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Apache Apex Fault Tolerance and Processing Semantics


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Components of an Apex application running on YARN, how they are made fault tolerant, how checkpointing works, recovery from failures, incremental recovery, processing guarantees.

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Apache Apex Fault Tolerance and Processing Semantics

  1. 1. Apache Apex Fault Tolerance and Processing Semantics San Francisco Apache Apex Meetup April 6th 2016 Thomas Weise, Apache Apex PPMC @thweise
  2. 2. Stream Processing • Paradigm shift in big data processing • Data from a variety of sources (Kafka, files, social media etc.) • Unbounded data sources • A batch can be processed as stream (but a stream is not a batch) • Processing with temporal boundaries (windows) • Results stored to a variety of sinks or destinations ᵒ Streaming application can also serve data with very low latency 2 Browser Web Server Kafka Input (logs) Decompress, Parse, Filter Dimensions Aggregate Kafka Logs Kafka
  3. 3. Apache Apex Features • In-memory Stream Processing • Scale out, Distributed, Parallel, High Throughput • Windowing (temporal boundary) • Reliability, Fault Tolerance • Operability • YARN native • Compute Locality • Dynamic updates 3
  4. 4. Apex Stack Overview 4
  5. 5. Native Hadoop Integration 5 • YARN is the resource manager • HDFS used for storing any persistent state
  6. 6. Recovery Demo 6
  7. 7. Streaming Windows 7  Application window  Sliding window and tumbling window  Checkpoint window  No artificial latency
  8. 8. Fault Tolerance 8 • 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
  9. 9. Checkpointing Operator State 9 • 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
  10. 10. • 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 10
  11. 11. Application Master State 11 • 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
  12. 12. • 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 12
  13. 13. NM NM Resource Manager Apex AM 3 2 1 Apex AM 1 2 3 NM Failure Scenarios NM 13
  14. 14. 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 14
  15. 15. Processing Guarantees 15 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
  16. 16. End-to-End Exactly Once 16 • 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
  17. 17. Files 17 • 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
  18. 18. Exactly Once Strategy 18 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
  19. 19. Transactional databases 19 • 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
  20. 20. Exactly Once Strategy 20 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
  21. 21. Stateful Message Queue 21 • 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
  22. 22. Resources 22 • Exactly-once processing: exactly-once-with-apache-apex/ • Examples with Kafka and Files: samples/tree/master/exactly-once • Learn more: • Subscribe - • Download - • Apex website - • Follow @ApacheApex - • Meetups -
  23. 23. Q&A 23