stable-25-3-1: Optimized serialization in discovery cache cron job (#26350)

Author: ydbot

ydb/core/discovery/discovery.cpp

@@ -281,8 +281,11 @@ namespace NDiscoveryPrivate {
 
     class TDiscoveryCache: public TActorBootstrapped<TDiscoveryCache> {
         THashMap<TString, std::shared_ptr<NDiscovery::TCachedMessageData>> CurrentCachedMessages;
+        THashMap<TString, std::shared_ptr<NDiscovery::TCachedMessageData>> DirtyCachedMessages;
+
         THashMap<TString, std::shared_ptr<NDiscovery::TCachedMessageData>> OldCachedMessages; // when subscriptions are disabled
         THashMap<TString, std::shared_ptr<NDiscovery::TCachedMessageData>> CachedNotAvailable; // for subscriptions
+
         THolder<TEvInterconnect::TEvNodeInfo> NameserviceResponse;
         TBridgeInfo::TPtr BridgeInfo;
 
@@ -354,7 +357,10 @@ namespace NDiscoveryPrivate {
             auto& currentCachedMessage = CurrentCachedMessages[path];
             Y_ABORT_UNLESS(currentCachedMessage);
 
-            currentCachedMessage->UpdateEntries(std::move(msg->Updates));
+            if (!msg->Updates.empty()) {
+                currentCachedMessage->UpdateEntries(std::move(msg->Updates));
+                DirtyCachedMessages[path] = currentCachedMessage;
+            }
 
             auto it = Requested.find(path);
             Y_ABORT_UNLESS(it == Requested.end());
@@ -382,6 +388,8 @@ namespace NDiscoveryPrivate {
                 EndpointId.GetOrElse({}), {}, msg->Status)
             );
 
+            DirtyCachedMessages.erase(path);
+
             if (AppData()->FeatureFlags.GetEnableSubscriptionsInDiscovery()) {
                 if (msg->Status != TEvStateStorage::TEvBoardInfo::EStatus::Ok) {
                     CurrentCachedMessages.erase(path);
@@ -416,10 +424,12 @@ namespace NDiscoveryPrivate {
             Y_ABORT_UNLESS(NameserviceResponse);
             Y_ABORT_UNLESS(!IsBridgeMode(ActorContext()) || BridgeInfo);
 
-            for (auto& [_, cachedData] : CurrentCachedMessages) {
+            for (auto& [_, cachedData] : DirtyCachedMessages) {
                 cachedData->UpdateCache(NameserviceResponse, BridgeInfo, EndpointId.GetOrElse({}));
             }
 
+            DirtyCachedMessages.clear();
+
             Schedule(TDuration::Seconds(1), new TEvents::TEvWakeup());
         }