diff --git a/dubbo-xds/src/main/java/org/apache/dubbo/registry/xds/util/protocol/AbstractProtocol.java b/dubbo-xds/src/main/java/org/apache/dubbo/registry/xds/util/protocol/AbstractProtocol.java
index 2d6dcb040b..4c87033109 100644
--- a/dubbo-xds/src/main/java/org/apache/dubbo/registry/xds/util/protocol/AbstractProtocol.java
+++ b/dubbo-xds/src/main/java/org/apache/dubbo/registry/xds/util/protocol/AbstractProtocol.java
@@ -147,7 +147,7 @@ public abstract class AbstractProtocol<T, S extends DeltaResource<T>> implements
                 // origin request, may changed by updateObserve
                 Set<String> names = requestParam.get(request);
 
-                // use future to get async result
+                // use future to get async result, future complete on StreamObserver onNext
                 CompletableFuture<T> future = new CompletableFuture<>();
                 streamResult.put(request, future);