Merge pull request #11598 from thalesmg/ds-fix-ps-router-m-20230912

fixes and improvements to persistent session router

apps/emqx/src/emqx_persistent_session_ds.erl

@@ -71,11 +71,9 @@ init() ->
     ok | {skipped, _Reason} | {error, _TODO}.
 persist_message(Msg) ->
     ?WHEN_ENABLED(
-        case needs_persistence(Msg) andalso find_subscribers(Msg) of
-            [_ | _] ->
+        case needs_persistence(Msg) andalso has_subscribers(Msg) of
+            true ->
                 store_message(Msg);
-            [] ->
-                {skipped, no_subscribers};
             false ->
                 {skipped, needs_no_persistence}
         end
@@ -90,8 +88,8 @@ store_message(Msg) ->
     Topic = emqx_topic:words(emqx_message:topic(Msg)),
     emqx_ds_storage_layer:store(?DS_SHARD, ID, Timestamp, Topic, serialize_message(Msg)).
 
-find_subscribers(#message{topic = Topic}) ->
-    emqx_persistent_session_ds_router:match_routes(Topic).
+has_subscribers(#message{topic = Topic}) ->
+    emqx_persistent_session_ds_router:has_any_route(Topic).
 
 open_session(ClientID) ->
     ?WHEN_ENABLED(emqx_ds:session_open(ClientID)).
@@ -101,6 +99,24 @@ open_session(ClientID) ->
 add_subscription(TopicFilterBin, DSSessionID) ->
     ?WHEN_ENABLED(
         begin
+            %% N.B.: we chose to update the router before adding the subscription to the
+            %% session/iterator table.  The reasoning for this is as follows:
+            %%
+            %% Messages matching this topic filter should start to be persisted as soon as
+            %% possible to avoid missing messages.  If this is the first such persistent
+            %% session subscription, it's important to do so early on.
+            %%
+            %% This could, in turn, lead to some inconsistency: if such a route gets
+            %% created but the session/iterator data fails to be updated accordingly, we
+            %% have a dangling route.  To remove such dangling routes, we may have a
+            %% periodic GC process that removes routes that do not have a matching
+            %% persistent subscription.  Also, route operations use dirty mnesia
+            %% operations, which inherently have room for inconsistencies.
+            %%
+            %% In practice, we use the iterator reference table as a source of truth,
+            %% since it is guarded by a transaction context: we consider a subscription
+            %% operation to be successful if it ended up changing this table.  Both router
+            %% and iterator information can be reconstructed from this table, if needed.
             ok = emqx_persistent_session_ds_router:do_add_route(TopicFilterBin, DSSessionID),
             TopicFilter = emqx_topic:words(TopicFilterBin),
             {ok, IteratorID, StartMS, IsNew} = emqx_ds:session_add_iterator(
@@ -148,6 +164,8 @@ do_open_iterator(TopicFilter, StartMS, IteratorID) ->
 del_subscription(TopicFilterBin, DSSessionID) ->
     ?WHEN_ENABLED(
         begin
+            %% N.B.: see comments in `?MODULE:add_subscription' for a discussion about the
+            %% order of operations here.
             TopicFilter = emqx_topic:words(TopicFilterBin),
             case emqx_ds:session_get_iterator_id(DSSessionID, TopicFilter) of
                 {error, not_found} ->

apps/emqx/src/emqx_persistent_session_ds_router.erl

@@ -25,6 +25,7 @@
 -export([
     do_add_route/2,
     do_delete_route/2,
+    has_any_route/1,
     match_routes/1,
     lookup_routes/1,
     foldr_routes/2,
@@ -52,7 +53,7 @@ init_tables() ->
     ok = mria:create_table(?PS_ROUTER_TAB, [
         {type, bag},
         {rlog_shard, ?PS_ROUTER_SHARD},
-        {storage, ram_copies},
+        {storage, disc_copies},
         {record_name, ps_route},
         {attributes, record_info(fields, ps_route)},
         {storage_properties, [
@@ -65,7 +66,7 @@ init_tables() ->
     ok = mria:create_table(?PS_FILTERS_TAB, [
         {type, ordered_set},
         {rlog_shard, ?PS_ROUTER_SHARD},
-        {storage, ram_copies},
+        {storage, disc_copies},
         {record_name, ps_routeidx},
         {attributes, record_info(fields, ps_routeidx)},
         {storage_properties, [
@@ -101,6 +102,19 @@ do_delete_route(Topic, Dest) ->
             mria_route_tab_delete(#ps_route{topic = Topic, dest = Dest})
     end.
 
+%% @doc Takes a real topic (not filter) as input, and returns whether there is any
+%% matching filters.
+-spec has_any_route(emqx_types:topic()) -> boolean().
+has_any_route(Topic) ->
+    DirectTopicMatch = lookup_route_tab(Topic),
+    WildcardMatch = emqx_topic_index:match(Topic, ?PS_FILTERS_TAB),
+    case {DirectTopicMatch, WildcardMatch} of
+        {[], false} ->
+            false;
+        {_, _} ->
+            true
+    end.
+
 %% @doc Take a real topic (not filter) as input, return the matching topics and topic
 %% filters associated with route destination.
 -spec match_routes(emqx_types:topic()) -> [emqx_types:route()].

apps/emqx/src/emqx_topic_index.erl

@@ -66,6 +66,7 @@ match(Topic, Tab) ->
 
 %% @doc Match given topic against the index and return _all_ matches.
 %% If `unique` option is given, return only unique matches by record ID.
+-spec matches(emqx_types:topic(), ets:table(), emqx_trie_search:opts()) -> [match(_ID)].
 matches(Topic, Tab, Opts) ->
     emqx_trie_search:matches(Topic, make_nextf(Tab), Opts).
 

apps/emqx/test/emqx_persistent_session_ds_router_SUITE.erl

@@ -80,12 +80,21 @@ delete_route(TopicFilter) ->
 %     error('TODO').
 
 t_add_delete(_) ->
+    ?assertNot(?R:has_any_route(<<"a/b/c">>)),
     add_route(<<"a/b/c">>),
+    ?assert(?R:has_any_route(<<"a/b/c">>)),
     add_route(<<"a/b/c">>),
+    ?assert(?R:has_any_route(<<"a/b/c">>)),
     add_route(<<"a/+/b">>),
+    ?assert(?R:has_any_route(<<"a/b/c">>)),
+    ?assert(?R:has_any_route(<<"a/c/b">>)),
     ?assertEqual([<<"a/+/b">>, <<"a/b/c">>], lists:sort(?R:topics())),
     delete_route(<<"a/b/c">>),
+    ?assertNot(?R:has_any_route(<<"a/b/c">>)),
+    ?assert(?R:has_any_route(<<"a/c/b">>)),
     delete_route(<<"a/+/b">>),
+    ?assertNot(?R:has_any_route(<<"a/b/c">>)),
+    ?assertNot(?R:has_any_route(<<"a/c/b">>)),
     ?assertEqual([], ?R:topics()).
 
 t_add_delete_incremental(_) ->
@@ -94,6 +103,7 @@ t_add_delete_incremental(_) ->
     add_route(<<"a/+/+">>),
     add_route(<<"a/b/#">>),
     add_route(<<"#">>),
+    ?assert(?R:has_any_route(<<"any/topic">>)),
     ?assertEqual(
         [
             #ps_route{topic = <<"#">>, dest = ?DEF_DS_SESSION_ID},

apps/emqx_durable_storage/src/emqx_ds_app.erl

@@ -6,7 +6,7 @@
 
 -dialyzer({nowarn_function, storage/0}).
 
--export([start/2]).
+-export([start/2, storage/0]).
 
 -include("emqx_ds_int.hrl").