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https://issues.apache.org/jira/browse/FLINK-4856?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15874578#comment-15874578
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ASF GitHub Bot commented on FLINK-4856:
---------------------------------------
Github user StefanRRichter commented on a diff in the pull request:
https://github.com/apache/flink/pull/3336#discussion_r101994611
--- Diff:
flink-runtime/src/main/java/org/apache/flink/runtime/state/HashMapSerializer.java
---
@@ -0,0 +1,179 @@
+/*
+ * 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.runtime.state;
+
+import org.apache.flink.api.common.typeutils.TypeSerializer;
+import org.apache.flink.core.memory.DataInputView;
+import org.apache.flink.core.memory.DataOutputView;
+import org.apache.flink.util.Preconditions;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * A serializer for {@link List Lists}. The serializer relies on an
element serializer
+ * for teh serialization of the list's elements.
+ *
+ * <p>The serialization format for the list is as follows: four bytes for
the length of the lost,
+ * followed by the serialized representation of each element.
+ *
+ * @param <K> The type of the keys in the map.
+ * @param <V> The type of the values in the map.
+ */
+public class HashMapSerializer<K, V> extends TypeSerializer<HashMap<K, V>>
{
--- End diff --
This could also be tested by extending `SerializerTestBase`.
> Add MapState for keyed streams
> ------------------------------
>
> Key: FLINK-4856
> URL: https://issues.apache.org/jira/browse/FLINK-4856
> Project: Flink
> Issue Type: New Feature
> Components: DataStream API, State Backends, Checkpointing
> Reporter: Xiaogang Shi
> Assignee: Xiaogang Shi
>
> Many states in keyed streams are organized as key-value pairs. Currently,
> these states are implemented by storing the entire map into a ValueState or a
> ListState. The implementation however is very costly because all entries have
> to be serialized/deserialized when updating a single entry. To improve the
> efficiency of these states, MapStates are urgently needed.
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