Uber uses a scalable real-time complex event processing system to analyze streaming data from its services. The system uses Apache Samza for distributed stream processing and WSO2 Siddhi for complex event processing. Key events are detected using Siddhi queries and then actions like notifications or indexing to databases are triggered. The system processes over 30 billion messages per day across many use cases. Maintaining scalability, fault tolerance, and low latency are ongoing challenges.

1. Scalable Real-Time Complex Event Processing @Uber
Shuyi Chen
Uber Technology Inc.

2. 6 continents, 70+ countries and 400+ cities
Transportation as reliable as running water, everywhere,
for everyone
Uber

3. Outline
● Motivation
● Architecture
● Limitations
● Challenges

4. Outline
● Motivation
● Architecture
● Limitations
● Challenges

5. Uber is a data-driven company

6. Thousands of Kafka topics from micro-services

7. We can extract a lot of useful information from this
rich set of logs in real-time!

8. Multiple logins from the same IP in the last 10
minutes

9. Partner accepted a trip
→ partner calls rider through the Uber APP
→ rider cancels the trip

10. Partners reject the second pickup of a UberPOOL
trip

11. Multiple logins from the same IP in the last 10
minutes
Window Aggregation

12. Partner accepted a trip
→ partner calls rider through the Uber APP
→ rider cancels the trip
Pattern detection

13. Partners reject the second pickup of a UberPOOL
trip
Filter

14. Can we use declarative languages to specify these
stream processing logics?

15. Complex event processing
● Combines data from multiple sources to infer events or patterns that suggest
more complicated circumstances
● CEP is used across many industries for various use cases, including:
○ Finance: Trade analysis, fraud detection
○ Airlines: Operations monitoring
○ Healthcare: Claims processing, patient monitoring
○ Energy and Telecommunications: Outage detection
● CEP uses declarative rule/query language to specify event processing logic

16. WSO2/Siddhi: Complex event processing engine
● Lightweight, extensible, open source, released as a Java library
● Features supported
○ Filter
○ Join
○ Aggregation
○ Group by
○ Window
○ Pattern processing
○ Sequence processing
○ Event tables
○ Event-time processing
○ UDF
○ Extensions
○ Declarative query language: SiddhiQL

17. How Siddhi works
● Specify processing logic declaratively with SiddhiQL

18. How Siddhi works
● Query is parsed at runtime into an execution plan runtime
● As events flow in, the execution plan runtime process events inside the CEP
engine according the query logic

19. How can we make it scalable at Uber scale?

20. Apache Samza
● A distributed stream processing framework
○ Distributed and Scalable
○ Built-in State management
○ Built-in fault tolerant
○ At-least-once message processing
○ Infrastructure support at Uber

21. How can we make the stream processing output
useful?

22. Actions
● Generalize a set of common action templates to make it easy for
micro-services and human to harness the power of realtime stream
processing
● Currently we support
○ Make an RPC call
○ Invoke a Webhook endpoint
○ Index to ElasticSearch
○ Index to Cassandra
○ Kafka
○ Statsd
○ Chat service
○ Email
○ Push notification

23. Actions
Real-time Scalable Complex Event Processing

24. Outline
● Motivation
● Architecture
● Limitations
● Challenges

27. Partitioner
● Re-shuffle events based on key
● Support predicate pushdown through query analysis
● Support column pruning through query analysis (WIP)

28. Query processor
● Parse Siddhi queries into execution plan runtime
● Process events in Siddhi execution plan runtime
● Checkpoint state regularly to ensure recovery upon crash/restart using
RocksDB

29. Action processor
● Execute actions upon the query processing output
● Support various kinds of actions for easy integration
● Implement action retry mechanism using RocksDB to provide at-least-once
delivery

30. How do we translate a query into psychical plan that
runs?

31. DAG (Directed Acyclic Graph) generation
● Analyze Siddhi query to automatically generate the stream processing DAG in
Samza using the processors
Filter, transformation

32. Join, window, pattern

33. More complicated

34. No stream processing logic is hard-coded in any of
the processors

35. REST API backend
● All queries, actions are stored externally in database.
● RESTFUL API for CRUD operations
● If query/action logic changed
○ Redeploy the Samza DAG if needed
○ Otherwise, the updated queries/actions will be loaded at runtime w/o interruption

36. Unified management and monitoring
● Every use case
○ share the same set of processors
○ Use queries and actions to describe its processing logic
● A single monitoring template can be reused across different use cases

37. Production status
● In production for >1.5 years
● 120+ production use cases
● 30+ billion messages processed per day

38. Applications
● Real-time fraud detection
● Real-time anomaly detection
● Real-time marketing campaign
● Real-time promotion
● Real-time monitoring
● Real-time feedback system
● Real-time analytics
● Real-time visualizations
● And etc.

39. Outline
● Motivation
● Architecture
● Limitations
● Challenges

40. Out-of-order event handling
● Not a big concern
○ Events of the same rider/partner are usually seconds aparts
● K-slack extension in Siddhi for out-of-order event processing

41. Auto-scaling
● Manually re-partition kafka topics to increase parallelism
● Manually tune container memory if needed
● Future
○ Use CPU/memory/IO stats to automate the process

42. Outline
● Motivation
● Architecture
● Limitations
● Challenges

43. Large checkpointing state
● Samza use Kafka to log state changes
● Siddhi engine snapshot can be large
● Kafka message size limit to 1MB by default
● Solution: we build logics to slice state into smaller pieces and checkpoint
them.

44. Synchronous checkpointing
● Samza checkpointing is synchronous with message processing
● If state is large, time to checkpoint can be long, might cause processing lag
● Incremental state checkpointing

45. Exactly once state processing?
● Can not commit state and offset atomically
● No exactly once state processing

46. Custom business logic
● Common logic implemented as Siddhi extensions
● Ad-hoc logic implemented as UDF in javascript or scalascript inline with the
query

47. Intermediate Kafka messages
● Samza uses Kafka as message queue for intermediate processing output
○ Each stage is independent of each other
○ This can create large load on Kafka if a heave topic is re-shuffled multiple times
■ Encode the intermediate messages to reduce footprint

48. Upgrading Samza jobs
● Upgrade Samza jobs require a full restart, and can take minutes due to
○ Offset checkpointing topic too large → set retention to hours or enable compaction
○ Changelog topic too large → set retention or enable compaction in Kafka or host affinity
● To minimize the interruption during upgrade, it would be nice to have
○ Rolling restart
○ Per container restart

49. Our solution: non-interrupted handoff
● For critical jobs, we use replication during upgrade
○ Start a shadow job
○ Upgrade shadow
○ Switch primary and shadow
○ Upgrade primary
○ Switch back
● Downside: require 2x capacity during upgrade

50. Thank You!