refactor: no need to sort timestamps

apps/emqx_dashboard/src/emqx_dashboard_monitor.erl

@@ -243,16 +243,11 @@ do_sample(Node, infinity) ->
     %% handle RPC from old version nodes
     do_sample(Node, 0);
 do_sample(all, Time) when is_integer(Time) ->
-    AllNodes = emqx:running_nodes() -- [node()],
-    Local = do_sample(node(), Time),
-    All = sample_nodes(AllNodes, Time, Local),
+    AllNodes = emqx:running_nodes(),
+    All = sample_nodes(AllNodes, Time, #{}),
     maps:map(fun(_, S) -> adjust_synthetic_cluster_metrics(S) end, All);
 do_sample(Node, Time) when Node == node() andalso is_integer(Time) ->
-    MS = ets:fun2ms(fun(#emqx_monit{time = T} = A) when T >= Time -> A end),
-    FromDB = ets:select(?TAB, MS),
-    Map = to_ts_data_map(FromDB),
-    %% downsample before return RPC calls for less data to merge by the caller nodes
-    downsample(Time, Map);
+    do_sample_local(Time);
 do_sample(Node, Time) when is_integer(Time) ->
     case emqx_dashboard_proto_v1:do_sample(Node, Time) of
         {badrpc, Reason} ->
@@ -261,6 +256,13 @@ do_sample(Node, Time) when is_integer(Time) ->
             Res
     end.
 
+do_sample_local(Time) ->
+    MS = ets:fun2ms(fun(#emqx_monit{time = T} = A) when T >= Time -> A end),
+    FromDB = ets:select(?TAB, MS),
+    Map = to_ts_data_map(FromDB),
+    %% downsample before return RPC calls for less data to merge by the caller nodes
+    downsample(Time, Map).
+
 sample_nodes(Nodes, Time, Local) ->
     ResList = concurrently_sample_nodes(Nodes, Time),
     {Failed, Success} = lists:partition(
@@ -293,19 +295,28 @@ merge_samplers_loop(TS, Increment, Base) when is_map(Increment) ->
     end.
 
 merge_sampler_maps(M1, M2) when is_map(M1) andalso is_map(M2) ->
-    Fun =
-        fun
-            (Key, Map) when
-                %% cluster-synced values
-                Key =:= topics;
-                Key =:= subscriptions_durable;
-                Key =:= disconnected_durable_sessions
-            ->
-                Map#{Key => max(maps:get(Key, M1, 0), maps:get(Key, M2, 0))};
-            (Key, Map) ->
-                Map#{Key => maps:get(Key, M1, 0) + maps:get(Key, M2, 0)}
-        end,
-    lists:foldl(Fun, #{}, ?SAMPLER_LIST).
+    Fun = fun(Key, Acc) -> merge_values(Key, M1, Acc) end,
+    lists:foldl(Fun, M2, ?SAMPLER_LIST).
+
+%% topics, subscriptions_durable and disconnected_durable_sessions are cluster synced
+merge_values(topics, M1, M2) ->
+    max_values(topics, M1, M2);
+merge_values(subscriptions_durable, M1, M2) ->
+    max_values(subscriptions_durable, M1, M2);
+merge_values(disconnected_durable_sessions, M1, M2) ->
+    max_values(disconnected_durable_sessions, M1, M2);
+merge_values(Key, M1, M2) ->
+    sum_values(Key, M1, M2).
+
+max_values(Key, M1, M2) when is_map_key(Key, M1) orelse is_map_key(Key, M2) ->
+    M2#{Key => max(maps:get(Key, M1, 0), maps:get(Key, M2, 0))};
+max_values(_Key, _M1, M2) ->
+    M2.
+
+sum_values(Key, M1, M2) when is_map_key(Key, M1) orelse is_map_key(Key, M2) ->
+    M2#{Key => maps:get(Key, M1, 0) + maps:get(Key, M2, 0)};
+sum_values(_Key, _M1, M2) ->
+    M2.
 
 merge_cluster_rate(Node, Cluster) ->
     Fun =
@@ -445,13 +456,16 @@ fill_gaps_loop(T, Interval, Latest, Samples) ->
     fill_gaps_loop(T + Interval, Interval, Latest, Samples1).
 
 downsample(SinceTs, TsDataMap) when map_size(TsDataMap) >= 2 ->
-    TsList = lists:sort(maps:keys(TsDataMap)),
-    Latest = lists:last(TsList),
+    TsList = ts_list(TsDataMap),
+    Latest = lists:max(TsList),
     Interval = sample_interval(Latest - SinceTs),
     downsample_loop(TsList, TsDataMap, Interval, #{});
 downsample(_Since, TsDataMap) ->
     TsDataMap.
 
+ts_list(TsDataMap) ->
+    maps:keys(TsDataMap).
+
 round_down(Ts, Interval) ->
     Ts - (Ts rem Interval).