feat(ds): allow isolated batches with preconditions

Namely, single message deletions and preconditions that can be used to
build complex "compare-and-swap"-style operations. Also allow user to
declare that atomic batches support is needed for a DB.

apps/emqx/src/emqx_types.erl

@@ -82,6 +82,7 @@
 -export_type([
     payload/0,
     message/0,
+    message/1,
     flag/0,
     flags/0,
     headers/0
@@ -238,7 +239,8 @@
 -type subscription() :: #subscription{}.
 -type subscriber() :: {pid(), subid()}.
 -type payload() :: binary() | iodata().
--type message() :: #message{}.
+-type message() :: #message{payload :: emqx_types:payload()}.
+-type message(Payload) :: #message{payload :: Payload}.
 -type flag() :: sys | dup | retain | atom().
 -type flags() :: #{flag() := boolean()}.
 -type headers() :: #{

apps/emqx_durable_storage/include/emqx_ds.hrl

@@ -16,4 +16,9 @@
 -ifndef(EMQX_DS_HRL).
 -define(EMQX_DS_HRL, true).
 
+-record(dsbatch, {
+    operations :: [emqx_ds:operation()],
+    preconditions = [] :: [emqx_ds:precondition()]
+}).
+
 -endif.

apps/emqx_durable_storage/src/emqx_ds.erl

@@ -19,6 +19,8 @@
 %% It takes care of forwarding calls to the underlying DBMS.
 -module(emqx_ds).
 
+-include_lib("emqx_durable_storage/include/emqx_ds.hrl").
+
 %% Management API:
 -export([
     register_backend/2,
@@ -52,6 +54,9 @@
     time/0,
     topic_filter/0,
     topic/0,
+    batch/0,
+    operation/0,
+    precondition/0,
     stream/0,
     delete_stream/0,
     delete_selector/0,
@@ -84,8 +89,6 @@
 %% Type declarations
 %%================================================================================
 
--define(APP, emqx_durable_storage).
-
 -type db() :: atom().
 
 %% Parsed topic.
@@ -94,6 +97,31 @@
 %% Parsed topic filter.
 -type topic_filter() :: list(binary() | '+' | '#' | '').
 
+-type message() :: emqx_types:message().
+
+%% Message pattern.
+-type message(Pattern) :: emqx_types:message(Pattern).
+
+%% A batch of storage operations.
+-type batch() :: [operation()] | #dsbatch{}.
+
+-type operation() ::
+    %% Store a message.
+    message()
+    %% Delete a message.
+    %% Does nothing if the message does not exist.
+    | {delete, message(_)}.
+
+%% Precondition.
+%% Fails whole batch if the message in question does not match (`while'), or
+%% does match (`unless'). Here "match" means that it either just exists (when
+%% pattern is '_') or has exactly the same payload, rest of the message fields are
+%% irrelevant. Useful to construct batches with "compare-and-set" semantics.
+%% Note: backends may not support this, but if they do only DBs with `atomic_batches'
+%% enabled are expected to support preconditions in batches.
+-type precondition() ::
+    {while | unless, message(iodata() | '_')}.
+
 -type rank_x() :: term().
 
 -type rank_y() :: integer().
@@ -157,13 +185,7 @@
         %% Whether to wait until the message storage has been acknowledged to return from
         %% `store_batch'.
         %% Default: `true'.
-        sync => boolean(),
-        %% Whether the whole batch given to `store_batch' should be inserted atomically as
-        %% a unit.  Note: the whole batch must be crafted so that it belongs to a single
-        %% shard (if applicable to the backend), as the batch will be split accordingly
-        %% even if this flag is `true'.
-        %% Default: `false'.
-        atomic => boolean()
+        sync => boolean()
     }.
 
 -type generic_db_opts() ::
@@ -176,6 +198,12 @@
         %% If `false' then message timestamps are respected; timestamp, topic and
         %% serialization key uniquely identify a message.
         force_monotonic_timestamps => boolean(),
+        %% Whether the whole batch given to `store_batch' should be processed and
+        %% inserted atomically as a unit, in isolation from other batches.
+        %% Default: `false'.
+        %% The whole batch must be crafted so that it belongs to a single shard (if
+        %% applicable to the backend).
+        atomic_batches => boolean(),
         serialize_by => clientid | topic,
         _ => _
     }.
@@ -313,11 +341,11 @@ drop_db(DB) ->
             Module:drop_db(DB)
     end.
 
--spec store_batch(db(), [emqx_types:message()], message_store_opts()) -> store_batch_result().
+-spec store_batch(db(), batch(), message_store_opts()) -> store_batch_result().
 store_batch(DB, Msgs, Opts) ->
     ?module(DB):store_batch(DB, Msgs, Opts).
 
--spec store_batch(db(), [emqx_types:message()]) -> store_batch_result().
+-spec store_batch(db(), batch()) -> store_batch_result().
 store_batch(DB, Msgs) ->
     store_batch(DB, Msgs, #{}).
 
@@ -418,7 +446,10 @@ timestamp_us() ->
 %%================================================================================
 
 set_db_defaults(Opts) ->
-    Defaults = #{force_monotonic_timestamps => true},
+    Defaults = #{
+        force_monotonic_timestamps => true,
+        atomic_batches => false
+    },
     maps:merge(Defaults, Opts).
 
 call_if_implemented(Mod, Fun, Args, Default) ->

apps/emqx_utils/include/emqx_message.hrl

@@ -33,7 +33,7 @@
     %% Topic that the message is published to
     topic :: emqx_types:topic(),
     %% Message Payload
-    payload :: emqx_types:payload(),
+    payload,
     %% Timestamp (Unit: millisecond)
     timestamp :: integer(),
     %% Miscellaneous extensions, currently used for OpenTelemetry context propagation