This document provides an overview of Cassandra, a decentralized structured storage system. It describes Cassandra as a distributed system that manages structured data with high availability and no single point of failure. The document outlines Cassandra's data model, system architecture including partitioning, replication and failure detection, local data persistence, and performance characteristics. It also provides examples of companies using Cassandra at scale.

1. Cassandra
A Decentralized Structured Storage System
By Sameera Nelson

2. Outline …
 Introduction
 Data Model
 System Architecture
 Failure Detection & Recovery
 Local Persistence
 Performance
 Statistics

3. What is Cassandra ?
 Distributed Storage System
 Manages Structured Data
 Highly available , No SPoF
 Not a Relational Data Model
 Handle high write throughput
◦ No impact on read efficiency

4. Motivation
 Operational Requirements in Facebook
◦ Performance
◦ Reliability/ Dealing with Failures
◦ Efficiency
◦ Continues Growth
 Application
◦ Inbox Search Problem, Facebook

5. Similar Work
 Google File System
◦ Distributed FS, Single master/Slave
 Ficus/ Coda
◦ Distributed FS
 Farsite
◦ Distributed FS, No centralized server
 Bayou
◦ Distributed Relational DB System
 Dynamo
◦ Distributed Storage system

6. Data Model

7. Data Model
Figure from Eben Hewitt’s slides.

8. Supported Operations
 insert(table; key; rowMutation)
 get(table; key; columnName)
 delete(table; key; columnName)

9. Query Language
CREATE TABLE users
( user_id int PRIMARY KEY,
fname text,
lname text );
INSERT INTO users
(user_id, fname, lname) VALUES
(1745, 'john', 'smith');
SELECT * FROM users;

10. Data Structure
 Log-Structured Merge Tree

11. System
Architecture

12. Architecture

13. Fully Distributed …
 No Single Point of Failure

14. Cassandra Architecture
 Partitioning
 Data distribution across nodes
 Replication
 Data duplication across nodes
 Cluster Membership
 Node management in cluster
 adding/ deleting

15. Partitioning
 The Token Ring

16. Partitioning
 Partitions using Consistent hashing

17. Partitioning
 Assignment in to the relevant partition

18. Partitioning, Vnodes

19. Replication
 Based on configured replication factor

20. Replication
 Different Replication Policies
◦ Rack Unaware
◦ Rack Aware
◦ Data center Aware

21. Cluster Membership
 Based on scuttlebutt
 Efficient Gossip based mechanism
 Inspired for real life rumor spreading.
 Anti Entropy protocol
◦ Repair replicated data by comparing &
reconciling differences

22. Cluster Membership
Gossip Based

23. Failure Detection
& Recovery

24. Failure Detection
 Track state
◦ Directly, Indirectly
 Accrual Detection mechanism
 Permanent Node change
◦ Admin should explicitly add or remove
 Hints
◦ Data to be replayed in replication
◦ Saved in system.hints table

25. Accrual Failure Detector
• Node is faulty, suspicion level
monotonically increases.
• Φ(t)  k
• k - threshold variable
• Node is correct
• Φ(t) = 0

26. Local
Persistence

27. Write Request

28. Write Operation

29. Write Operation
 Logging data in commit log/ memtable
 Flushing data from the memtable
◦ Flushing data on threshold
 Storing data on disk in SSTables
 Mark with tombstone
 Compaction
 Remove deletes, Sorts, Merges data, consolidation

30. Write Operation
 Compaction

31. Read Request
 Direct/ Background (Read repair)

32. Read Operation

33. Delete Operation
 Data not removed immediately
 Only Tombstone is written
 Deleted in Compacting Process

34. Additional Features
 Adding compression
 Snappy Compression
 Secondary index support
 SSL support
◦ Client/ Node
◦ Node/ Node
 Rolling commit logs
 SSTable data file merging

35. Performance

36. Performance
 High Throughput & Low Latency
◦ Eliminating on-disk data modification
◦ Eliminate erase-block cycles
◦ No Locking for concurrency control
◦ Maintaining integrity not required
 High Availability
 Linear Scalability
 Fault Tolerant

37. Statistics

38. Stats from Netflix
 Liner scalability

39. Stats from Netflix

40. Some users

41. Thank you

42. Read Detailed Structure