Hi Ning, Apologies for the delay in response. I realized after publishing the KIP that there were some finer points I hadn't considered in my design and that it was far from providing exactly-once guarantees. In response to your questions:
1) The goal of the KIP is to ensure the accuracy of the offsets that the framework provides to source tasks; if tasks choose to manage offsets outside of the framework, they're on their own. So, the source records and their offsets will be written/committed to Kafka, and the task will be provided them on startup, but it (or really, its predecessor) may not have had time to do cleanup on resources associated with those records before being killed. 2) I've cleaned up this section and removed the pseudocode as it seems too low-level to be worth discussing in a KIP. I'll try to summarize here, though: task.commit() is not what causes offsets provided to the framework by tasks to be committed; it's simply a follow-up hook provided out of convenience to tasks so that they can clean up resources associated with the most recent batch of records (by ack'ing JMS messages, for example). The Connect framework uses an internal Kafka topic to store source task offsets. 3) In order to benefit from the improvements proposed in this KIP, yes, the single source-of-truth should be the OffsetStorageReader provided to the task by the Connect framework, at least at startup. After startup, tasks should ideally bookkeep their own offset progress as each request to read offsets requires a read to the end of the offsets topic, which can be expensive in some cases. I've since expanded the KIP to include general exactly-once support for source connectors that should cover the points I neglected in my initial design, so it should be ready for review again. Cheers, Chris On Mon, Jul 27, 2020 at 11:42 PM Ning Zhang <ning2008w...@gmail.com> wrote: > Hello Chris, > > That is an interesting KIP. I have a couple of questions: > > (1) in section of pseudo-code, what if the failure happens between 4(b) > and 5(a), meaning after the producer commit the transaction, and before > task.commitRecord(). > > (2) in section "source task life time", what is the difference between > "commit offset" and "offsets to commit"? Given that the offset storage can > be a Kafka topic (/KafkaOffsetBackingStore.java) and producer could only > produce to a kafka topic, are / is the topic(s) the same ? (the topic that > producer writes offsets to and the topic task.commit() to) > > (3) for JDBC source task, it relies on `context.offsetStorageReader()` ( > https://github.com/confluentinc/kafka-connect-jdbc/blob/master/src/main/java/io/confluent/connect/jdbc/source/JdbcSourceTask.java#L140) > to retrieve the previously committed offset (if from a fresh start or > resume from failure). so it seems that the single-source-of-truth of where > to consume from last known / committed position stored in offset storage > (e.g. kafka topic) managed by the periodic task.commit()? > > On 2020/05/22 06:20:51, Chris Egerton <chr...@confluent.io> wrote: > > Hi all, > > > > I know it's a busy time with the upcoming 2.6 release and I don't expect > > this to get a lot of traction until that's done, but I've published a KIP > > for allowing atomic commit of offsets and records for source connectors > and > > would appreciate your feedback: > > > https://cwiki.apache.org/confluence/display/KAFKA/KIP-618%3A+Atomic+commit+of+source+connector+records+and+offsets > > > > This feature should make it possible to implement source connectors with > > exactly-once delivery guarantees, and even allow a wide range of existing > > source connectors to provide exactly-once delivery guarantees with no > > changes required. > > > > Cheers, > > > > Chris > > >
