Apache Apex Fault Tolerance and Processing Semantics
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The document discusses Apache Apex's fault tolerance and processing semantics, highlighting features such as in-memory stream processing, stateful reliability, and automatic recovery mechanisms. It details various strategies for achieving at-least-once, at-most-once, and exactly-once processing guarantees, along with implementation examples for different output operators and their handling of failure scenarios. Additionally, it outlines integration with Hadoop and the importance of checkpointing to maintain data integrity and consistency during processing.
Apache Apex Fault Tolerance and Processing Semantics
- 1. Apache Apex (incubating) Fault Tolerance and Processing Semantics Thomas Weise, Architect & Co-founder, PPMC member Pramod Immaneni, Architect, PPMC member March 24th 2016
- 2. Apache Apex Features • In-memory Stream Processing • Partitioning and Scaling out • Windowing (temporal boundary) • Reliability ᵒ Stateful ᵒ Automatic Recovery ᵒ Processing Guarantees • Operability • Compute Locality • Dynamic updates 2
- 4. Native Hadoop Integration 4 • YARN is the resource manager • HDFS used for storing any persistent state
- 5. Streaming Windows 5 Application window Sliding window and tumbling window Checkpoint window No artificial latency
- 6. Fault Tolerance 6 • Operator state is checkpointed to persistent store ᵒ Automatically performed by engine, no additional coding needed ᵒ Asynchronous and distributed ᵒ In case of failure operators are restarted from checkpoint state • Automatic detection and recovery of failed containers ᵒ Heartbeat mechanism ᵒ YARN process status notification • Buffering to enable replay of data from recovered point ᵒ Fast, incremental recovery, spike handling • Application master state checkpointed ᵒ Snapshot of physical (and logical) plan ᵒ Execution layer change log
- 7. Checkpointing Operator State 7 • Save state of operator so that it can be recovered on failure • Pluggable storage handler • Default implementation ᵒ Serialization with Kryo ᵒ All non-transient fields serialized ᵒ Serialized state written to HDFS ᵒ Writes asynchronous, non-blocking • Possible to implement custom handlers for alternative approach to extract state or different storage backend (such as IMDG) • For operators that rely on previous state for computation ᵒ Operators can be marked @Stateless to skip checkpointing • Checkpoint frequency tunable (by default 30s) ᵒ Based on streaming windows for consistent state
- 8. • In-memory PubSub • Stores results emitted by operator until committed • Handles backpressure / spillover to local disk • Ordering, idempotency Operator 1 Container 1 Buffer Server Node 1 Operator 2 Container 2 Node 2 Buffer Server 8
- 9. Application Master State 9 • Snapshot state on plan change ᵒ Serialize Physical Plan (includes logical plan) ᵒ Infrequent, expensive operation • WAL (Write-ahead-Log) for state changes ᵒ Execution layer changes ᵒ Container, operator state, property changes • Containers locate master through DFS ᵒ AM can fail and restart, other containers need to find it ᵒ Work preserving restart • Recovery ᵒ YARN restarts application master ᵒ Apex restores state from snapshot and replays log
- 10. • Container process fails • NM detects • In case of AM (Apex Application Master), YARN launches replacement container (for attempt count < max) • Node Manager Process fails • RM detects NM failure and notifies AM • Machine fails • RM detects NM/AM failure and recovers or notifies AM • RM fails - RM HA option • Entire YARN cluster down – stateful restart of Apex application Failure Scenarios 10
- 11. NM NM Resource Manager Apex AM 3 2 1 Apex AM 1 2 3 NM Failure Scenarios NM 11
- 12. Failure Scenarios … EW2, 1, 3, BW2, EW1, 4, 2, 1, BW1 sum 0 … EW2, 1, 3, BW2, EW1, 4, 2, 1, BW1 sum 7 … EW2, 1, 3, BW2, EW1, 4, 2, 1, BW1 sum 10 … EW2, 1, 3, BW2, EW1, 4, 2, 1, BW1 sum 7 12
- 13. Processing Guarantees 13 At-least-once • On recovery data will be replayed from a previous checkpoint ᵒ No messages lost ᵒ Default, suitable for most applications • Can be used to ensure data is written once to store ᵒ Transactions with meta information, Rewinding output, Feedback from external entity, Idempotent operations At-most-once • On recovery the latest data is made available to operator ᵒ Useful in use cases where some data loss is acceptable and latest data is sufficient Exactly-once ᵒ At-least-once + idempotency + transactional mechanisms (operator logic) to achieve end-to-end exactly once behavior
- 14. End-to-End Exactly Once 14 • Becomes important when writing to external systems • Data should not be duplicated or lost in the external system even in case of application failures • Common external systems ᵒ Databases ᵒ Files ᵒ Message queues • Platform support for at least once is a must so that no data is lost • Data duplication must still be avoided when data is replayed from checkpoint ᵒ Operators implement the logic dependent on the external system • Aid of platform features such as stateful checkpointing and windowing • Three different mechanisms with implementations explained in next slides
- 15. Files 15 • Streaming data is being written to file on a continuous basis • Failure at a random point results in file with an unknown amount of data • Operator works with platform to ensure exactly once ᵒ Platform responsibility • Restores state and restarts operator from an earlier checkpoint • Platform replays data from the exact point after checkpoint ᵒ Operator responsibility • Replayed data doesn’t get duplicated in the file • Accomplishes by keeping track of file offset as state ᵒ Details in next slide • Implemented in operator AbstractFileOutputOperator in apache/incubator- apex-malhar github repository available here • Example application AtomicFileOutputApp available here
- 16. Exactly Once Strategy 16 File Data Offset • Operator saves file offset during checkpoint • File contents are flushed before checkpoint to ensure there is no pending data in buffer • On recovery platform restores the file offset value from checkpoint • Operator truncates the file to the offset • Starts writing data again • Ensures no data is duplicated or lost Chk
- 17. Transactional databases 17 • Use of streaming windows • For exactly once in failure scenarios ᵒ Operator uses transactions ᵒ Stores window id in a separate table in the database ᵒ Details in next slide • Implemented in operator AbstractJdbcTransactionableOutputOperator in apache/incubator-apex-malhar github repository available here • Example application streaming data in from kafka and writing to a JDBC database is available here
- 18. Exactly Once Strategy 18 d11 d12 d13 d21 d22 d23 lwn1 lwn2 lwn3 op-id wn chk wn wn+1 Lwn+11 Lwn+12 Lwn+13 op-id wn+1 Data Table Meta Table • Data in a window is written out in a single transaction • Window id is also written to a meta table as part of the same transaction • Operator reads the window id from meta table on recovery • Ignores data for windows less than the recovered window id and writes new data • Partial window data before failure will not appear in data table as transaction was not committed • Assumes idempotency for replay
- 19. Stateful Message Queue 19 • Data is being sent to a stateful message queue like Apache Kafka • On failure data already sent to message queue should not be re-sent • Exactly once strategy ᵒ Sends a key along with data that is monotonically increasing ᵒ On recovery operator asks the message queue for the last sent message • Gets the recovery key from the message ᵒ Ignores all replayed data with key that is less than or equal to the recovered key ᵒ If the key is not monotonically increasing then data can be sorted on the key at the end of the window and sent to message queue • Implemented in operator AbstractExactlyOnceKafkaOutputOperator in apache/incubator-apex-malhar github repository available here
- 20. Resources 20 • Subscribe - http://apex.incubator.apache.org/community.html • Download - http://apex.incubator.apache.org/downloads.html • Apex website - http://apex.incubator.apache.org/ • Twitter - @ApacheApex; Follow - https://twitter.com/apacheapex • Facebook - https://www.facebook.com/ApacheApex/ • Meetup - http://www.meetup.com/topics/apache-apex
- 21. Q&A 21