Add detailed comments for dataSyncService in DataNode (#11285)

Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>

internal/datanode/data_sync_service.go

@@ -36,17 +36,17 @@ import (
 type dataSyncService struct {
 	ctx          context.Context
 	cancelFn     context.CancelFunc
-	fg           *flowgraph.TimeTickedFlowGraph
-	flushCh      chan flushMsg
-	replica      Replica
-	idAllocator  allocatorInterface
+	fg           *flowgraph.TimeTickedFlowGraph // internal flowgraph processes insert/delta messages
+	flushCh      chan flushMsg                  // chan to notify flush
+	replica      Replica                        // segment replica stores meta
+	idAllocator  allocatorInterface             // id/timestamp allocator
 	msFactory    msgstream.Factory
-	collectionID UniqueID
-	dataCoord    types.DataCoord
-	clearSignal  chan<- UniqueID
+	collectionID UniqueID        // collection id of vchan for which this data sync service serves
+	dataCoord    types.DataCoord // DataCoord instance to interact with
+	clearSignal  chan<- UniqueID // signal channel to notify flowgraph close for collection/partition drop msg consumed
 
-	flushingSegCache *Cache
-	flushManager     flushManager
+	flushingSegCache *Cache       // a guarding cache stores currently flushing segment ids
+	flushManager     flushManager // flush manager handles flush process
 }
 
 func newDataSyncService(ctx context.Context,
@@ -156,6 +156,7 @@ func (dsService *dataSyncService) initNodes(vchanInfo *datapb.VchannelInfo) erro
 		return err
 	}
 
+	// initialize flush manager for DataSync Service
 	dsService.flushManager = NewRendezvousFlushManager(dsService.idAllocator, minIOKV, dsService.replica, func(pack *segmentFlushPack) error {
 		fieldInsert := []*datapb.FieldBinlog{}
 		fieldStats := []*datapb.FieldBinlog{}