flink sink kafka的事务提交现象猜想

现象

查看flink源码时 sink kafka有事务提交机制，查看源码发现是使用两阶段提交策略，而事务提交是checkpoint完成后才执行，那么如果checkpoint设置间隔时间比较长时，事务未提交之前，后端应该消费不到数据，而观察实际现象为写入kafka的消费数据可以立马消费。

测试用例

测试流程

编写任务1，设置较长的checkpoint时间，并且指定 CheckpointingMode.EXACTLY_ONCE，输出输出到kafka。
编写任务2消费任务的结果topic，打印控制台，验证结果。
根据现象查看源码，分析原因。

测试用例

测试任务1

 StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();env.setParallelism(1);env.enableCheckpointing(1000*60l, CheckpointingMode.EXACTLY_ONCE);env.getCheckpointConfig().setCheckpointStorage("file:///flink/checkpoint");//        超时时间，checkpoint没在时间内完成则丢弃env.getCheckpointConfig().setCheckpointTimeout(50000L); //10秒env.getCheckpointConfig().setMaxConcurrentCheckpoints(2);env.getCheckpointConfig().setTolerableCheckpointFailureNumber(1);//最小间隔时间（前一次结束时间，与下一次开始时间间隔）env.getCheckpointConfig().setMinPauseBetweenCheckpoints(1000);
//        当 Flink 任务取消时，保留外部保存的 checkpoint 信息KafkaSource<String> source = KafkaSource.<String>builder().setBootstrapServers("127.0.0.1:9092").setTopics("test001").setGroupId("my-group")
//                .setStartingOffsets(OffsetsInitializer()).setStartingOffsets(OffsetsInitializer.committedOffsets()).setValueOnlyDeserializer(new SimpleStringSchema()).build();DataStreamSource<String> kafkaSource = env.fromSource(source, WatermarkStrategy.noWatermarks(), "Kafka Source");// 从文件读取数据
//        DataStream<SensorReading> dataStream = env.addSource( new SourceTest4.MySensorSource() );DataStream<String> map = kafkaSource.map(new MapFunction<String, String>() {@Overridepublic String map(String s) throws Exception {return s;}});Properties properties = new Properties();
// 根据上面的介绍自己计算这边的超时时间，满足条件即可properties.setProperty("transaction.timeout.ms","900000");
//        properties.setProperty("bootstrap.servers", "127.0.0.1:9092");KafkaSink<String> sink = KafkaSink.<String>builder().setBootstrapServers("192.168.65.128:9092").setRecordSerializer(KafkaRecordSerializationSchema.<String>builder().setTopic("test002").setValueSerializationSchema(new SimpleStringSchema()).build()).setKafkaProducerConfig(properties).setDeliverGuarantee(DeliveryGuarantee.EXACTLY_ONCE)
//                .setDeliveryGuarantee(DeliveryGuarantee.EXACTLY_ONCE).setTransactionalIdPrefix("flink-xhaodream-").build();map.sinkTo(sink);// 打印输出env.execute();

测试任务2

        StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();env.setParallelism(1);env.enableCheckpointing(1000*150l, CheckpointingMode.EXACTLY_ONCE);env.getCheckpointConfig().setCheckpointStorage("file:///flink/checkpoint");
//        当 Flink 任务取消时，保留外部保存的 checkpoint 信息Properties properties1 = new Properties();
//        properties1.put("isolation.level","read_committed");KafkaSource<String> source = KafkaSource.<String>builder().setBootstrapServers("127.0.0.1:9092").setTopics("test002").setGroupId("my-group2").setProperties(properties1).setStartingOffsets(OffsetsInitializer.committedOffsets(OffsetResetStrategy.LATEST)).setValueOnlyDeserializer(new SimpleStringSchema()).build();DataStreamSource<String> kafkaSource = env.fromSource(source, WatermarkStrategy.noWatermarks(), "Kafka Source");kafkaSource.print(" test2接受数据");// 打印输出env.execute();

测试结果分析

测试结果：

任务1开启后，无论是否执行checkpoint，任务checkpoint都可以正常消费数据，与预期不符合。

原因排查

查看kafkaSink 的源码，找到跟与两阶段提交相关的代码，1.18源码中TwoPhaseCommittingSink有重构。kafkasink实现TwoPhaseCommittingSink接口实现，创建Commiter和Writer。

@PublicEvolving
public interface TwoPhaseCommittingSink<InputT, CommT> extends Sink<InputT> {PrecommittingSinkWriter<InputT, CommT> createWriter(Sink.InitContext var1) throws IOException;Committer<CommT> createCommitter() throws IOException;SimpleVersionedSerializer<CommT> getCommittableSerializer();@PublicEvolvingpublic interface PrecommittingSinkWriter<InputT, CommT> extends SinkWriter<InputT> {Collection<CommT> prepareCommit() throws IOException, InterruptedException;}
}--------------------------------------
public class KafkaSink<IN>implements StatefulSink<IN, KafkaWriterState>,TwoPhaseCommittingSink<IN, KafkaCommittable> {private final DeliveryGuarantee deliveryGuarantee;private final KafkaRecordSerializationSchema<IN> recordSerializer;private final Properties kafkaProducerConfig;private final String transactionalIdPrefix;KafkaSink(DeliveryGuarantee deliveryGuarantee,Properties kafkaProducerConfig,String transactionalIdPrefix,KafkaRecordSerializationSchema<IN> recordSerializer) {this.deliveryGuarantee = deliveryGuarantee;this.kafkaProducerConfig = kafkaProducerConfig;this.transactionalIdPrefix = transactionalIdPrefix;this.recordSerializer = recordSerializer;}/*** Create a {@link KafkaSinkBuilder} to construct a new {@link KafkaSink}.** @param <IN> type of incoming records* @return {@link KafkaSinkBuilder}*/public static <IN> KafkaSinkBuilder<IN> builder() {return new KafkaSinkBuilder<>();}
-- 创建Committer@Internal@Overridepublic Committer<KafkaCommittable> createCommitter() throws IOException {return new KafkaCommitter(kafkaProducerConfig);}@Internal@Overridepublic SimpleVersionedSerializer<KafkaCommittable> getCommittableSerializer() {return new KafkaCommittableSerializer();}
-- 创建writer@Internal@Overridepublic KafkaWriter<IN> createWriter(InitContext context) throws IOException {return new KafkaWriter<IN>(deliveryGuarantee,kafkaProducerConfig,transactionalIdPrefix,context,recordSerializer,context.asSerializationSchemaInitializationContext(),Collections.emptyList());}@Internal@Overridepublic KafkaWriter<IN> restoreWriter(InitContext context, Collection<KafkaWriterState> recoveredState) throws IOException {return new KafkaWriter<>(deliveryGuarantee,kafkaProducerConfig,transactionalIdPrefix,context,recordSerializer,context.asSerializationSchemaInitializationContext(),recoveredState);}@Internal@Overridepublic SimpleVersionedSerializer<KafkaWriterState> getWriterStateSerializer() {return new KafkaWriterStateSerializer();}@VisibleForTestingprotected Properties getKafkaProducerConfig() {return kafkaProducerConfig;}
}

KafkaWriter和KafkaCommitter源码，

在KafkaWriter中snapshotState方法中发现如果deliveryGuarantee == DeliveryGuarantee.EXACTLY_ONCE的开启事务的判断逻辑。

class KafkaWriter<IN>implements StatefulSink.StatefulSinkWriter<IN, KafkaWriterState>,TwoPhaseCommittingSink.PrecommittingSinkWriter<IN, KafkaCommittable> {
.... 省略代码  @Overridepublic Collection<KafkaCommittable> prepareCommit() {if (deliveryGuarantee != DeliveryGuarantee.EXACTLY_ONCE) {return Collections.emptyList();}// only return a KafkaCommittable if the current transaction has been written some dataif (currentProducer.hasRecordsInTransaction()) {final List<KafkaCommittable> committables =Collections.singletonList(KafkaCommittable.of(currentProducer, producerPool::add));LOG.debug("Committing {} committables.", committables);return committables;}// otherwise, we commit the empty transaction as is (no-op) and just recycle the producercurrentProducer.commitTransaction();producerPool.add(currentProducer);return Collections.emptyList();}@Overridepublic List<KafkaWriterState> snapshotState(long checkpointId) throws IOException {
-- 开启事务判断        
if (deliveryGuarantee == DeliveryGuarantee.EXACTLY_ONCE) {currentProducer = getTransactionalProducer(checkpointId + 1);currentProducer.beginTransaction();}return Collections.singletonList(kafkaWriterState);}
。。。。。
}

查看 KafkaCommitter的commit()方法发现producer.commitTransaction();操作

/*** Committer implementation for {@link KafkaSink}** <p>The committer is responsible to finalize the Kafka transactions by committing them.*/
class KafkaCommitter implements Committer<KafkaCommittable>, Closeable {private static final Logger LOG = LoggerFactory.getLogger(KafkaCommitter.class);public static final String UNKNOWN_PRODUCER_ID_ERROR_MESSAGE ="because of a bug in the Kafka broker (KAFKA-9310). Please upgrade to Kafka 2.5+. If you are running with concurrent checkpoints, you also may want to try without them.\n"+ "To avoid data loss, the application will restart.";private final Properties kafkaProducerConfig;@Nullable private FlinkKafkaInternalProducer<?, ?> recoveryProducer;KafkaCommitter(Properties kafkaProducerConfig) {this.kafkaProducerConfig = kafkaProducerConfig;}@Overridepublic void commit(Collection<CommitRequest<KafkaCommittable>> requests)throws IOException, InterruptedException {for (CommitRequest<KafkaCommittable> request : requests) {final KafkaCommittable committable = request.getCommittable();final String transactionalId = committable.getTransactionalId();LOG.debug("Committing Kafka transaction {}", transactionalId);Optional<Recyclable<? extends FlinkKafkaInternalProducer<?, ?>>> recyclable =committable.getProducer();FlinkKafkaInternalProducer<?, ?> producer;try {producer =recyclable.<FlinkKafkaInternalProducer<?, ?>>map(Recyclable::getObject).orElseGet(() -> getRecoveryProducer(committable));--- 事务提交producer.commitTransaction();producer.flush();recyclable.ifPresent(Recyclable::close);} catch (RetriableException e) {LOG.warn("Encountered retriable exception while committing {}.", transactionalId, e);request.retryLater();} catch (ProducerFencedException e) {......}}}
。。。。
}

分析结果

发现除了设置checkpoint还需要kafkasink单独设置.才会实现输出端的开启事务，因此在任务1中添加设置setDeliverGuarantee(DeliveryGuarantee.EXACTLY_ONCE)

 KafkaSink<String> sink = KafkaSink.<String>builder().setBootstrapServers("192.168.65.128:9092").setRecordSerializer(KafkaRecordSerializationSchema.<String>builder().setTopic("test002").setValueSerializationSchema(new SimpleStringSchema()).build()).setKafkaProducerConfig(properties).setDeliverGuarantee(DeliveryGuarantee.EXACTLY_ONCE)
//                .setDeliveryGuarantee(DeliveryGuarantee.EXACTLY_ONCE).setTransactionalIdPrefix("flink-xhaodream-").build();

再次验证任务任务2依然可以正常消费。这是有一点头大，不明白为什么？想到既然开启事务肯定有事务的隔离级别，查询了kafka的事务隔离级别，有两种，分别是读已提交和读未提交，默认消费事务是读未提交。


kafka的事务隔离级别：
读已提交（Read committed）：此隔离级别保证消费者只能读取已经提交的消息。这意味着事务中的消息在提交之前对消费者是不可见的。使用此隔离级别可以避免消费者读取到未提交的事务消息，确保消费者只读取到已经持久化的消息。读未提交（Read Uncommitted）：此隔离级别允许消费者读取未提交的消息。这意味着事务中的消息在提交之前就对消费者可见。使用此隔离级别可以实现更低的延迟，但可能会导致消费者读取到未提交的事务消息。

在任务2中添加isolation.level="read_committed"，设定读取消费事务级别为读已提交，再次测试，发现任务1执行完checkpoint前任务2消费不到数据。而命令行可以及时消费任务1的输出topic可可以消费到数据。结果与预期相同。

 Properties properties1 = new Properties();properties1.put("isolation.level","read_committed");KafkaSource<String> source = KafkaSource.<String>builder().setBootstrapServers("127.0.0.1:9092").setTopics("test002").setGroupId("my-group2").setProperties(properties1)

注意事项

Kafka | Apache Flink

FlinkKafkaProducer 已被弃用并将在 Flink 1.15 中移除，请改用 KafkaSink。

官网文档信息

Kafka | Apache Flink

Kafka Consumer 提交 Offset 的行为配置 #

Flink Kafka Consumer 允许有配置如何将 offset 提交回 Kafka broker 的行为。请注意：Flink Kafka Consumer 不依赖于提交的 offset 来实现容错保证。提交的 offset 只是一种方法，用于公开 consumer 的进度以便进行监控。

配置 offset 提交行为的方法是否相同，取决于是否为 job 启用了 checkpointing。

禁用 Checkpointing： 如果禁用了 checkpointing，则 Flink Kafka Consumer 依赖于内部使用的 Kafka client 自动定期 offset 提交功能。因此，要禁用或启用 offset 的提交，只需将 enable.auto.commit 或者 auto.commit.interval.ms 的Key 值设置为提供的 Properties 配置中的适当值。
启用 Checkpointing： 如果启用了 checkpointing，那么当 checkpointing 完成时，Flink Kafka Consumer 将提交的 offset 存储在 checkpoint 状态中。这确保 Kafka broker 中提交的 offset 与 checkpoint 状态中的 offset 一致。用户可以通过调用 consumer 上的 setCommitOffsetsOnCheckpoints(boolean) 方法来禁用或启用 offset 的提交(默认情况下，这个值是 true )。注意，在这个场景中，Properties 中的自动定期 offset 提交设置会被完全忽略。

kafkasink支持语义保证

kafkaSink 总共支持三种不同的语义保证（DeliveryGuarantee）。对于 DeliveryGuarantee.AT_LEAST_ONCE 和 DeliveryGuarantee.EXACTLY_ONCE，Flink checkpoint 必须启用。默认情况下 KafkaSink 使用 DeliveryGuarantee.NONE。以下是对不同语义保证的解释：

DeliveryGuarantee.NONE 不提供任何保证：消息有可能会因 Kafka broker 的原因发生丢失或因 Flink 的故障发生重复。
DeliveryGuarantee.AT_LEAST_ONCE: sink 在 checkpoint 时会等待 Kafka 缓冲区中的数据全部被 Kafka producer 确认。消息不会因 Kafka broker 端发生的事件而丢失，但可能会在 Flink 重启时重复，因为 Flink 会重新处理旧数据。
DeliveryGuarantee.EXACTLY_ONCE: 该模式下，Kafka sink 会将所有数据通过在 checkpoint 时提交的事务写入。因此，如果 consumer 只读取已提交的数据（参见 Kafka consumer 配置 isolation.level），在 Flink 发生重启时不会发生数据重复。然而这会使数据在 checkpoint 完成时才会可见，因此请按需调整 checkpoint 的间隔。请确认事务 ID 的前缀（transactionIdPrefix）对不同的应用是唯一的，以保证不同作业的事务不会互相影响！此外，强烈建议将 Kafka 的事务超时时间调整至远大于 checkpoint 最大间隔 + 最大重启时间，否则 Kafka 对未提交事务的过期处理会导致数据丢失。

推荐查看1.14版本和1.18版本结合起来看，在一些细节处理上有差异。

Kafka | Apache Flink

其他源码简介

如果查看1.18版本源码不太好理解两阶段提交，可以查看1.14.5的源码，发现FlinkKafkaProducer被标记废除请改用 KafkaSink，并将在 Flink 1.15 中移除，在1.14.5中TwoPhaseCommitSinkFunction为抽象类，有明确定开启事务、预提交和提交的抽象方法，比较好理解。

查看1.14.5版本的KafkaSink 的依赖，发现没有直接使用TwoPhaseCommitSinkFunction，但是查看源码可以看到使用了commiter和kafkawriter对象

public class KafkaSink<IN> implements Sink<IN, KafkaCommittable, KafkaWriterState, Void> {   public static <IN> KafkaSinkBuilder<IN> builder() {return new KafkaSinkBuilder<>();}
-- KafkaWriter 中会判断是否需要开启事务@Overridepublic SinkWriter<IN, KafkaCommittable, KafkaWriterState> createWriter(InitContext context, List<KafkaWriterState> states) throws IOException {final Supplier<MetricGroup> metricGroupSupplier =() -> context.metricGroup().addGroup("user");return new KafkaWriter<>(deliveryGuarantee,kafkaProducerConfig,transactionalIdPrefix,context,recordSerializer,new InitContextInitializationContextAdapter(context.getUserCodeClassLoader(), metricGroupSupplier),states);}-- 事务提交在kafkaCommitter@Overridepublic Optional<Committer<KafkaCommittable>> createCommitter() throws IOException {return Optional.of(new KafkaCommitter(kafkaProducerConfig));}@Overridepublic Optional<GlobalCommitter<KafkaCommittable, Void>> createGlobalCommitter()throws IOException {return Optional.empty();}...
}

KafkaWriter源码

@Overridepublic List<KafkaCommittable> prepareCommit(boolean flush) {if (deliveryGuarantee != DeliveryGuarantee.NONE || flush) {currentProducer.flush();}if (deliveryGuarantee == DeliveryGuarantee.EXACTLY_ONCE) {final List<KafkaCommittable> committables =Collections.singletonList(KafkaCommittable.of(currentProducer, producerPool::add));LOG.debug("Committing {} committables, final commit={}.", committables, flush);return committables;}return Collections.emptyList();}
-- 快照状态开启事务@Overridepublic List<KafkaWriterState> snapshotState(long checkpointId) throws IOException {if (deliveryGuarantee == DeliveryGuarantee.EXACTLY_ONCE) {currentProducer = getTransactionalProducer(checkpointId + 1);currentProducer.beginTransaction();}return ImmutableList.of(kafkaWriterState);}

1.14.5 版本TwoPhaseCommitSinkFunction是一个抽象类在1.18 中是接口

/*** Flink Sink to produce data into a Kafka topic. By default producer will use {@link* FlinkKafkaProducer.Semantic#AT_LEAST_ONCE} semantic. Before using {@link* FlinkKafkaProducer.Semantic#EXACTLY_ONCE} please refer to Flink's Kafka connector documentation.** @deprecated Please use {@link org.apache.flink.connector.kafka.sink.KafkaSink}.*/
@Deprecated
@PublicEvolving
public class FlinkKafkaProducer<IN>extends TwoPhaseCommitSinkFunction<IN,FlinkKafkaProducer.KafkaTransactionState,FlinkKafkaProducer.KafkaTransactionContext> {。。。}
-- 1.14 版本TwoPhaseCommitSinkFunction 为抽象类@PublicEvolving
public abstract class TwoPhaseCommitSinkFunction<IN, TXN, CONTEXT> extends RichSinkFunction<IN>implements CheckpointedFunction, CheckpointListener { }-- 1.18 版本
@PublicEvolving
public interface TwoPhaseCommittingSink<InputT, CommT> extends Sink<InputT> {PrecommittingSinkWriter<InputT, CommT> createWriter(Sink.InitContext var1) throws IOException;Committer<CommT> createCommitter() throws IOException;SimpleVersionedSerializer<CommT> getCommittableSerializer();@PublicEvolvingpublic interface PrecommittingSinkWriter<InputT, CommT> extends SinkWriter<InputT> {Collection<CommT> prepareCommit() throws IOException, InterruptedException;}
}

FlinkKafkaProducer继承TwoPhaseCommitSinkFunction，会重写其中的方法，查看重写开启事务的方法

  -- FlinkKafkaProducer 中重写beginTransaction 方法@Overrideprotected FlinkKafkaProducer.KafkaTransactionState beginTransaction()throws FlinkKafkaException {switch (semantic) {case EXACTLY_ONCE:FlinkKafkaInternalProducer<byte[], byte[]> producer = createTransactionalProducer();
-- 开启kafka的procder的事务producer.beginTransaction();return new FlinkKafkaProducer.KafkaTransactionState(producer.getTransactionalId(), producer);case AT_LEAST_ONCE:case NONE:// Do not create new producer on each beginTransaction() if it is not necessaryfinal FlinkKafkaProducer.KafkaTransactionState currentTransaction =currentTransaction();if (currentTransaction != null && currentTransaction.producer != null) {return new FlinkKafkaProducer.KafkaTransactionState(currentTransaction.producer);}return new FlinkKafkaProducer.KafkaTransactionState(initNonTransactionalProducer(true));default:throw new UnsupportedOperationException("Not implemented semantic");}}

只有当FlinkKafkaProducer.Semantic 为EXACTLY_ONCE时才会开启事务，查看其构造方法

 public FlinkKafkaProducer(String topicId,SerializationSchema<IN> serializationSchema,Properties producerConfig,@Nullable FlinkKafkaPartitioner<IN> customPartitioner,FlinkKafkaProducer.Semantic semantic,int kafkaProducersPoolSize) {this(topicId,null,null,new KafkaSerializationSchemaWrapper<>(topicId, customPartitioner, false, serializationSchema),producerConfig,semantic,kafkaProducersPoolSize);}