Params);
         _ ->
             {400, 'BAD_REQUEST', <<"This API only support built in database">>}

apps/emqx_retainer/src/emqx_retainer_app.erl

@@ -26,12 +26,12 @@
 ]).
 
 start(_Type, _Args) ->
-    ok = emqx_retainer_mnesia_cli:load(),
+    ok = emqx_retainer_cli:load(),
     init_bucket(),
     emqx_retainer_sup:start_link().
 
 stop(_State) ->
-    ok = emqx_retainer_mnesia_cli:unload(),
+    ok = emqx_retainer_cli:unload(),
     delete_bucket(),
     ok.
 

apps/emqx_retainer/src/emqx_retainer_mnesia_cli.erl

@@ -14,7 +14,7 @@
 %% limitations under the License.
 %%--------------------------------------------------------------------
 
--module(emqx_retainer_mnesia_cli).
+-module(emqx_retainer_cli).
 
 -include_lib("emqx/include/logger.hrl").
 
@@ -32,36 +32,52 @@ load() ->
     ok = emqx_ctl:register_command(retainer, {?MODULE, retainer}, []).
 
 retainer(["info"]) ->
-    count();
+    if_enabled(fun() ->
+        count()
+    end);
 retainer(["topics"]) ->
-    topic(1, 1000);
+    if_enabled(fun() ->
+        topic(1, 1000)
+    end);
 retainer(["topics", Start, Len]) ->
-    topic(list_to_integer(Start), list_to_integer(Len));
+    if_enabled(fun() ->
+        topic(list_to_integer(Start), list_to_integer(Len))
+    end);
 retainer(["clean", Topic]) ->
-    emqx_retainer:delete(list_to_binary(Topic));
+    if_enabled(fun() ->
+        emqx_retainer:delete(list_to_binary(Topic))
+    end);
 retainer(["clean"]) ->
-    emqx_retainer:clean();
+    if_enabled(fun() ->
+        emqx_retainer:clean()
+    end);
 retainer(["reindex", "status"]) ->
-    case emqx_retainer_mnesia:reindex_status() of
-        true ->
-            ?PRINT_MSG("Reindexing is in progress~n");
-        false ->
-            ?PRINT_MSG("Reindexing is not running~n")
-    end;
+    if_mnesia_enabled(fun() ->
+        case emqx_retainer_mnesia:reindex_status() of
+            true ->
+                ?PRINT_MSG("Reindexing is in progress~n");
+            false ->
+                ?PRINT_MSG("Reindexing is not running~n")
+        end
+    end);
 retainer(["reindex", "start"]) ->
-    retainer(["reindex", "start", "false"]);
+    if_mnesia_enabled(fun() ->
+        retainer(["reindex", "start", "false"])
+    end);
 retainer(["reindex", "start", ForceParam]) ->
-    case mria_rlog:role() of
-        core ->
-            Force =
-                case ForceParam of
-                    "true" -> true;
-                    _ -> false
-                end,
-            do_reindex(Force);
-        replicant ->
-            ?PRINT_MSG("Can't run reindex on a replicant node")
-    end;
+    if_mnesia_enabled(fun() ->
+        case mria_rlog:role() of
+            core ->
+                Force =
+                    case ForceParam of
+                        "true" -> true;
+                        _ -> false
+                    end,
+                do_reindex(Force);
+            replicant ->
+                ?PRINT_MSG("Can't run reindex on a replicant node")
+        end
+    end);
 retainer(_) ->
     emqx_ctl:usage(
         [
@@ -71,10 +87,12 @@ retainer(_) ->
                 "Show topics of retained messages by the specified range"},
             {"retainer clean", "Clean all retained messages"},
             {"retainer clean <Topic>", "Clean retained messages by the specified topic filter"},
-            {"retainer reindex status", "Show reindex status"},
+            {"retainer reindex status",
+                "Show reindex status.\nOnly available for built-in backend."},
             {"retainer reindex start [force]",
                 "Generate new retainer topic indices from config settings.\n"
-                "Pass true as <Force> to ignore previously started reindexing"}
+                "Pass true as <Force> to ignore previously started reindexing.\n"
+                "Only available for built-in backend."}
         ]
     ).
 
@@ -107,6 +125,23 @@ count() ->
 
 topic(Start, Len) ->
     count(),
-    Topics = lists:sublist(emqx_retainer_mnesia:topics(), Start, Len),
-    [?PRINT("~ts~n", [I]) || I <- Topics],
+    {ok, _HasNext, Messages} = emqx_retainer:page_read(<<"#">>, Start, Len),
+    [?PRINT("~ts~n", [emqx_message:topic(M)]) || M <- Messages],
     ok.
+
+if_enabled(Fun) ->
+    case emqx_retainer:enabled() of
+        true -> Fun();
+        false -> ?PRINT_MSG("Retainer is not enabled~n")
+    end.
+
+if_mnesia_backend(Fun) ->
+    case emqx_retainer:backend_module() of
+        emqx_retainer_mnesia -> Fun();
+        _ -> ?PRINT_MSG("Command only applicable for builtin backend~n")
+    end.
+
+if_mnesia_enabled(Fun) ->
+    if_enabled(fun() ->
+        if_mnesia_backend(Fun)
+    end).

apps/emqx_retainer/src/emqx_retainer_dispatcher.erl

@@ -236,7 +236,7 @@ cast(Msg) ->
 
 -spec dispatch(context(), pid(), topic(), cursor(), limiter()) -> {ok, limiter()}.
 dispatch(Context, Pid, Topic, Cursor, Limiter) ->
-    Mod = emqx_retainer:get_backend_module(),
+    Mod = emqx_retainer:backend_module(Context),
     case Cursor =/= undefined orelse emqx_topic:wildcard(Topic) of
         false ->
             {ok, Result} = erlang:apply(Mod, read_message, [Context, Topic]),

apps/emqx_retainer/src/emqx_retainer_mnesia.erl

@@ -51,10 +51,8 @@
 
 -export([reindex/2, reindex_status/0]).
 
--ifdef(TEST).
 -export([populate_index_meta/0]).
 -export([reindex/3]).
--endif.
 
 -record(retained_message, {topic, msg, expiry_time}).
 -record(retained_index, {key, expiry_time}).
@@ -79,7 +77,7 @@ topics() ->
 %% emqx_retainer callbacks
 %%--------------------------------------------------------------------
 
-create(#{storage_type := StorageType}) ->
+create(#{storage_type := StorageType, max_retained_messages := MaxRetainedMessages} = Config) ->
     ok = create_table(
         ?TAB_INDEX_META,
         retained_index_meta,
@@ -87,7 +85,7 @@ create(#{storage_type := StorageType}) ->
         set,
         StorageType
     ),
-    ok = populate_index_meta(),
+    ok = populate_index_meta(Config),
     ok = create_table(
         ?TAB_MESSAGE,
         retained_message,
@@ -102,8 +100,7 @@ create(#{storage_type := StorageType}) ->
         ordered_set,
         StorageType
     ),
-    %% The context is not used by this backend
-    #{storage_type => StorageType}.
+    #{storage_type => StorageType, max_retained_messages => MaxRetainedMessages}.
 
 create_table(Table, RecordName, Attributes, Type, StorageType) ->
     Copies =
@@ -139,17 +136,15 @@ create_table(Table, RecordName, Attributes, Type, StorageType) ->
             ok
     end.
 
-update(#{storage_type := StorageType}, #{storage_type := StorageType}) ->
-    ok;
-update(_Context, _NewConfig) ->
+update(_State, _NewConfig) ->
     need_recreate.
 
-close(_Context) -> ok.
+close(_State) -> ok.
 
-store_retained(_Context, Msg = #message{topic = Topic}) ->
+store_retained(State, Msg = #message{topic = Topic}) ->
     ExpiryTime = emqx_retainer:get_expiry_time(Msg),
     Tokens = topic_to_tokens(Topic),
-    case is_table_full() andalso is_new_topic(Tokens) of
+    case is_table_full(State) andalso is_new_topic(Tokens) of
         true ->
             ?SLOG(error, #{
                 msg => "failed_to_retain_message",
@@ -182,7 +177,7 @@ clear_expired() ->
     QC = qlc:cursor(QH),
     clear_batch(dirty_indices(write), QC).
 
-delete_message(_Context, Topic) ->
+delete_message(_State, Topic) ->
     Tokens = topic_to_tokens(Topic),
     case emqx_topic:wildcard(Topic) of
         false ->
@@ -198,10 +193,10 @@ delete_message(_Context, Topic) ->
             )
     end.
 
-read_message(_Context, Topic) ->
+read_message(_State, Topic) ->
     {ok, read_messages(Topic)}.
 
-match_messages(_Context, Topic, undefined) ->
+match_messages(_State, Topic, undefined) ->
     Tokens = topic_to_tokens(Topic),
     Now = erlang:system_time(millisecond),
     QH = msg_table(search_table(Tokens, Now)),
@@ -212,7 +207,7 @@ match_messages(_Context, Topic, undefined) ->
             Cursor = qlc:cursor(QH),
             match_messages(undefined, Topic, {Cursor, BatchNum})
     end;
-match_messages(_Context, _Topic, {Cursor, BatchNum}) ->
+match_messages(_State, _Topic, {Cursor, BatchNum}) ->
     case qlc_next_answers(Cursor, BatchNum) of
         {closed, Rows} ->
             {ok, Rows, undefined};
@@ -220,7 +215,7 @@ match_messages(_Context, _Topic, {Cursor, BatchNum}) ->
             {ok, Rows, {Cursor, BatchNum}}
     end.
 
-page_read(_Context, Topic, Page, Limit) ->
+page_read(_State, Topic, Page, Limit) ->
     Now = erlang:system_time(millisecond),
     QH =
         case Topic of
@@ -263,7 +258,8 @@ size(_) ->
     table_size().
 
 reindex(Force, StatusFun) ->
-    reindex(config_indices(), Force, StatusFun).
+    Config = emqx:get_config([retainer, backend]),
+    reindex(config_indices(Config), Force, StatusFun).
 
 reindex_status() ->
     case mnesia:dirty_read(?TAB_INDEX_META, ?META_KEY) of
@@ -438,9 +434,8 @@ make_index_match_spec(Index, Tokens, NowMs) ->
     MsHd = #retained_index{key = Cond, expiry_time = '$3'},
     {[{MsHd, [{'orelse', {'=:=', '$3', 0}, {'>', '$3', NowMs}}], ['$_']}], IsExact}.
 
-is_table_full() ->
-    Limit = emqx:get_config([retainer, backend, max_retained_messages]),
-    Limit > 0 andalso (table_size() >= Limit).
+is_table_full(#{max_retained_messages := MaxRetainedMessages} = _State) ->
+    MaxRetainedMessages > 0 andalso (table_size() >= MaxRetainedMessages).
 
 is_new_topic(Tokens) ->
     case mnesia:dirty_read(?TAB_MESSAGE, Tokens) of
@@ -453,11 +448,15 @@ is_new_topic(Tokens) ->
 table_size() ->
     mnesia:table_info(?TAB_MESSAGE, size).
 
-config_indices() ->
-    lists:sort(emqx_config:get([retainer, backend, index_specs])).
+config_indices(#{index_specs := IndexSpecs}) ->
+    IndexSpecs.
 
 populate_index_meta() ->
-    ConfigIndices = config_indices(),
+    Config = emqx:get_config([retainer, backend]),
+    populate_index_meta(Config).
+
+populate_index_meta(Config) ->
+    ConfigIndices = config_indices(Config),
     case mria:transaction(?RETAINER_SHARD, fun ?MODULE:do_populate_index_meta/1, [ConfigIndices]) of
         {atomic, ok} ->
             ok;

apps/emqx_retainer/src/emqx_retainer_schema.erl

@@ -36,7 +36,8 @@ roots() ->
     [
         {"retainer",
             hoconsc:mk(hoconsc:ref(?MODULE, "retainer"), #{
-                converter => fun retainer_converter/2
+                converter => fun retainer_converter/2,
+                validator => fun validate_backends_enabled/1
             })}
     ].
 
@@ -86,11 +87,29 @@ fields("retainer") ->
                     aliases => [deliver_rate]
                 }
             )},
-        {backend, backend_config()}
+        {backend, backend_config()},
+        {external_backends,
+            ?HOCON(
+                hoconsc:ref(?MODULE, external_backends),
+                #{
+                    desc => ?DESC(backends),
+                    required => false,
+                    default => #{},
+                    importance => ?IMPORTANCE_HIDDEN
+                }
+            )}
     ];
 fields(mnesia_config) ->
     [
-        {type, sc(built_in_database, mnesia_config_type, built_in_database)},
+        {type,
+            ?HOCON(
+                built_in_database,
+                #{
+                    desc => ?DESC(mnesia_config_type),
+                    required => false,
+                    default => built_in_database
+                }
+            )},
         {storage_type,
             sc(
                 hoconsc:enum([ram, disc]),
@@ -103,7 +122,13 @@ fields(mnesia_config) ->
                 max_retained_messages,
                 0
             )},
-        {index_specs, fun retainer_indices/1}
+        {index_specs, fun retainer_indices/1},
+        {enable,
+            ?HOCON(boolean(), #{
+                desc => ?DESC(mnesia_enable),
+                required => false,
+                default => true
+            })}
     ];
 fields(flow_control) ->
     [
@@ -125,7 +150,9 @@ fields(flow_control) ->
                 batch_deliver_limiter,
                 undefined
             )}
-    ].
+    ];
+fields(external_backends) ->
+    emqx_schema_hooks:injection_point('retainer.external_backends').
 
 desc("retainer") ->
     "Configuration related to handling `PUBLISH` packets with a `retain` flag set to 1.";
@@ -140,9 +167,6 @@ desc(_) ->
 %% Internal functions
 %%--------------------------------------------------------------------
 
-%%sc(Type, DescId) ->
-%%    hoconsc:mk(Type, #{desc => ?DESC(DescId)}).
-
 sc(Type, DescId, Default) ->
     sc(Type, DescId, Default, ?DEFAULT_IMPORTANCE).
 sc(Type, DescId, Default, Importance) ->
@@ -214,3 +238,16 @@ retainer_converter(#{<<"deliver_rate">> := Delivery} = Conf, Opts) ->
     retainer_converter(Conf1#{<<"delivery_rate">> => Delivery}, Opts);
 retainer_converter(Conf, _Opts) ->
     Conf.
+
+validate_backends_enabled(Config) ->
+    BuiltInBackend = maps:get(<<"backend">>, Config, #{}),
+    ExternalBackends = maps:values(maps:get(<<"external_backends">>, Config, #{})),
+    Enabled = lists:filter(fun(#{<<"enable">> := E}) -> E end, [BuiltInBackend | ExternalBackends]),
+    case Enabled of
+        [#{}] ->
+            ok;
+        _Conflicts = [_ | _] ->
+            {error, multiple_enabled_backends};
+        _None = [] ->
+            {error, no_enabled_backend}
+    end.

apps/emqx_retainer/test/emqx_retainer_SUITE.erl

@@ -644,7 +644,7 @@ t_reindex(_) ->
 
 t_get_basic_usage_info(_Config) ->
     ?assertEqual(#{retained_messages => 0}, emqx_retainer:get_basic_usage_info()),
-    Context = undefined,
+    Context = emqx_retainer:context(),
     lists:foreach(
         fun(N) ->
             Num = integer_to_binary(N),
@@ -788,8 +788,8 @@ t_compatibility_for_deliver_rate(_) ->
     ).
 
 t_update_config(_) ->
-    OldConf = emqx_config:get([retainer]),
-    NewConf = emqx_utils_maps:deep_put([backend, storage_type], OldConf, disk),
+    OldConf = emqx_config:get_raw([retainer]),
+    NewConf = emqx_utils_maps:deep_put([<<"backend">>, <<"storage_type">>], OldConf, <<"disk">>),
     emqx_retainer:update_config(NewConf).
 
 %%--------------------------------------------------------------------
@@ -836,7 +836,7 @@ with_conf(ConfMod, Case) ->
     emqx_retainer:update_config(NewConf),
     try
         Case(),
-        emqx_retainer:update_config(Conf)
+        {ok, _} = emqx_retainer:update_config(Conf)
     catch
         Type:Error:Strace ->
             emqx_retainer:update_config(Conf),

apps/emqx_retainer/test/emqx_retainer_backend_SUITE.erl

@@ -0,0 +1,104 @@
+%%--------------------------------------------------------------------
+%% Copyright (c) 2024 EMQ Technologies Co., Ltd. All Rights Reserved.
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%%
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
+%%--------------------------------------------------------------------
+
+-module(emqx_retainer_backend_SUITE).
+
+-compile(export_all).
+-compile(nowarn_export_all).
+
+-include_lib("eunit/include/eunit.hrl").
+-include_lib("common_test/include/ct.hrl").
+
+%%--------------------------------------------------------------------
+%% Setups
+%%--------------------------------------------------------------------
+
+all() -> emqx_common_test_helpers:all(?MODULE).
+
+-define(BASE_CONF, #{
+    <<"retainer">> =>
+        #{
+            <<"enable">> => true,
+            <<"backend">> =>
+                #{
+                    <<"type">> => <<"built_in_database">>,
+                    <<"storage_type">> => <<"ram">>,
+                    <<"max_retained_messages">> => 0
+                }
+        }
+}).
+
+init_per_suite(Config) ->
+    Apps = emqx_cth_suite:start(
+        [
+            {emqx, #{
+                before_start => fun() ->
+                    ok = emqx_schema_hooks:inject_from_modules([emqx_retainer_dummy])
+                end
+            }},
+            emqx_conf,
+            {emqx_retainer, ?BASE_CONF}
+        ],
+        #{work_dir => emqx_cth_suite:work_dir(Config)}
+    ),
+    [{suite_apps, Apps} | Config].
+
+end_per_suite(Config) ->
+    emqx_cth_suite:stop(?config(suite_apps, Config)).
+
+%%--------------------------------------------------------------------
+%% Test Cases
+%%--------------------------------------------------------------------
+
+t_external_backend(_Config) ->
+    {ok, _} = emqx_retainer:update_config(#{
+        <<"enable">> => true,
+        <<"backend">> => #{
+            <<"enable">> => false
+        },
+        <<"external_backends">> =>
+            #{
+                <<"dummy">> => #{<<"enable">> => true}
+            }
+    }),
+    ?assertMatch(
+        ok,
+        emqx_retainer:clean()
+    ),
+    ?assertMatch(
+        ok,
+        emqx_retainer:delete(<<"topic">>)
+    ),
+    ?assertMatch(
+        {ok, []},
+        emqx_retainer:read_message(<<"topic">>)
+    ),
+    ?assertMatch(
+        {ok, false, []},
+        emqx_retainer:page_read(<<"topic">>, 0, 10)
+    ),
+    ?assertEqual(
+        0,
+        emqx_retainer:retained_count()
+    ),
+    ?assertEqual(
+        emqx_retainer_dummy,
+        emqx_retainer:backend_module()
+    ),
+    ?assertEqual(
+        true,
+        emqx_retainer:enabled()
+    ).

apps/emqx_retainer/test/emqx_retainer_cli_SUITE.erl

@@ -38,22 +38,22 @@ end_per_suite(Config) ->
     emqx_cth_suite:stop(?config(suite_apps, Config)).
 
 t_reindex_status(_Config) ->
-    ok = emqx_retainer_mnesia_cli:retainer(["reindex", "status"]).
+    ok = emqx_retainer_cli:retainer(["reindex", "status"]).
 
 t_info(_Config) ->
-    ok = emqx_retainer_mnesia_cli:retainer(["info"]).
+    ok = emqx_retainer_cli:retainer(["info"]).
 
 t_topics(_Config) ->
-    ok = emqx_retainer_mnesia_cli:retainer(["topics"]).
+    ok = emqx_retainer_cli:retainer(["topics"]).
 
 t_topics_with_len(_Config) ->
-    ok = emqx_retainer_mnesia_cli:retainer(["topics", "100", "200"]).
+    ok = emqx_retainer_cli:retainer(["topics", "100", "200"]).
 
 t_clean(_Config) ->
-    ok = emqx_retainer_mnesia_cli:retainer(["clean"]).
+    ok = emqx_retainer_cli:retainer(["clean"]).
 
 t_topic(_Config) ->
-    ok = emqx_retainer_mnesia_cli:retainer(["clean", "foo/bar"]).
+    ok = emqx_retainer_cli:retainer(["clean", "foo/bar"]).
 
 t_reindex(_Config) ->
     {ok, C} = emqtt:start_link([{clean_start, true}, {proto_ver, v5}]),
@@ -84,6 +84,6 @@ t_reindex(_Config) ->
     ),
 
     emqx_config:put([retainer, backend, index_specs], [[4, 5]]),
-    ok = emqx_retainer_mnesia_cli:retainer(["reindex", "start"]),
+    ok = emqx_retainer_cli:retainer(["reindex", "start"]),
 
     ?assertEqual(1000, mnesia:table_info(?TAB_INDEX, size)).

apps/emqx_retainer/test/emqx_retainer_dummy.erl

@@ -0,0 +1,98 @@
+%%--------------------------------------------------------------------
+%% Copyright (c) 2024 EMQ Technologies Co., Ltd. All Rights Reserved.
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%%
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
+%%--------------------------------------------------------------------
+
+-module(emqx_retainer_dummy).
+
+-include_lib("typerefl/include/types.hrl").
+-include_lib("hocon/include/hoconsc.hrl").
+
+-behaviour(emqx_retainer).
+
+-export([
+    create/1,
+    update/2,
+    close/1,
+    delete_message/2,
+    store_retained/2,
+    read_message/2,
+    page_read/4,
+    match_messages/3,
+    clear_expired/1,
+    clean/1,
+    size/1
+]).
+
+-behaviour(emqx_schema_hooks).
+
+-export([
+    fields/1,
+    injected_fields/0
+]).
+
+injected_fields() ->
+    #{
+        'retainer.external_backends' => external_backend_fields()
+    }.
+
+create(_Config) -> #{}.
+
+update(_Context, _Config) -> ok.
+
+close(_Context) -> ok.
+
+delete_message(_Context, _Topic) -> ok.
+
+store_retained(_Context, _Message) -> ok.
+
+read_message(_Context, _Topic) -> {ok, []}.
+
+page_read(_Context, _Topic, _Offset, _Limit) -> {ok, false, []}.
+
+match_messages(_Context, _Topic, _Cursor) -> {ok, [], 0}.
+
+clear_expired(_Context) -> ok.
+
+clean(_Context) -> ok.
+
+size(_Context) -> 0.
+
+external_backend_fields() ->
+    [
+        {dummy, hoconsc:ref(?MODULE, dummy)}
+    ].
+
+fields(dummy) ->
+    [
+        {module,
+            hoconsc:mk(
+                emqx_retainer_dummy,
+                #{
+                    desc => <<"dummy backend mod">>,
+                    required => false,
+                    default => <<"emqx_retainer_dummy">>,
+                    importance => ?IMPORTANCE_HIDDEN
+                }
+            )},
+        {enable,
+            hoconsc:mk(
+                boolean(),
+                #{
+                    desc => <<"enable dummy backend">>,
+                    required => true,
+                    default => false
+                }
+            )}
+    ].

rel/i18n/emqx_retainer_schema.hocon

@@ -33,6 +33,9 @@ mnesia_config_storage_type.desc:
 mnesia_config_type.desc:
 """Backend type."""
 
+mnesia_enable.desc:
+"""Enable built-in Mnesia backend."""
+
 msg_clear_interval.desc:
 """Interval for EMQX to scan expired messages and delete them. Never scan if the value is 0."""