This post will highlight some practical aspects of using kafka transactions. I will not dive deep into theory on how Kafka transactions are implemented. Instead, let’s discuss main “gotchas” someone could encounter when trying to use Kafka transactions first time. For detailed theoretical reference please refer to this great article or actual KIP.

Full code mentioned in article can be found here

Transactions in Kafka can be divided into 3 big groups: transactions when producing messages, transactions when consuming and exactly once processing, when both consuming and producing are done within one single transaction. In this post we will discuss only transactional producing.

Just one more note before we start. In this article we will discuss only transaction semantics, but not delivery semantics. These may differ, especially when transaction also involves systems external to Kafka.

Usage example

Let’s say we have an application that allows users to trade chairs between each other. Users submit buy/sell orders and our app finds matching orders so that trades can occur. We also want to aggregate order data to figure out daily volumes, most traded instruments and other fancy business intelligence data. However we do not want to submit customer specific data to business intelligence pipeline because it is personal information and should not be read by unauthorized personnel. One way to tackle that is to have a microservice that forwards orders to matcher queue and submits orders stripped of sensitive data into aggregator queue.

Successful scenario

To submit messages into different queues we use following code:

public void sendTransactionally(List<TopicAndMessage> messages) {
    producer.beginTransaction();

    for (TopicAndMessage topicAndMessage : messages) {
        producer.send(new ProducerRecord<>(
                topicAndMessage.getTopic(),
                getTransactionNr(),
                topicAndMessage.getMessage()));
    }

    producer.commitTransaction();
}

Now lets simulate successful write in our test scenario:

private static final List<TopicAndMessage> MESSAGES = asList(
            new TopicAndMessage(TRADE_MATCHER_TOPIC, "id: 123, buy 42 chairs"),
            new TopicAndMessage(AGGREGATOR_TOPIC, "buy 42 chairs"));

private KafkaConsumer<Integer, String> consumer = ... // create or get read_uncommitted consumer here
private KafkaProducer<Integer, String> producer = ... // create or get transactional producer here

@Test
public void noFailuresOccurred_readAllConsumer() {
    CrashableTransactionalKafkaProducer crashableProducer
            = new CrashableTransactionalKafkaProducer(producer);
    crashableProducer.sendTransactionally(MESSAGES);
    crashableProducer.sendTransactionally(MESSAGES);

    ConsumerRecords<Integer, String> records = poll();

    assertEquals(4, records.count());
    print(records);
}

Output:

Topic: orders, offset: 0, transaction: 1, message:  id: 123, buy 42 chairs
Topic: anonymous-orders, offset: 0, transaction: 1, message:  buy 42 chairs
Topic: orders, offset: 2, transaction: 2, message:  id: 123, buy 42 chairs
Topic: anonymous-orders, offset: 2, transaction: 2, message:  buy 42 chairs

Looks good! Very attentive reader will notice that 2nd transaction offsets start from 2 even though we submitted only 1 message before that. Also what does read_uncommitted mean in comment above? Let’s discuss this in a moment after checking out failure scenario.

Transactional KafkaProducer initialisation is stuck

If something is set incorrectly, most of the time Kafka will throw descriptive exception, for example:

java.lang.IllegalStateException: Cannot use transactional methods without enabling transactions by setting the transactional.id configuration property

However one setting may cause indefinite blocking and “hanging”. This is broker setting transaction.state.log.min.isr (see also transaction.state.log.replication.factor when changing this). To keep track of which transactions were committed Kafka keeps internal topic called __transaction_state. This topic should be replicated for fault-tolerance as any other with Kafka. Now when you call initTransactions() on producer, this call will block until topic has all in-sync replicas acknowledgement.

For example, if broker has only 2 machines online and transaction.state.log.min.isr = 3, initTransactions() will block until one more broker node gets online. If that 3rd node will go offline after producer has been initialized successfully, everything will keep working. However for startup it is necessary to have minimum broker node count online.

Failure scenario

Now following scenario:

@Test
public void crashOnSecondTransaction_readAllConsumer() {
    crashableProducer.sendTransactionally(MESSAGES);
    crashableProducer.setShouldCrash(true);
    try {
        crashableProducer.sendTransactionally(MESSAGES);
    }
    catch (ForceCrashException e) {
        System.out.println("Producer crashed!");
    }

    List<ConsumerRecord<Integer, String>> records = poll();

    assertEquals(3, records.size());
    print(records);
}

gives us:

Producer crashed!
Topic: orders, offset: 0, transaction: 1, message:  id: 123, buy 42 chairs
Topic: anonymous-orders, offset: 0, transaction: 1, message:  buy 42 chairs
Topic: orders, offset: 2, transaction: 2, message:  id: 123, buy 42 chairs

Oops! Looks like transactions didn’t work. But they actually do. The problem here is that consumer was created with isolation.level = read_uncommitted setting, which is it’s default value. Every time producer commits a transaction it submits a commit marker to the log. If consumer’s isolation.level is set to read_uncommitted it just reads all messages it can. In case setting is read_committed consumer will wait until commit marker is submitted to decide what to do with preceding messages.

This commit marker contains all necessary data for consumer to read from __transactions_state topic and decide, if message must be returned. In case when producer abort transaction commit marker will represents abort and will be ignore by consumer. In case producer dies, broker will ultimately expire transaction and abort marker itself. This is why if you submit n messages to topic the next batch will always start at n+1 offset.

Now if we set consumer’s isolation.level = read_committed, we will get:

Producer crashed!
Topic: orders, offset: 4, transaction: 1, message:  id: 123, buy 42 chairs
Topic: anonymous-orders, offset: 2, transaction: 1, message:  buy 42 chairs

So important note here is that transactional producing is dependent on the readers. Therefore when building pipelines with Kafka, every node in chain settings must be carefully considered, because one error may cause change in all pipeline delivery semantics!

Producer fencing and expiration

To protect cluster from intermittent connection errors Kafka uses epochs tied to producer’s transaction ID. Every time producer calls initTransactions(), cluster will assign it an epoch = previous_epoch + 1. This means that if you start producer with transaction id and cluster has active producer with same id, the latter one will be disconnected (fenced in Kafka terms) and throw an exception next time it tries to send message:

org.apache.kafka.common.errors.ProducerFencedException: Producer attempted an operation with an old epoch. Either there is a newer producer with the same transactionalId, or the producer's transaction has been expired by the broker.

Sometimes producer throws another exception, with ProducerFencedException as cause.

But this exception may occur even without connectivity errors. Internally broker keeps a map of producer IDs to transaction IDs to maintain aforementioned fencing functionality. However broker will expire these entries based on transactional.id.expiration.ms setting which is by default 604800000 ms or 1 week.

This means that if your producer sends messages slower than once a week it will be fenced (crash) every time it tries to send after that prolonged period. One could put that value to Integer.MAX, but that still will be around 24 days. In case of very rare events, this should be solved differently, e.g. by having separate producer ping topic, that producer will periodically send messages to.

Instead of conclusion

That’s it for Kafka producers! Hopefully this will help you to understand, setup Kafka transactions and successfully keep them producing. For more implementation details how Kafka transactions work, I recommend blog mentioned in this post’s intro and this Kafka KIP.

Stay tuned for article on transactional consuming!

Categories: Messaging

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