三、kafka中事务的使用

本来以为我用过幂等性了，再使用事务应该很简单，不知道幂等性使用的可以去查看上一篇。

首先肯定是照着文档配置生产者，配置如下：

Properties props = new Properties();
        props.put("bootstrap.servers", "localhost:9092,localhost:9093,localhost:9094");
        props.put("acks", "all");
        props.put("retries", 3);
        props.put("batch.size", 163840);
        props.put("linger.ms", 10);
        props.put("buffer.memory", 33554432);
        props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        props.put("enable.idempotence",true);
        props.put("max.in.flight.requests.per.connection",5);
        props.put("transactional.id","aaa");
        Producer producer = new KafkaProducer<>(props);

就比幂等性多配个transactional.id，这样配好之后启动生产者一看，竟然报错了：

Exception in thread "main" java.lang.IllegalStateException: Cannot perform a 'send' before completing a call to initTransactions when transactions are enabled.
	at org.apache.kafka.clients.producer.internals.TransactionManager.failIfNotReadyForSend(TransactionManager.java:422)
	at org.apache.kafka.clients.producer.KafkaProducer.doSend(KafkaProducer.java:925)
	at org.apache.kafka.clients.producer.KafkaProducer.send(KafkaProducer.java:862)
	at org.apache.kafka.clients.producer.KafkaProducer.send(KafkaProducer.java:750)
	at org.apache.kafka.clients.producer.MyProducer.main(MyProducer.java:24)

错误所在的源码如下：

if (isTransactional()) {
            if (!hasProducerId())
                throw new IllegalStateException("Cannot perform a 'send' before completing a call to initTransactions " +
                        "when transactions are enabled.");

            if (currentState != State.IN_TRANSACTION)
                throw new IllegalStateException("Cannot call send in state " + currentState);
        }

源码大体意思是说，如果事务id不为空，而producerId未初始化，则抛错，看到这里我就疑惑了，怎么样才能初始化producerId呢，原来我用幂等性不也好好的吗，后来一查才知道，事务一定要有这一步producer.initTransactions();具体生产者代码如下：

Properties props = new Properties();
        props.put("bootstrap.servers", "localhost:9092,localhost:9093,localhost:9094");
        props.put("acks", "all");
        props.put("retries", 3);
        props.put("batch.size", 163840);
        props.put("linger.ms", 10);
        props.put("buffer.memory", 33554432);
        props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");
        props.put("enable.idempotence",true);
        props.put("max.in.flight.requests.per.connection",5);
        props.put("transactional.id","aaa");
        Producer producer = new KafkaProducer<>(props);
        producer.initTransactions();
        producer.beginTransaction();
        for(int i = 0; i < 5; i++){
            byte[] log = new byte[2];
            String slog = new String(log);
            producer.send(new ProducerRecord("te_transaction",0, Integer.toString(i),  slog));
        }
        producer.commitTransaction();
        producer.close();