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Status

Current state: Under Discussion

Discussion thread:

JIRA:

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

Motivation

Idempotent/transactional semantics depend on the broker retaining state for each active producer id (e.g. epoch and sequence number). When the broker loses that state–due to segment deletion or a call to DeleteRecords–then additional produce requests will result in the UNKNOWN_PRODUCER_ID error.

Currently the producer attempts to handle this error by comparing the last acknowledged offset with the log start offset from the produce response with the error. If the last acknowledged offset is smaller than the log start offset, then the producer assumes that the error is spurious. It resets the sequence number to 0 and retries using the existing epoch.

There are several problems with this approach: 

  1. Resetting the sequence number is safe only if we are guaranteed that the produce request was not written to the log. If we had to retry the request–e.g. due to a disconnect–then we have no way to know whether the first attempt was successful or not. The only option at the moment is to treat this as a fatal error for the producer.
  2. Currently there are not any strict guarantees that the log start offset will increase monotonically. In particular, a new leader can be elected without having seen the latest log start offset from the previous leader. The impact for the producer is that state that was once lost may resurface after a leader failover. This can lead to an out of sequence error when the producer has already reset its sequence number. This is a fatal error for the producer.
  3. The broker accepts produce requests without prior state if the sequence number is 0. There is no guarantee that the producer hasn't been previously fenced. In the worst case, this can lead to a dangling transaction. For example, suppose a zombie producer has its transaction aborted by the coordinator and then a call to DeleteRecords causes this state to be lost. The fenced zombie may still be able to write with sequence 0, but it cannot complete the transaction since it is fenced and the transaction coordinator does not know about the write.

Resetting the sequence number is fundamentally unsafe because it violates the uniqueness of produced records. Additionally, the lack of validation on the first write of a producer introduces the possibility of non-monotonic updates and hence, dangling transactions. In this KIP, we propose to address these problems and simplify the client handling.  

Proposed Changes

Our proposal has three parts: 1) safe epoch incrementing, 2) unknown producer fencing, and 3) simplified client error handling.

Safe Epoch Incrementing: When the producer receives an UNKNOWN_PRODUCER_ID error, in addition to resetting the sequence number, we propose to bump the epoch. For the idempotent producer, bumping the epoch can be done locally since its producer id is unique. The gap at the moment is a safe way for the transactional producer to do so. The basic problem is that the producer may have already been fenced by another instance, so we do not want to allow it to continue.

We propose to alter the InitProducerId API to accept an optional current epoch. When provided, the transaction coordinator will verify that it matches the current epoch and only allow the version bump if it does.

To simplify the handling, the producer will take the following steps upon receiving the UNKNOWN_PRODUCER_ID error:

  1. Enter the ABORTABLE_ERROR state. The only way to recover is for the user to abort the current transaction.
  2. Use the InitProducerId API to request an epoch bump using the current epoch.
  3. If another producer has already bumped the epoch, this will result in a fatal PRODUCER_FENCED error.
  4. If the epoch bump succeeds, the producer will reset sequence numbers back to 0 and continue after the next transaction begins.

Of course the producer may fail to receive the response from the InitProducerId call, so we need to make this API safe for retries. In the worst case, a retry may span coordinator failover, so we need to record in the transaction log whether the bump was the result of a new producer instance or not. We propose to add a new field to the transaction state message for the last epoch that was assigned to a producer instance. When the coordinator receives a new InitProducerId request, we will use the following logic to update the epoch:

  1. No epoch is provided: the current epoch will be bumped and the last epoch will be set to -1.
  2. Epoch is provided:
    1. Epoch matches current epoch: the current epoch is bumped, but the instance epoch will stay the same.
    2. Epoch matches last epoch: the current epoch will be returned
    3. Else: return INVALID_PRODUCER_EPOCH

Unknown Producer Fencing: We propose to introduce a new DescribeTransactionState API which allows a broker to verify with the transaction coordinator whether a producer id has been fenced. This is used only when the broker sees a write with a sequence number 0 from an unknown producer. 

In practice, we expect the need for this API to be rare. As proposed in KAFKA-7190, we will alter the behavior of the broker to retain the cached producer state even after it has been removed from the log. Instead it will be removed only when the transactional id expiration time has passed. Under some circumstances we may have to rebuild the producer state using the log. One example is partition reassignment. A new replica will only see the producers which have state in the log. If one of these replicas becomes a leader, we may see the UNKNOWN_PRODUCER_ID error, which will result in an epoch bump. But the monotonicity of producer writes will never be violated.

Note that it is possible for a transaction to be completed while the DescribeTransactionState response is still inflight. The broker must verify after receiving the response that the producer state is still unknown.

Simplified error handling: Much of the complexity in the error handling of the idempotent/transactional producer is a result of the UNKNOWN_PRODUCER_ID case. Since we are proposing to cache producer state for as long as the transactional id expiration time even after removal from the log, this should become a rare error, so we propose to simplify our handling of it. The current handling attempts to reason about the log start offset and whether or not the batch had been previously retried. If we are sure it is safe, then we attempt to adjust the sequence number of the failed request (and any inflight requests which followed). Not only is this behavior complex to implement, but continuing with subsequent batches introduces the potential for reordering. Currently there is no easy way to prevent this from happening.

We propose the following simplifications:

  1. Assignment of the epoch/sequence number to a record batch is permanent and happens at the time of the record send. We will remove the logic to adjust sequence numbers after failing a batch.
  2. When we encounter a fatal error for a batch, we will fail all subsequent batches for that partition. 

This will be simpler to implement and easier for users to reason about. Records will be guaranteed to be delivered in order. If any record fails delivery, then all subsequently sent records fail as well. For the transactional producer, the user can proceed by aborting the current transaction. Internally, the producer will bump the epoch and reset sequence numbers for the next transaction. For the idempotent producer, the user can choose to fail or they can continue (with the possibility of duplication or reordering). If the user continues, the epoch will be bumped locally and the sequence number will be reset.

Public Interfaces

We will bump the InitProducerId API. The new schemas are provided below:

InitProducerIdRequest => TransactionalId TransactionTimeoutMs ProducerId Epoch
 TransactionalId => NULLABLE_STRING
 TransactionTimeoutMs => INT32
 ProducerId => INT32           // NEW
 Epoch => INT16                // NEW

InitProducerIdResponse => Error ProducerId Epoch
 Error => INT16
 ProducerId => INT64
 Epoch => INT16

The producerId in the request is used to disambiguate requests following expiration of the transactionalId. After a transactional id has expired, its state is removed from the log. If the id is used again in the future, a new producerId will be generated. 

As mentioned above, we will bump the version of the transaction state message to include the instance epoch.

Value => Version ProducerId ProducerEpoch TxnTimeoutDuration TxnStatus [TxnPartitions] TxnEntryLastUpdateTime TxnStartTime
  Version => 1 (INT16)
  LastEpoch => INT16  // NEW
  ProducerId => INT16
  ProducerEpoch => INT16
  TxnTimeoutDuration => INT32
  TxnStatus => INT8
  TxnPartitions => [Topic [Partition]]
     Topic => STRING
     Partition => INT32
  TxnLastUpdateTime => INT64
  TxnStartTime => INT64

As described above, the last epoch is initialized based on the epoch provided in the InitProducerId call. For a new producer instance, the value will be -1.

Additionally, this proposal introduces a new API to query transaction state. This will be used to check whether a 

DescribeTransactionState => [TransactionalId]
  TransactionalId => STRING

DescribeTransactionState => [Error ProducerId Epoch State Partitions]
  Error => INT16
  ProducerId => INT64
  Epoch => INT16
  State => STRING
  Partitions => [TopicName [PartitionId]]
    TopicName => STRING
    PartitionId => INT32

The response includes the latest producer id and the latest epoch. This API is analogous to the DescribeGroup API. The following errors are possible:

  1. COORDINATOR_LOADING
  2. NOT_COORDINATOR
  3. COORDINATOR_NOT_AVAILABLE

Compatibility, Deprecation, and Migration Plan

The main problem from a compatibility perspective is dealing with the existing producers which reset the sequence number to 0 but continue to use the same epoch. We believe that caching the producer state even after it is no longer retained in the log will make the UNKNOWN_PRODUCER_ID error unlikely in practice. Furthermore, even if the sequence number is reset, the fencing check should still be valid. So we expect the behavior to continue to work as expected even with the additional protection. 

The new CheckProducerEpoch API and the new version of the transaction state message will not be used until the inter-broker version supports it. We expect the usual two rolling bounce process for updating the cluster.

Rejected Alternatives

We considered fixing this problem in streams by being less aggressive with record deletion for repartition topics. This might make the problem less likely, but it does not fix it and we would like to have a general solution for all EOS users.


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