[skip ci]Add comment on flowgraph dmstream inputnode (#9359)

Signed-off-by: NYang Xuan <xuan.yang@zilliz.com>

internal/datanode/flow_graph_dmstream_input_node.go

@@ -22,6 +22,9 @@ import (
 	"github.com/milvus-io/milvus/internal/util/flowgraph"
 )
 
+// DmInputNode receives messages from message streams, packs messages between two timeticks, and passes all
+//  messages between two timeticks to the following flowgraph node. In DataNode, the following flow graph node is
+//  flowgraph ddNode.
 func newDmInputNode(ctx context.Context, factory msgstream.Factory, collID UniqueID, chanName string, seekPos *internalpb.MsgPosition) (*flowgraph.InputNode, error) {
 	maxQueueLength := Params.FlowGraphMaxQueueLength
 	maxParallelism := Params.FlowGraphMaxParallelism
@@ -33,12 +36,13 @@ func newDmInputNode(ctx context.Context, factory msgstream.Factory, collID Uniqu
 		return nil, err
 	}
 
+	// MsgStream needs a physical channel name, but the channel name in seek position from DataCoord
+	//  is virtual channel name, so we need to convert vchannel name into pchannel neme here.
 	pchannelName := rootcoord.ToPhysicalChannel(chanName)
 	insertStream.AsConsumer([]string{pchannelName}, consumeSubName)
 	log.Debug("datanode AsConsumer physical channel: " + pchannelName + " : " + consumeSubName)
 
 	if seekPos != nil {
-		// ChannelName in seek position is virtual channel name.
 		seekPos.ChannelName = pchannelName
 		log.Debug("datanode Seek: " + seekPos.GetChannelName())
 		insertStream.Seek([]*internalpb.MsgPosition{seekPos})