恰好一次发送和事务消息(译)

public interface Producer<K,V> extends Closeable {
   
  /**
   * Needs to be called before any of the other transaction methods. Assumes that
   * the transactional.id is specified in the producer configuration.
   *
   * This method does the following:
   *   1. Ensures any transactions initiated by previous instances of the producer
   *      are completed. If the previous instance had failed with a transaction in
   *      progress, it will be aborted. If the last transaction had begun completion,
   *      but not yet finished, this method awaits its completion.
   *   2. Gets the internal producer id and epoch, used in all future transactional
   *      messages issued by the producer.
   *
   * @throws IllegalStateException if the TransactionalId for the producer is not set
   *         in the configuration.
   */
  void initTransactions() throws IllegalStateException;
   
  /**
   * Should be called before the start of each new transaction.
   *
   * @throws ProducerFencedException if another producer is with the same
   *         transactional.id is active.
   */
  void beginTransaction() throws ProducerFencedException;
   
  /**
   * Sends a list of consumed offsets to the consumer group coordinator, and also marks
   * those offsets as part of the current transaction. These offsets will be considered
   * consumed only if the transaction is committed successfully.
   *
   * This method should be used when you need to batch consumed and produced messages
   * together, typically in a consume-transform-produce pattern.
   *
   * @throws ProducerFencedException if another producer is with the same
   *         transactional.id is active.
   */
  void sendOffsetsToTransaction(Map<TopicPartition, OffsetAndMetadata> offsets,
                                String consumerGroupId) throws ProducerFencedException;
   
  /**
   * Commits the ongoing transaction.
   *
   * @throws ProducerFencedException if another producer is with the same
   *         transactional.id is active.
   */
  void commitTransaction() throws ProducerFencedException;
   
  /**
   * Aborts the ongoing transaction.
   *
   * @throws ProducerFencedException if another producer is with the same
   *         transactional.id is active.
 
 
   */
  void abortTransaction() throws ProducerFencedException;
 
 
  /**
   * Send the given record asynchronously and return a future which will eventually contain the response information.
   *
   * @param record The record to send
   * @return A future which will eventually contain the response information
   *
   */
  public Future<RecordMetadata> send(ProducerRecord<K, V> record);
 
  /**
   * Send a record and invoke the given callback when the record has been acknowledged by the server
   */
  public Future<RecordMetadata> send(ProducerRecord<K, V> record, Callback callback);
}

public class KafkaTransactionsExample {
  
  public static void main(String args[]) {
    KafkaConsumer<String, String> consumer = new KafkaConsumer<>(consumerConfig);
 
 
    // Note that the ‘transactional.id’ configuration _must_ be specified in the
    // producer config in order to use transactions.
    KafkaProducer<String, String> producer = new KafkaProducer<>(producerConfig);
 
    // We need to initialize transactions once per producer instance. To use transactions,
    // it is assumed that the application id is specified in the config with the key
    // transactional.id.
    //
    // This method will recover or abort transactions initiated by previous instances of a
    // producer with the same app id. Any other transactional messages will report an error
    // if initialization was not performed.
    //
    // The response indicates success or failure. Some failures are irrecoverable and will
    // require a new producer  instance. See the documentation for TransactionMetadata for a
    // list of error codes.
    producer.initTransactions();
     
    while(true) {
      ConsumerRecords<String, String> records = consumer.poll(CONSUMER_POLL_TIMEOUT);
      if (!records.isEmpty()) {
        // Start a new transaction. This will begin the process of batching the consumed
        // records as well
        // as an records produced as a result of processing the input records.
        //
        // We need to check the response to make sure that this producer is able to initiate
        // a new transaction.
        producer.beginTransaction();
         
        // Process the input records and send them to the output topic(s).
        List<ProducerRecord<String, String>> outputRecords = processRecords(records);
        for (ProducerRecord<String, String> outputRecord : outputRecords) {
          producer.send(outputRecord);
        }
         
        // To ensure that the consumed and produced messages are batched, we need to commit
        // the offsets through
        // the producer and not the consumer.
        //
        // If this returns an error, we should abort the transaction.
         
        sendOffsetsResult = producer.sendOffsetsToTransaction(getUncommittedOffsets());
         
      
        // Now that we have consumed, processed, and produced a batch of messages, let's
        // commit the results.
        // If this does not report success, then the transaction will be rolled back.
        producer.endTransaction();
      }
    }
  }
}