Github user aljoscha commented on a diff in the pull request:
https://github.com/apache/flink/pull/3412#discussion_r102940901
--- Diff:
flink-streaming-java/src/main/java/org/apache/flink/streaming/api/operators/KeyRegistry.java
---
@@ -0,0 +1,257 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.streaming.api.operators;
+
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+import org.apache.flink.core.memory.DataInputViewStreamWrapper;
+import org.apache.flink.core.memory.DataOutputViewStreamWrapper;
+import org.apache.flink.runtime.state.KeyGroupRangeAssignment;
+import org.apache.flink.runtime.state.KeyGroupsList;
+import org.apache.flink.streaming.api.watermark.Watermark;
+import org.apache.flink.util.InstantiationUtil;
+
+import java.io.IOException;
+import java.util.HashSet;
+import java.util.Set;
+
+import static org.apache.flink.util.Preconditions.checkArgument;
+import static org.apache.flink.util.Preconditions.checkNotNull;
+
+/**
+ * The key registry allows to register a {@link OnWatermarkCallback} and
multiple keys, for which
+ * the callback will be invoked periodically, upon reception of each
subsequent {@link Watermark},
+ * after the registration of the key.
+ * <p>
+ * <b>NOTE: </b> This service is only available to <b>keyed</b> operators.
+ *
+ * @param <K> The type of key returned by the {@code KeySelector}.
+ * */
+public class KeyRegistry<K> {
--- End diff --
To keep it in line with the other services this could be called
`InternalWatermarkCallbackService`.
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