In the discussion of KIP-185: Make exactly once in order delivery per partition the default producer setting, the following point regarding the OutOfOrderSequenceException
was raised:
OutOfOrderSequenceException
indicates that there has been data loss on the broker.. ie. a previously acknowledged message no longer exists. For most part, this should only occur in rare situations (simultaneous power outages, multiple disk losses, software bugs resulting in data corruption, etc.).OutOfOrderSequence
which is a false positive: the data is removed through valid processes, and this isn't an error.OutOfOrderSequenceException
when we get a duplicate of a batch which is not at the tail of the log. This is also confusing, and a more clear error would be the DuplicateSequenceException
in this case.We would like to eliminate the possibility of getting spurious OutOfOrderSequenceExceptions
– when you get it, it should always mean data loss and should be taken very seriously.
Essentially, we want to distinguish between the case where a producer's state is removed from the broker because the retention time has elapsed, and when the state is lost due to some problem in the system.
One solution is described here:
ProducerStateManager
on the broker due to retention, the next ProduceRequest
from the client will arrive with the existing producer id and with a non-zero sequence. Currently this results in an OutOfOrderSequenceException
returned by the broker, since the broker can't find any metadata and gets a non-zero sequence. This isn't strictly correct, and we propose introducing a new UnknownProducerException
and returning this instead. UnknownProducerException
as a non-fatal error and just reinitialize the producer and continue on its merry way in most cases.UknownProducerException
and with the protocol above would simply be retried. Hence the fact that a message was lost would never be raised to the application. This applies to the first write because it is only at the front of the log where there could be a confusion between removal due to retention or loss due to an unforeseen circumstance.ProduceResponse
. With these two pieces of information, we can be sure that an UknownProducerException
is valid if the log start offset returned along with the error code is greater than the last ack'd offset. This means that the front of the log has been truncated, causing the producer to become unknown. In this case, there is no unwanted data loss and the last batch can simply be retried. If we get an UnkownProducerException
but the log start offset is not greater than the last ack'd offset, then the record has been not been lost due to the retention period elapsing, and this should be treated as a fatal error. DuplicateSequenceException
instead of an OutOfOrderSequencException
OutOfOrderSequenceException
would always mean real data loss. An UnkownProducerException
may mean data loss, and the information of the last ack'd offset and log start offset will enable us to disambiguate.Client side changes to track the last ack'd offset and correctly interpret an UnknownProducerException
and either retry it or raise it as an error – 1 day.
Broker side changes to raise the UnkownProducerException
– 0.25 days.
logStartOffset
per partition (with KIP) - 2 days.Total : 1.25 weeks.