Transactions in Action: the Story of Exactly Once in Apache Kafka
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The document discusses the implementation of exactly-once semantics (EOS) in Apache Kafka, focusing on its transaction model that ensures either all operations complete or none at all. It details the roles of transaction producers, coordinators, brokers, and group coordinators, as well as challenges like idempotency, error handling, and managing hanging transactions. Various Kafka Improvement Proposals (KIPs) are referenced, addressing the evolution and enhancement of transactions, including mechanisms to prevent data loss and improve the reliability of message delivery.
![baseOffset: 123456 lastOffset: 123456 count: 1 baseSequence: -1 lastSequence: -1
producerId: 123456 producerEpoch: 0 partitionLeaderEpoch: 0 isTransactional: true
isControl: true deleteHorizonMs: OptionalLong.empty position: 123456789 CreateTime:
1695839400 size: 78 magic: 2 compresscodec: none crc: 1234567890 isvalid: true
| offset: 123456 CreateTime: 1691433940165 keySize: 4 valueSize: 6 sequence: -1
headerKeys: [] endTxnMarker: COMMIT coordinatorEpoch: 12](https://image.slidesharecdn.com/bs58-20230927-cur23-olshanjustine-adv1final-231026160532-8942e016/85/Transactions-in-Action-the-Story-of-Exactly-Once-in-Apache-Kafka-25-320.jpg)
Transactions in Action: the Story of Exactly Once in Apache Kafka
- 1. Transactions in Action The Story of Exactly Once in Apache Kafka®
- 2. What are transactions? ● All or nothing ● Well-known in databases ● Similar notion in Kafka ○ Read then write using streams ● We expect failures!
- 3. ● At most once semantics ● At least once semantics ● If you can fulfill both, you get exactly once! What are “EXACTLY ONCE” semantics? At most once At least once EOS!
- 4. What is IDEMPOTENCY? An operation can be performed multiple times and will always result in the same outcome. x=20 x++
- 5. Can we have transactions and EOS in Kafka? Enter KIP-98! (https://cwiki.apache.org/confluence/display/KAFKA/KIP-98+-+Exactly+Once+Delivery+an d+Transactional+Messaging) Producer Broker Transaction Coordinator Group Coordinator 1 2 4.1 4.1a 2a 4.2 4.2a 4.3 4.3a 5.1 5.1a 5.3 4.4 4.4a 5.2a 5.2a Tp0-offset- x (PID) Tp0-offset- y (PID) Commit (PID) Txn Id -> PID Insert tp(PID) Prepare (PID) Commit (PID) M0 (PID) M1 (PID) Commit (PID)
- 6. How do transactions work? ● Producer initiates transaction and sends data ○ Add data ○ Commit offsets ○ Abort or commit the transaction ● Transaction Coordinator tracks transactional metadata ○ Stores + persists ongoing transactions, transaction metadata ○ Sends markers to appropriate data partitions ○ Calls off the transaction if it is ongoing too long
- 7. How do transactions work? ● Brokers host data partitions ○ Contains transactional record data ○ Records contain producer metadata ● Group Coordinators host group metadata and offsets ○ Optional transactions participant ○ Supports streams/read + write EOS applications
- 8. How do transactions work? 1. Producer initializes transactions 2. Producer produces data a. Add partition b. Produce data 3. Producer may add offsets 4. Producer requests to abort or commit 5. Transaction coordinator sends markers to partitions
- 9. Producer Broker Transaction Coordinator Group Coordinator 1 2 4.1 4.1a 2a 4.2 4.2a 4.3 4.3a 5.1 5.1a 5.3 4.4 4.4a 5.2a 5.2a tp0-offset-x (PID) tp0-offset-y (PID) Commit (PID) Txn Id -> PID Insert tp(PID) Prepare (PID) Commit (PID) M0 (PID) M1 (PID) Commit (PID)
- 10. What’s in the logs? Transaction Coordinator tp0-offset- x (PID) tp0-offset- y (PID) Commit (PID) Txn Id -> PID Insert tp (PID) Prepare (PID) Commit (PID) M0 (PID) M1 (PID) Commit (PID) Group Coordinator Data Partition
- 11. How to ensure ordering and no duplicates? ● Each message is assigned a sequence number ● Broker keeps track of the last 5 batches to deduplicate ○ If incoming message matches stored sequence, rejected as duplicate ● Not next sequence – OutOfOrderSequence ● Do idempotent producers guarantee idempotency? ○ KIP-98 promises EOS, but there are some scenarios that were missed.
- 12. What happened to KIP-185? ● Can we make idempotency the default? ● OutOfOrderSequence error was too ambiguous ○ real data loss, retriable, or unclear – all seen as fatal! ● KAFKA-5793 tried to address known scenarios ○ OutOfOrderSequence should represent real data loss ○ UnknownProducerId to cover retention errors, trigger epoch bump (https://cwiki.apache.org/confluence/display/KAFKA/Kafka+Exactly+Once+-+Solving+the +problem+of+spurious+OutOfOrderSequence+errors)
- 13. Idempotent Producer problems continue... ● UnknownProducerId error could lead to more confusing scenarios ○ Sequence would be reset in ambiguous cases – was the record written? ○ Still some fatal cases ● Leader failovers can result in OutOfOrderSequence errors
- 14. How does KIP-360 change the EOS producer? ● Safe epoch bumping (KAFKA-8710) ● UnknownProducerId becomes rare and abortable, never fatal ○ Removed from the server code entirely! ○ We accept nonzero sequences in most cases ○ Can get stuck in retriable loop (KAFKA-14359) ● Store producer state for longer (https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=89068820)
- 15. KIP-484 and Slow Loading ● Transaction coordinator partition persists metadata ● Reassign partition or restart → load from disk ● More data on disk → slower loading time (https://cwiki.apache.org/confluence/display/KAFKA/KIP-484%3A+Expose+metrics+for+gro up+and+transaction+metadata+loading+duration)
- 16. KIP-691 and error handling ● Error handling not consistent ● Compatibility vs consistency (https://cwiki.apache.org/confluence/display/KAFKA/KIP-691%3A+Enhance+Transactional+ Producer+Exception+Handling)
- 17. KIP-679, KIP-854, OOM, and you ● KIP-679 established idempotency as the default ● Some users spin up one-time use producers ● Storing too much producer state causes OOM ● Transactional and Producer state use same config ○ Transactional metadata deletion can cause InvalidPidMapping errors ● KIP-854 separated these configs ○ 1 day default
- 18. References to previous KIPs (https://cwiki.apache.org/confluence/display/KAFKA/KIP-679%3A+Producer+will+enable+t he+strongest+delivery+guarantee+by+default) (https://cwiki.apache.org/confluence/display/KAFKA/KIP-854+Separate+configuration+fo r+producer+ID+expiry)
- 19. What are hanging transactions? ● Txn Coordinator sends markers to all added partitions ● What if partition wasn’t added? ● How could it not be added?! ○ Buggy client ○ Race conditions
- 20. Why are hanging transactions an issue? ● Last Stable Offset (LSO) gets stuck ● READ_COMMITTED consumers can’t read past ● Log cleaner can’t clean past, large partitions 0 PID-2 1 PID-2 Commit PID-2 0 PID-7 0 PID-5 2 PID-2 Commit PID-2 Commit PID-5 LSO LSO LSO LSO LSO 3 PID-2
- 21. KIP-890 ● Part 1 eliminates hanging transactions on all clients ○ Extra interbroker hop ○ Can still add records to the wrong transaction ● Part 2 includes new client changes to strengthen EOS ○ Implicitly adds partition, eliminates extra hop ○ Epoch + transaction id will uniquely identify txn (https://cwiki.apache.org/confluence/display/KAFKA/KIP-890%3A+Transactions+Server-Si de+Defense)
- 22. KIP-936 ● Does producer ID expiration cause loss of idempotency? ● Can we throttle the creation of new producer IDs? ● Availability vs correctness ● How can we identify “misbehaving” clients? ○ KIP-936 suggests throttling by user (https://cwiki.apache.org/confluence/display/KAFKA/KIP-936%3A+Throttle+number+of+act ive+PIDs)
- 23. Idempotent Producers V2 ● Clear semantics ○ Ordering ○ No duplicates ● Bounded memory usage ● Contract between client and server ● Use across sessions?
- 24. KIP-939 and the future of 2PC in Kafka ● Outside system (ie. DB) and Kafka need to stay in sync? ● 2 phase commit ○ Prepare, then commit ○ Kafka does this implicitly internally ○ Want to extend to external systems/coordinators ● FLIP-319 for Flink to use Kafka 2PC transactions (https://cwiki.apache.org/confluence/display/KAFKA/KIP-939%3A+Support+Participation+ in+2PC) (https://cwiki.apache.org/confluence/pages/viewpage.action?pageId=255071710)
- 25. baseOffset: 123456 lastOffset: 123456 count: 1 baseSequence: -1 lastSequence: -1 producerId: 123456 producerEpoch: 0 partitionLeaderEpoch: 0 isTransactional: true isControl: true deleteHorizonMs: OptionalLong.empty position: 123456789 CreateTime: 1695839400 size: 78 magic: 2 compresscodec: none crc: 1234567890 isvalid: true | offset: 123456 CreateTime: 1691433940165 keySize: 4 valueSize: 6 sequence: -1 headerKeys: [] endTxnMarker: COMMIT coordinatorEpoch: 12