Github user kl0u commented on a diff in the pull request:
https://github.com/apache/flink/pull/5481#discussion_r171235356
--- Diff:
flink-streaming-java/src/main/java/org/apache/flink/streaming/api/functions/KeyedProcessFunction.java
---
@@ -0,0 +1,130 @@
+/*
+ * 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.functions;
+
+import org.apache.flink.annotation.PublicEvolving;
+import org.apache.flink.api.common.functions.AbstractRichFunction;
+import org.apache.flink.streaming.api.TimeDomain;
+import org.apache.flink.streaming.api.TimerService;
+import org.apache.flink.util.Collector;
+import org.apache.flink.util.OutputTag;
+
+/**
+ * A keyed function that processes elements of a stream.
+ *
+ * <p>For every element in the input stream {@link #processElement(Object,
Context, Collector)}
+ * is invoked. This can produce zero or more elements as output.
Implementations can also
+ * query the time and set timers through the provided {@link Context}. For
firing timers
+ * {@link #onTimer(long, OnTimerContext, Collector)} will be invoked. This
can again produce
+ * zero or more elements as output and register further timers.
+ *
+ * <p><b>NOTE:</b> Access to keyed state and timers (which are also scoped
to a key) is only
+ * available if the {@code KeyedProcessFunction} is applied on a {@code
KeyedStream}.
+ *
+ * <p><b>NOTE:</b> A {@code KeyedProcessFunction} is always a
+ * {@link org.apache.flink.api.common.functions.RichFunction}. Therefore,
access to the
+ * {@link org.apache.flink.api.common.functions.RuntimeContext} is always
available and setup and
+ * teardown methods can be implemented. See
+ * {@link
org.apache.flink.api.common.functions.RichFunction#open(org.apache.flink.configuration.Configuration)}
+ * and {@link org.apache.flink.api.common.functions.RichFunction#close()}.
+ *
+ * @param <K> Type of the key.
+ * @param <I> Type of the input elements.
+ * @param <O> Type of the output elements.
+ */
+@PublicEvolving
+public abstract class KeyedProcessFunction<K, I, O> extends
AbstractRichFunction {
+
+ private static final long serialVersionUID = 1L;
+
+ /**
+ * Process one element from the input stream.
+ *
+ * <p>This function can output zero or more elements using the {@link
Collector} parameter
+ * and also update internal state or set timers using the {@link
Context} parameter.
+ *
+ * @param value The input value.
+ * @param ctx A {@link Context} that allows querying the timestamp of
the element and getting
+ * a {@link TimerService} for registering timers and
querying the time. The
+ * context is only valid during the invocation of this
method, do not store it.
+ * @param out The collector for returning result values.
+ *
+ * @throws Exception This method may throw exceptions. Throwing an
exception will cause the operation
+ * to fail and may trigger recovery.
+ */
+ public abstract void processElement(I value, Context ctx, Collector<O>
out) throws Exception;
+
+ /**
+ * Called when a timer set using {@link TimerService} fires.
+ *
+ * @param timestamp The timestamp of the firing timer.
+ * @param ctx An {@link OnTimerContext} that allows querying the
timestamp, the {@link TimeDomain}, and the key
+ * of the firing timer and getting a {@link TimerService}
for registering timers and querying the time.
+ * The context is only valid during the invocation of this
method, do not store it.
+ * @param out The collector for returning result values.
+ *
+ * @throws Exception This method may throw exceptions. Throwing an
exception will cause the operation
+ * to fail and may trigger recovery.
+ */
+ public void onTimer(long timestamp, OnTimerContext<K> ctx, Collector<O>
out) throws Exception {}
+
--- End diff --
Remove the `<K>` (see comment below).
---
