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docs(ps_session): add notes explaining rationale about operation order and consistency

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Thales Macedo Garitezi 2023-09-13 10:11:00 -03:00
parent e081abb9e6
commit f55f6dc779
1 changed files with 20 additions and 0 deletions
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20
apps/emqx/src/emqx_persistent_session_ds.erl
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@ -99,6 +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(
@ -146,6 +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} ->