聊聊flink的Managed Keyed State-优快云博客

本文深入解析Flink中几种关键的ManagedKeyedState类型，包括ValueState、ListState、ReducingState、AggregatingState等，阐述了它们的继承关系及各自特点。

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本文主要研究一下flink的Managed Keyed State

State

flink-core-1.7.0-sources.jar!/org/apache/flink/api/common/state/State.java

/**
 * Interface that different types of partitioned state must implement.
 *
 * <p>The state is only accessible by functions applied on a {@code KeyedStream}. 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.
 */
@PublicEvolving
public interface State {

	/**
	 * Removes the value mapped under the current key.
	 */
	void clear();
}
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State是所有不同类型的State必须实现的接口，它定义了clear方法

ValueState

flink-core-1.7.0-sources.jar!/org/apache/flink/api/common/state/ValueState.java

@PublicEvolving
public interface ValueState<T> extends State {

	/**
	 * Returns the current value for the state. When the state is not
	 * partitioned the returned value is the same for all inputs in a given
	 * operator instance. If state partitioning is applied, the value returned
	 * depends on the current operator input, as the operator maintains an
	 * independent state for each partition.
	 *
	 * <p>If you didn't specify a default value when creating the {@link ValueStateDescriptor}
	 * this will return {@code null} when to value was previously set using {@link #update(Object)}.
	 *
	 * @return The state value corresponding to the current input.
	 *
	 * @throws IOException Thrown if the system cannot access the state.
	 */
	T value() throws IOException;

	/**
	 * Updates the operator state accessible by {@link #value()} to the given
	 * value. The next time {@link #value()} is called (for the same state
	 * partition) the returned state will represent the updated value. When a
	 * partitioned state is updated with null, the state for the current key
	 * will be removed and the default value is returned on the next access.
	 *
	 * @param value The new value for the state.
	 *
	 * @throws IOException Thrown if the system cannot access the state.
	 */
	void update(T value) throws IOException;

}
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ValueState继承了State接口，它定义了value、update两个方法，一个用于取值，一个用于更新值

AppendingState

flink-core-1.7.0-sources.jar!/org/apache/flink/api/common/state/AppendingState.java

@PublicEvolving
public interface AppendingState<IN, OUT> extends State {

	/**
	 * Returns the current value for the state. When the state is not
	 * partitioned the returned value is the same for all inputs in a given
	 * operator instance. If state partitioning is applied, the value returned
	 * depends on the current operator input, as the operator maintains an
	 * independent state for each partition.
	 *
	 * <p><b>NOTE TO IMPLEMENTERS:</b> if the state is empty, then this method
	 * should return {@code null}.
	 *
	 * @return The operator state value corresponding to the current input or {@code null}
	 * if the state is empty.
	 *
	 * @throws Exception Thrown if the system cannot access the state.
	 */
	OUT get() throws Exception;

	/**
	 * Updates the operator state accessible by {@link #get()} by adding the given value
	 * to the list of values. The next time {@link #get()} is called (for the same state
	 * partition) the returned state will represent the updated list.
	 *
	 * <p>If null is passed in, the state value will remain unchanged.
	 *
	 * @param value The new value for the state.
	 *
	 * @throws Exception Thrown if the system cannot access the state.
	 */
	void add(IN value) throws Exception;

}
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AppendingState继承了State接口，它定义了get、add方法，该State接收IN、OUT两个泛型

FoldingState

flink-core-1.7.0-sources.jar!/org/apache/flink/api/common/state/FoldingState.java

@PublicEvolving
@Deprecated
public interface FoldingState<T, ACC> extends AppendingState<T, ACC> {}
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FoldingState继承了AppendingState，其中OUT泛型表示ACC，即累积值；FoldingState在Flink 1.4版本被标记为废弃，后续会被移除掉，可使用AggregatingState替代

MergingState

flink-core-1.7.0-sources.jar!/org/apache/flink/api/common/state/MergingState.java

/**
 * Extension of {@link AppendingState} that allows merging of state. That is, two instances
 * of {@link MergingState} can be combined into a single instance that contains all the
 * information of the two merged states.
 *
 * @param <IN> Type of the value that can be added to the state.
 * @param <OUT> Type of the value that can be retrieved from the state.
 */
@PublicEvolving
public interface MergingState<IN, OUT> extends AppendingState<IN, OUT> { }
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MergingState继承了AppendingState，这里用命名表达merge state的意思，它有几个子接口，分别是ListState、ReducingState、AggregatingState

ListState

flink-core-1.7.0-sources.jar!/org/apache/flink/api/common/state/ListState.java

@PublicEvolving
public interface ListState<T> extends MergingState<T, Iterable<T>> {

	/**
	 * Updates the operator state accessible by {@link #get()} by updating existing values to
	 * to the given list of values. The next time {@link #get()} is called (for the same state
	 * partition) the returned state will represent the updated list.
	 *
	 * <p>If null or an empty list is passed in, the state value will be null.
	 *
	 * @param values The new values for the state.
	 *
	 * @throws Exception The method may forward exception thrown internally (by I/O or functions).
	 */
	void update(List<T> values) throws Exception;

	/**
	 * Updates the operator state accessible by {@link #get()} by adding the given values
	 * to existing list of values. The next time {@link #get()} is called (for the same state
	 * partition) the returned state will represent the updated list.
	 *
	 * <p>If null or an empty list is passed in, the state value remains unchanged.
	 *
	 * @param values The new values to be added to the state.
	 *
	 * @throws Exception The method may forward exception thrown internally (by I/O or functions).
	 */
	void addAll(List<T> values) throws Exception;
}
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ListState继承了MergingState，它的OUT类型为Iterable；它主要用于operation存储partitioned list state，它继承了MergingState接口(指定OUT的泛型为Iterable)，同时声明了两个方法；其中update用于全量更新state，如果参数为null或者empty，那么state会被清空；addAll方法用于增量更新，如果参数为null或者empty，则保持不变，否则则新增给定的values

ReducingState

flink-core/1.7.0/flink-core-1.7.0-sources.jar!/org/apache/flink/api/common/state/ReducingState.java

@PublicEvolving
public interface ReducingState<T> extends MergingState<T, T> {}
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ReducingState继承了MergingState，它的IN、OUT类型相同

AggregatingState

flink-core/1.7.0/flink-core-1.7.0-sources.jar!/org/apache/flink/api/common/state/AggregatingState.java

@PublicEvolving
public interface AggregatingState<IN, OUT> extends MergingState<IN, OUT> {}
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AggregatingState继承了MergingState，它与ReducingState不同，IN、OUT类型可以不同

MapState

flink-core-1.7.0-sources.jar!/org/apache/flink/api/common/state/MapState.java

@PublicEvolving
public interface MapState<UK, UV> extends State {

	/**
	 * Returns the current value associated with the given key.
	 *
	 * @param key The key of the mapping
	 * @return The value of the mapping with the given key
	 *
	 * @throws Exception Thrown if the system cannot access the state.
	 */
	UV get(UK key) throws Exception;

	/**
	 * Associates a new value with the given key.
	 *
	 * @param key The key of the mapping
	 * @param value The new value of the mapping
	 *
	 * @throws Exception Thrown if the system cannot access the state.
	 */
	void put(UK key, UV value) throws Exception;

	/**
	 * Copies all of the mappings from the given map into the state.
	 *
	 * @param map The mappings to be stored in this state
	 *
	 * @throws Exception Thrown if the system cannot access the state.
	 */
	void putAll(Map<UK, UV> map) throws Exception;

	/**
	 * Deletes the mapping of the given key.
	 *
	 * @param key The key of the mapping
	 *
	 * @throws Exception Thrown if the system cannot access the state.
	 */
	void remove(UK key) throws Exception;

	/**
	 * Returns whether there exists the given mapping.
	 *
	 * @param key The key of the mapping
	 * @return True if there exists a mapping whose key equals to the given key
	 *
	 * @throws Exception Thrown if the system cannot access the state.
	 */
	boolean contains(UK key) throws Exception;

	/**
	 * Returns all the mappings in the state.
	 *
	 * @return An iterable view of all the key-value pairs in the state.
	 *
	 * @throws Exception Thrown if the system cannot access the state.
	 */
	Iterable<Map.Entry<UK, UV>> entries() throws Exception;

	/**
	 * Returns all the keys in the state.
	 *
	 * @return An iterable view of all the keys in the state.
	 *
	 * @throws Exception Thrown if the system cannot access the state.
	 */
	Iterable<UK> keys() throws Exception;

	/**
	 * Returns all the values in the state.
	 *
	 * @return An iterable view of all the values in the state.
	 *
	 * @throws Exception Thrown if the system cannot access the state.
	 */
	Iterable<UV> values() throws Exception;

	/**
	 * Iterates over all the mappings in the state.
	 *
	 * @return An iterator over all the mappings in the state
	 *
	 * @throws Exception Thrown if the system cannot access the state.
	 */
	Iterator<Map.Entry<UK, UV>> iterator() throws Exception;
}
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MapState直接继承了State，它接收UK、UV两个泛型，分别是map的key和value的类型

小结

flink提供了好几个不同类型的Managed Keyed State，有ValueState<T>、ListState<T>、ReducingState<T>、AggregatingState<IN, OUT>、FoldingState<T, ACC>、MapState<UK, UV>
ValueState<T>和MapState<UK, UV>是直接继承State接口；FoldingState继承了AppendingState<IN, OUT>(AppendingState直接继承了State)；ListState、ReducingState、AggregatingState继承了MergingState<IN, OUT>(MergingState继承了AppendingState)
FoldingState在Flink 1.4版本被标记为废弃，后续会被移除掉，可使用AggregatingState替代

doc

Using Managed Keyed State