Flink 状态和 CheckPoint 的区别和联系(附源码)

1. 本质区别：运行时 vs 持久化

1.1 State（状态）：运行时的"工作内存"

package org.apache.flink.api.common.state;import org.apache.flink.annotation.PublicEvolving;/*** 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();
}

State 的特征：

• 位置：存储在 TaskManager 的内存或本地磁盘（RocksDB）
• 目的：算子处理数据时的"工作记忆"
• 访问：频繁、实时、微秒级
• 生命周期：作业运行期间一直存在
• 可变性：每处理一条数据可能就会更新

1.2 Checkpoint：状态的"快照存档"

@Internal
public interface Snapshotable<S extends StateObject> {/*** Operation that writes a snapshot into a stream that is provided by the given {@link* CheckpointStreamFactory} and returns a @{@link RunnableFuture} that gives a state handle to* the snapshot. It is up to the implementation if the operation is performed synchronous or* asynchronous. In the later case, the returned Runnable must be executed first before* obtaining the handle.** @param checkpointId The ID of the checkpoint.* @param timestamp The timestamp of the checkpoint.* @param streamFactory The factory that we can use for writing our state to streams.* @param checkpointOptions Options for how to perform this checkpoint.* @return A runnable future that will yield a {@link StateObject}.*/@NonnullRunnableFuture<S> snapshot(long checkpointId,long timestamp,@Nonnull CheckpointStreamFactory streamFactory,@Nonnull CheckpointOptions checkpointOptions)throws Exception;
}

Checkpoint 的特征：

• 位置：持久化存储（HDFS、S3、OSS 等）
• 目的：容错恢复的"存档点"
• 访问：低频、定期（如每分钟）
• 生命周期：独立于作业运行，故障恢复时使用
• 不可变性：一旦完成就不再改变

2. 形象类比

State = 你正在编辑的 Word 文档（内存中）↓ 每隔一段时间
Checkpoint = 保存到磁盘的文档副本（硬盘上）↓ 如果程序崩溃
Recovery = 从最近的保存恢复（重新加载到内存）

3. 源码层面的关系

3.1 CheckpointableKeyedStateBackend：连接两者的桥梁

/*** Interface that combines both, the {@link KeyedStateBackend} interface, which encapsulates methods* responsible for keyed state management and the {@link Snapshotable} which tells the system how to* snapshot the underlying state.** <p><b>NOTE:</b> State backends that need to be notified of completed checkpoints can additionally* implement the {@link CheckpointListener} interface.** @param <K> Type of the key by which state is keyed.*/
public interface CheckpointableKeyedStateBackend<K>extends KeyedStateBackend<K>, Snapshotable<SnapshotResult<KeyedStateHandle>>, Closeable {/** Returns the key groups which this state backend is responsible for. */KeyGroupRange getKeyGroupRange();/*** Returns a {@link SavepointResources} that can be used by {@link SavepointSnapshotStrategy} to* write out a savepoint in the common/unified format.*/@NonnullSavepointResources<K> savepoint() throws Exception;
}

设计理念：

• KeyedStateBackend：管理运行时状态的读写
• Snapshotable：提供状态快照能力
• CheckpointableKeyedStateBackend：同时具备两种能力

3.2 StateHandle：Checkpoint 的元数据

/*** Base for the handles of the checkpointed states in keyed streams. When recovering from failures,* the handle will be passed to all tasks whose key group ranges overlap with it.*/
public interface KeyedStateHandle extends CompositeStateHandle {/** Returns the range of the key groups contained in the state. */KeyGroupRange getKeyGroupRange();/*** Returns a state over a range that is the intersection between this handle's key-group range* and the provided key-group range.** @param keyGroupRange The key group range to intersect with, will return null if the*     intersection of this handle's key-group and the provided key-group is empty.*/@NullableKeyedStateHandle getIntersection(KeyGroupRange keyGroupRange);/*** Returns a unique state handle id to distinguish with other keyed state handles.