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https://issues.apache.org/jira/browse/FLINK-4856?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15874307#comment-15874307
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ASF GitHub Bot commented on FLINK-4856:
---------------------------------------
Github user aljoscha commented on a diff in the pull request:
https://github.com/apache/flink/pull/3336#discussion_r101983507
--- Diff:
flink-core/src/main/java/org/apache/flink/api/common/state/MapState.java ---
@@ -0,0 +1,111 @@
+/*
+ * 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.api.common.state;
+
+import org.apache.flink.annotation.PublicEvolving;
+
+import java.io.IOException;
+import java.util.Iterator;
+import java.util.Map;
+
+/**
+ * {@link State} interface for partitioned key-value state. The key-value
pair can be
+ * added, updated and retrieved.
+ *
+ * <p>The state is accessed and modified by user functions, and
checkpointed consistently
+ * by the system as part of the distributed snapshots.
+ *
+ * <p>The state is only accessible by functions applied on a
KeyedDataStream. The key is
+ * automatically supplied by the system, so the function always sees the
value mapped to the
+ * key of the current element. That way, the system can handle stream and
state partitioning
+ * consistently together.
+ *
+ * @param <UK> Type of the keys in the state.
+ * @param <UV> Type of the values in the state.
+ */
+@PublicEvolving
+public interface MapState<UK, UV> extends AppendingState<Map<UK, UV>,
Iterable<Map.Entry<UK, UV>>> {
--- End diff --
Exactly, the semantics are different from how they are described on
`AppendingState` and from how it is used. `MapState` can still have a
`add(Map<UK, UV> map)` method but I don't think it makes sense that it is an
`AppendingState`. What are the use cases where a user is expected to have an
`AppendingState<Map<K, V>, Map<K, V>>` instead of simply having a `MapState<K,
V>`?
As I said before, the semantics of `MapState.add()` would be like
`ValueState.update()` and `ValueState` is also not an `AppendingState`. I think
a `MultiMapState` that would be like a `Map<K, Iterable<V>>` (and would have
the semantics of a `ListState`) would be able to satisfy `AppendingState` but
I'm not sure whether we would need the interface there.
> 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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