Flink

https://ci.apache.org/projects/flink/flink-docs-release-1.3/dev/connectors/kafka.html

使用的方式，

DataStream<String> stream = ...;

FlinkKafkaProducer010Configuration myProducerConfig = FlinkKafkaProducer010.writeToKafkaWithTimestamps(
        stream,                     // input stream
        "my-topic",                 // target topic
        new SimpleStringSchema(),   // serialization schema
        properties);                // custom configuration for KafkaProducer (including broker list)

// the following is necessary for at-least-once delivery guarantee
myProducerConfig.setLogFailuresOnly(false);   // "false" by default
myProducerConfig.setFlushOnCheckpoint(true);  // "false" by default

 

Besides enabling Flink’s checkpointing, you should also configure the setter methods setLogFailuresOnly(boolean) andsetFlushOnCheckpoint(boolean) appropriately, as shown in the above examples in the previous section:

• setLogFailuresOnly(boolean): enabling this will let the producer log failures only instead of catching and rethrowing them. This essentially accounts the record to have succeeded, even if it was never written to the target Kafka topic. This must be disabled for at-least-once.
• setFlushOnCheckpoint(boolean): with this enabled, Flink’s checkpoints will wait for any on-the-fly records at the time of the checkpoint to be acknowledged by Kafka before succeeding the checkpoint. This ensures that all records before the checkpoint have been written to Kafka. This must be enabled for at-least-once.

Note: By default, the number of retries is set to “0”. This means that when setLogFailuresOnly is set to false, the producer fails immediately on errors, including leader changes. The value is set to “0” by default to avoid duplicate messages in the target topic that are caused by retries. For most production environments with frequent broker changes, we recommend setting the number of retries to a higher value.

setLogFailuresOnly，如果true，发送kafka失败时，只是log，不会中断执行，这样可能丢数据

                                如果false，发送kafka失败时，抛异常，这样job会restart，不会丢数据，但是会中断执行；这里最好把produer的retires设成3，这样避免kafka临时不可用导致job中断，比如leader切换

setFlushOnCheckpoint，如果true，在做checkpoint的时候，会等待所有pending的record被发送成功，这样保证数据不丢

 

首先FlinkKafkaProducer010是一种sink，

一般的使用方式是，steam.addSink(RichSinkFunction)

    public DataStreamSink<T> addSink(SinkFunction<T> sinkFunction) {
        this.transformation.getOutputType();
        if(sinkFunction instanceof InputTypeConfigurable) {
            ((InputTypeConfigurable)sinkFunction).setInputType(this.getType(), this.getExecutionConfig());
        }

        StreamSink sinkOperator = new StreamSink((SinkFunction)this.clean(sinkFunction));
        DataStreamSink sink = new DataStreamSink(this, sinkOperator);
        this.getExecutionEnvironment().addOperator(sink.getTransformation());
        return sink;
    }

 

这里用FlinkKafkaProducer010.writeToKafkaWithTimestamps封装这部分，比较tricky

   /**
     * Creates a FlinkKafkaProducer for a given topic. The sink produces a DataStream to
     * the topic.
     *
     * This constructor allows writing timestamps to Kafka, it follow approach (b) (see above)
     *
     *  @param inStream The stream to write to Kafka
     *  @param topicId The name of the target topic
     *  @param serializationSchema A serializable serialization schema for turning user objects into a kafka-consumable byte[] supporting key/value messages
     *  @param producerConfig Configuration properties for the KafkaProducer. 'bootstrap.servers.' is the only required argument.
     *  @param customPartitioner A serializable partitioner for assigning messages to Kafka partitions.
     */
    public static <T> FlinkKafkaProducer010Configuration<T> writeToKafkaWithTimestamps(DataStream<T> inStream,
                                                                                    String topicId,
                                                                                    KeyedSerializationSchema<T> serializationSchema,
                                                                                    Properties producerConfig,
                                                                                    KafkaPartitioner<T> customPartitioner) {

        GenericTypeInfo<Object> objectTypeInfo = new GenericTypeInfo<>(Object.class);
        FlinkKafkaProducer010<T> kafkaProducer = new FlinkKafkaProducer010<>(topicId, serializationSchema, producerConfig, customPartitioner);
        SingleOutputStreamOperator<Object> transformation = inStream.transform("FlinKafkaProducer 0.10.x", objectTypeInfo, kafkaProducer);
        return new FlinkKafkaProducer010Configuration<>(transformation, kafkaProducer);
    }

 

可以看到这里实现了addSink的逻辑，返回FlinkKafkaProducer010Configuration，其实就是DataStreamSink

    public static class FlinkKafkaProducer010Configuration<T> extends DataStreamSink<T> {

        private final FlinkKafkaProducerBase wrappedProducerBase;
        private final FlinkKafkaProducer010 producer;

        private FlinkKafkaProducer010Configuration(DataStream stream, FlinkKafkaProducer010<T> producer) {
            //noinspection unchecked
            super(stream, producer);
            this.producer = producer;
            this.wrappedProducerBase = (FlinkKafkaProducerBase) producer.userFunction;
        }

 

关键是FlinkKafkaProducer010扩展StreamSink并重写

processElement

public class FlinkKafkaProducer010<T> extends StreamSink<T> implements SinkFunction<T>, RichFunction {

    public FlinkKafkaProducer010(String topicId, KeyedSerializationSchema<T> serializationSchema, Properties producerConfig, KafkaPartitioner<T> customPartitioner) {
        // We create a Kafka 09 producer instance here and only "override" (by intercepting) the
        // invoke call.
        super(new FlinkKafkaProducer09<>(topicId, serializationSchema, producerConfig, customPartitioner));
        
    }
    
    @Override
    public void processElement(StreamRecord<T> element) throws Exception {
        invokeInternal(element.getValue(), element.getTimestamp());
    }

StreamSink中processElement是这样实现的，

public class StreamSink<IN> extends AbstractUdfStreamOperator<Object, SinkFunction<IN>>
        implements OneInputStreamOperator<IN, Object> {

    @Override
    public void processElement(StreamRecord<IN> element) throws Exception {
        userFunction.invoke(element.getValue());
    }

可以看到FlinkKafkaProducer010绕开了对SinkFunction的调用，直接调用invokeInternal

所以SinkFunction的实现是无用的，不会被调用到

    public void invoke(T value) throws Exception {
        invokeInternal(value, Long.MAX_VALUE);
    }

invokeInternal

    private void invokeInternal(T next, long elementTimestamp) throws Exception {

        final FlinkKafkaProducerBase<T> internalProducer = (FlinkKafkaProducerBase<T>) userFunction;

        internalProducer.checkErroneous();

        byte[] serializedKey = internalProducer.schema.serializeKey(next);
        byte[] serializedValue = internalProducer.schema.serializeValue(next);
        String targetTopic = internalProducer.schema.getTargetTopic(next);
        if (targetTopic == null) {
            targetTopic = internalProducer.defaultTopicId;
        }

        Long timestamp = null;
        if(this.writeTimestampToKafka) {
            timestamp = elementTimestamp;
        }

        ProducerRecord<byte[], byte[]> record;
        if (internalProducer.partitioner == null) {
            record = new ProducerRecord<>(targetTopic, null, timestamp, serializedKey, serializedValue);
        } else {
            record = new ProducerRecord<>(targetTopic, internalProducer.partitioner.partition(next, serializedKey, serializedValue, internalProducer.partitions.length), timestamp, serializedKey, serializedValue);
        }
        if (internalProducer.flushOnCheckpoint) {
            synchronized (internalProducer.pendingRecordsLock) {
                internalProducer.pendingRecords++;  // 如果flushOnCheckpoint打开，需要记录正在发送的record数目
            }
        }
        internalProducer.producer.send(record, internalProducer.callback);
    }

代码很容易理解，正常的producer发送流程，

除了，

internalProducer.checkErroneous();

internalProducer.callback

 

internalProducer.callback是用来处理kafka返回的ack的

FlinkKafkaProducerBase

    @Override
    public void open(Configuration configuration) {if (logFailuresOnly) {
            callback = new Callback() {
                @Override
                public void onCompletion(RecordMetadata metadata, Exception e) {
                    if (e != null) {
                        LOG.error("Error while sending record to Kafka: " + e.getMessage(), e);
                    }
                    acknowledgeMessage();
                }
            };
        }
        else {
            callback = new Callback() {
                @Override
                public void onCompletion(RecordMetadata metadata, Exception exception) {
                    if (exception != null && asyncException == null) {
                        asyncException = exception;
                    }
                    acknowledgeMessage();
                }
            };
        }
    }

可以看到logFailuresOnly是true的时候，对于Exception只是，log

如果是false，就会记录下这个Exception到asyncException

 

acknowledgeMessage，无论是否有错都需要ack

    private void acknowledgeMessage() {
        if (flushOnCheckpoint) {
            synchronized (pendingRecordsLock) {
                pendingRecords--;
                if (pendingRecords == 0) {
                    pendingRecordsLock.notifyAll();
                }
            }
        }
    }

逻辑就是计数--，如果pendingRecords == 0，即没有正在发送的record，通知所有在等锁的

 

checkErroneous()

    protected void checkErroneous() throws Exception {
        Exception e = asyncException;
        if (e != null) {
            // prevent double throwing
            asyncException = null;
            throw new Exception("Failed to send data to Kafka: " + e.getMessage(), e);
        }
    }

就是把asyncException里面的异常抛出去