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From aljoscha <...@git.apache.org>
Subject [GitHub] flink pull request #3336: [FLINK-4856][state] Add MapState in KeyedState
Date Mon, 20 Feb 2017 10:11:28 GMT
Github user aljoscha commented on a diff in the pull request:

    --- 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.
    + */
    +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.

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