Status

Current state: Accepted

Discussion thread: TBD

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Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

Kafka Producer supports transactional semantics since 0.11, including the following APIs:

• InitTransaction for transactional producer identity initialization
• beginTransaction to start a new transaction 
• sendOffsetsToTransaction to commit consumer offsets advanced within the current transaction
• commitTransaction commit the ongoing transaction
• abortTransaction abort the ongoing transaction

Within a transaction session, the internal state tracks the last fatal/abortable error. When the Producer hits a fatal/abortable exception, it will transit to the error state, and when next time someone uses a transactional API, it will throw the buffered exception. The caveat is that today we wrap many non-fatal exceptions as KafkaException, which does not make a clear boundary on whether the thrown exception is fatal – should fail fast, or just abortable – should catch and abort the ongoing transaction to resume. It affects the stability of upstream users as well such as Streams EOS. This KIP tries to address this gap by revamping Producer APIs to make it more robust and let exception handling coherent and consistent.

As of 08/16/2022, we (guozhang Wang) made a pass inside the KafkaProducer on how we categorizes those exceptions, and also on how we wrapped those exceptions.

First of all, the exceptions can be thrown in the following ways:

1) Thrown from TransactionManager#maybeFailWithError, which is thrown from KafkaProducer#initTxn, #beginTxn, #commitTxn, #abortTxn, #sendOffsetsToTransaction, #send. Except #send, all other functions would trigger maybeFailWithError right at the beginning of the function call. However, KafkaProducer#send would only call this check AFTER the record is appended to the accumulator, and hence it's possible that the sender has already sent this record out for idempotent producer (for transactional producer the record would not be sent until we successfully added the partition to the txn coordinator), this exception may be thrown directly from the #send call or maybe thrown from the #send callback of the future (KAFKA-14138). See below for more details. When they are thrown, they are mostly wrapped as KafkaExceptions

2) Thrown from KafkaProducer#send's callback, or the returned future.get(). Note that only produce request related exceptions may be thrown here, but these exceptions may also be thrown in 1) above since some of these exceptions would also be cause TransactionManager to transit to error states.

Now here's a full summary of all possible exceptions below:

Besides the detailed suggestions on each of the above category lines, there are also a few meta-level proposals:

1. We should have a clear distinguishment between fatal and abortable errors. Today they are all wrapped as KafkaExceptions and hence users cannot really tell the difference, so they have to just treat them all as fatal. One idea is to wrap them differently: we use KafkaException to wrap fatal errors, while use AbortableException to wrap abortable errors.
2. We should have consistent mechanisms on wrapping errors. More specifically:
   1. All exceptions should be wrapped at most once.
   2. We should not have a nested wrapping like KE(KE(e)). Instead we should just have KE(e).
   3. For IllegalState / Runtime errors which indicate a bug, we should not wrap but directly throw.
3. For errors that can be thrown from both scenario 1) and 2) above, we should have a clear guidance on how EOS users should handle them. More specifically:
   1. Users should be safely ignore the returned future since all errors that should be set inside the TransactionManager as well and hence users should just capture the function itself.
   2. The only error that's thrown only in 2) is TransactionAbortedException, and it could just be ignored since the user asked the producer to close.

Proposed Changes

We are proposing a new transactional API usage template which makes EOS processing safer from handling a mix of fatal and non-fatal exceptions:

KafkaConsumer consumer = new KafkaConsumer<>(consumerConfig);
producer.initTransactions();
volatile boolean isRunning = true;

try {
    while (isRunning) {
        ConsumerRecords<String, String> records = consumer.poll(CONSUMER_POLL_TIMEOUT);
        final boolean shouldCommit;
        try {
            producer.beginTransaction();

            // Do some processing and build the records we want to produce.
            List<ProducerRecord> processed = process(consumed);

            for (ProducerRecord record : processed)
                producer.send(record, (metadata, exception) -> {
                    // not required to capture the exception here.
                });
            producer.sendOffsetsToTransaction(consumedOffsets, consumer.groupMetadata());

            shouldCommit = true;
        } catch (Exception e) {
            // Catch any exception thrown from the data transmission phase.
            shouldCommit = false;
        }

        try {
            if (shouldCommit) {
                producer.commitTransaction();
            } else {
                resetToLastCommittedPositions(consumer);
                producer.abortTransaction();
            }
        } catch (CommitFailedException e) {
            // Transaction commit failed with abortable error, user could reset
            // the application state and resume with a new transaction. The root
            // cause was wrapped in the thrown exception.
            resetToLastCommittedPositions(consumer);
            producer.abortTransaction();
        }
    }
} catch (KafkaException e) { // Recommended closing producer/consumer for fatal exceptions
    producer.close();
    consumer.close();
    throw e;
}

In the above example, we separate the transactional processing into two phases: the data transmission phase, and the commit phase. In data transmission phase, any exception thrown would be an indication of the ongoing transaction failure, so that we got a clear signal for the next stage whether to commit or abort the ongoing transaction.

In the commit phase, we should decide whether to commit or abort transaction based on the previous stage result. In new Producer API, commitTransaction() will no longer throw non-fatal exceptions in their raw formats. Instead, it would try to wrap all non-fatal exceptions as `CommitFailedException`. This means any exception other than `CommitFailedException` caught during the commit phase will be definitely fatal, so user's error handling experience could be simplified by just doing a controlled shutdown.

The only debatable case is timeout exception within commit/abort transaction. It could be treated either fatal or not, as strictly speaking producer would have already done the retrying for max.block.ms, so a timeout here may be suggesting a fatal state to a basic user's perspective. Blindly call abortTxn upon timeout could result in illegal state as well when the previous commit already writes `prepare_commit` on the broker side. Usually caller level could have more sophisticated handling to do an application level retry if necessary, but we don't do any recommendations here. It is highly recommended to increase the request timeout here instead of relying on unreliable retries. 

We also put another try-catch block outside of the whole processing loop to get a chance catching all fatal exceptions and close producer and consumer modules. It is a recommended way to handle fatal exceptions when the application still wants to proceed without any memory leak, but is optional to users.

Unify Wrapped KafkaException

As discussed in the motivation section, in KafkaProducer we have a logic to wrap all thrown exceptions as KafkaException. To make the semantic clear and for advanced users such as Kafka Streams to better understand the root cause, we shall no longer wrap any fatal exceptions, but instead only wrap non-fatal ones as KafkaException. We also detect certain cases where we did a double-wrap of KafkaException internally, which will be addressed to ensure only one layer wrapping is supported.

Callback Exception Improvement

As we have seen, there is a callback mechanism in the producer#send which carries the exception type. In EOS setup, it is not required to handle the exception, but for non-EOS cases, the current exception type mechanism is complicated as it throws raw exceptions. Although in the callback function comments we listed all the fatal and non-fatal exceptions, in users' perspective they still need to maintain an exhausting list for checking exception types and take proper actions. The application code is fragile when facing ever-changing underlying producer logic with new exception types, and we have seen the difficulty to classify populated exceptions in application level such as Streams.

On the other thread, there are proposals around making producer API adopt more modern return types than a mere Future. This is a potential good opportunity to merge these two efforts. 

To make the handling easier and consistent, we suggest to add a new exception type called ProduceFailedException which wraps the thrown exception and contains an enum field indicating failure type. Right now there would be 3 types of failures:

1. message rejected: this error associates with this specific record being produced, such as InvalidTopic or RecordTooLarge
2. delivery failed: suggests a failure to produce last record, such as NotEnoughReplicas or Timeout
3. transaction error: an exception relating specifically to transactional semantic, such as ProducerFenced or InvalidTransactionState

With additional flagging, producer users would be in a much better position interpreting the callback and take proper actions with less effort to diagnose the failures by themselves. 

This new exception type would be thrown back to the user only in the new producer API depicted in KIP-706.

Stream side change

For EOS Kafka Streams case, we would adopt these simplified exception throwing logic by catching all exceptions in the data transmission phase to decide for Streams commit. Furthermore, these changes leave to door open for us to analyze the non-fatal exceptions thrown as well by unwrapping KafkaException's cause and reading failure type through callback.

For known exceptions such as ProducerFenced, the handling shall be simplified as we no longer need to wrap them as TaskMigratedException in the send callback, since they should not crash the stream thread if thrown in raw format, once we adopt the new processing model in the send phase.

For exception such as TimeoutException, we already have KIP-572 to cover the application level retry, which should be ok to let it throw to the upper level.

Public Interfaces

As mentioned in the proposed changes section, we would be doing the following public API changes:

• The commitTransaction() API will  throw CommitFailedException to wrap non-fatal exceptions
• All the non-fatal exceptions thrown from data transmission APIs will be wrapped as KafkaException, which we will be documented clearly. This includes:
  • beginTransaction 
  • sendOffsetsToTransaction 
  • send

We would also let commitTransaction API only throw CommitFailedException with wrapped cause when hitting non-fatal exceptions, to simply the exception try-catching.

We would add a new Producer error type called ProduceFailedException which tries to wrap send/produce exceptions with root cause and reasoning.

/**
 * Exception indicating a produce failure for the given record, with root cause and reasoning embedded.
 */
public ProduceFailedException extends ApiException {

    private final FailureType type;
    
    public ProduceFailedException(Throwable cause, FailureType type) {
        super(cause);
        this.type = type;
    }
    
    public FailureType failureType() {
        return type;
    }

    enum FailureType {
        MESSSAGE_REJECTED, // the specific record being produced was rejected, such as InvalidTopic or RecordTooLarge
		DELIVERY_FAILED, // a failure to produce last record, such as NotEnoughReplicas or Timeout
		TRANSACTION_FAILED // a transactional processing failure, such as ProducerFenced or InvalidTransactionState
    }
}

The failure type in the embed exception should be helpful, for example, they could choose to ignore transactional errors since other txn APIs are already taking care of them. We expect this error code to be implemented once KIP-706 is accepted, which would provide a more user-friendly send API with CompletableFuture or similar.

Documentation change

We shall put the newly marked fatal exceptions on the public Producer API docs correspondingly, including 

• beginTransaction 
• sendOffsetsToTransaction
• commitTransaction
• abortTransaction
• send

Compatibility, Deprecation, and Migration Plan

This is a pure client side change which only affects the resiliency of new Producer client and Streams. For customized EOS use case, user needs to change their exception catching logic to take actions against their exception handling around commitTransaction(), since it no longer throws non-fatal exception, which means it does not indicate a success of commit when not throwing. However, all the thrown exceptions' base type would still be KafkaException, so the effect should be minimal.

Rejected Alternatives

We thought about exhausting all the possible exception types on the Streams level for resiliency, but abandoned the approach pretty soon as it would require a joint code change every time the underlying Producer client throws a new exception. The encapsulation should help reduce the amount of work on the caller side for exception handling. 

We also proposed to add a non-fatal exception wrapper type called TransactionStateCorruptedException to help users distinguish thrown exception types. This solution has compatibility issue and is not necessarily making the developer and user's life easier.

We proposed to to add a return boolean in commitTransaction, so that even if the commit failed internally with non-fatal exception but wasn't throwing, we still got a clear returned signal from commitTransaction to know whether the last commit was successful, as certain EOS users rely on external data storage component and need to perform non-rollback commit operation as necessary. This approach was abandoned as it broke the compatibility since old users would not assume a commitTxn not to be indicating a success when it passes without throwing non-fatal exceptions.

We discussed about throwing ProduceFailedException within the send callback, however there are compatibility concerns. The other approach is to make failure reason as part of the callback function, which is less optimized than introducing a true async method like KIP-706 did.