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 two main problems with this approach:
- 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.
- 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.
- 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 so 1) this error condition becomes rare, and 2) it is no longer fatal. For transactional producers, it will be possible to simply abort the current transaction and continue. We also make some effort to simplify error handling in the client handlingproducer.
Our proposal has two three parts: 1) safe epoch incrementing, and 2) unknown producer fencingprolonged producer state retention, 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.
- Enter the ABORTABLE_ERROR state. The only way to recover is for the user to abort the current transaction.
- Use the InitProducerId API to request an epoch bump using the current epoch.
- If another producer has already bumped the epoch, this will result in a fatal INVALID_PRODUCER_FENCED EPOCH error.
- 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 "instance_epoch," which is the first epoch last epoch that was assigned to a new producer instance. When the coordinator receives a new InitProducerId request, we will use the following logic to update the epoch:
- No epoch is provided: the current epoch will be bumped and the instance last epoch will be set to the new value-1.
- Epoch is provided:
- Matches Provided epoch matches current epoch: the last epoch will be set to the current epoch is bumped, but and the instance current epoch will stay the samebe bumped .
- Epoch is greater than or equal to instance epochProvided epoch matches last epoch: the current epoch will be returned
- Epoch is less than instance epochElse: return INVALID_PRODUCER_EPOCH
- Epoch is greater than current epoch: 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. Another case we want to handle is InvalidProducerIdMapping. This error occurs following expiration of the producerId. It's possible that another producerId has been installed in its place following expiration (if another producer instance has become active), or the mapping is empty. We can safely retry the InitProducerId with the logic in this KIP in order to detect which case it is:
- After receiving INVALID_PRODUCER_ID_MAPPING, the producer can send InitProducerId using the current producerId and epoch.
- If no mapping exists, the coordinator can generate a new producerId and return it. If a transaction is in progress on the client, it will have to be aborted, but the producer can continue afterwards.
- Otherwise if a different producerId has been assigned, then we can return INVALID_PRODUCER_EPOCH since that is effectively what has happened. This is intended to simplify error handling. The point is that there is a newer producer and it doesn't matter whether it has the same producer id or not.
Prolonged producer state retention: 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 Previously we attempted to keep the producer state consistent with the the contents of the log so that we could rebuild it from the log if needed. However, it is rarely necessary to rebuild producer state, and it is more useful to retain the state we have as long as possible. Here we propose to remove it 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
Note that it is possible for some disagreement on the current producer state between the replicas. One example is following a 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., so this may lead to an unexpected UNKNOWN_PRODUCER_ID error if the new replica becomes a leader before any additional writes occur for that producer id. However, any inconsistency between the leader and follower about producer state does not cause any problems from the perspective of replication because followers always assume the leader's state is correct. Furthermore, since we will now have a way to safely bump the epoch on the producer when we see UNKNOWN_PRODUCER_ID, these edge cases are not fatal for the producer.
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 duplicates. Currently there is no easy way to prevent this from happening.
We propose the following simplifications:
- 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.
- When we encounter a fatal error for a batch, we will fail all subsequent batches for that partition which have been assigned a sequence number.
Records will be guaranteed to be delivered in order up until the first fatal error and there will be no duplicates. For the transactional producer, the user can proceed by aborting the current transaction and ordering can still be guaranteed going forward. 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.
We will bump the InitProducerId API. The new schemas are provided below:
As mentioned above, we will bump the version of the transaction state message to include the instance last epoch.
Value => Version ProducerId ProducerEpochCurrentEpoch LastBump TxnTimeoutDuration TxnStatus [TxnPartitions] TxnEntryLastUpdateTime TxnStartTime Version => 1 (INT16) InstanceEpochProducerId => INT16 // NEWCurrentEpoch ProducerId => INT16 ProducerEpochLastEpoch => INT16 // NEW TxnTimeoutDuration => INT32 TxnStatus => INT8 TxnPartitions => [Topic [Partition]] Topic => STRING Partition => INT32 TxnLastUpdateTime => INT64 TxnStartTime => INT64
As described above, the instance last epoch is assigned initialized based on the first call to InitProducerId of a producer instance The first call uses an epoch of -1 and a producer id of -1 and will fence any active instance. The instance epoch does not change until epoch provided in the InitProducerId call. For a new producer instance with the same transactional id is initialized.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:
Only in the latter two cases will the latest producer id and epoch be provided in the responsethe value will be -1.
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 , so this resetting logic should be less frequently relied upon. When it is used, the broker will 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.
One key question is how the producer will interoperate with older brokers which do not support the new version of the `InitProducerId` request. For idempotent producers, we can safely bump the epoch without broker intervention, but there is no way to do do so for transactional producers. We propose in this case to immediately fail pending requests and enter the ABORTABLE_ERROR state. After the transaction is aborted, we will reset the sequence number to 0 and continue. So the only difference is that we skip the epoch bump.
- 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.
- When the broker has no state for a given producerId, it will only accept new messages as long as they begin with sequence=0. There is no guarantee that such messages aren't duplicates which were previously removed from the log or that the producer id hasn't been fenced. The initial version of this KIP attempted to add some additional validation in such cases to prevent these edge cases. After some discussion, we felt the proposed fix still had some holes and other changes in this KIP made this sufficiently unlikely in any case. This will be reconsidered in a future KIP.