Playlists at Spotify - Using Cassandra to store version controlled objects
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Slides from my talk at the Cassandra Summit Europe 2013 in London.
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Playlists at Spotify - Using Cassandra to store version controlled objects
- 1. Playlists at Spotify Using Cassandra to store version controlled objects at large scale Jimmy Mårdell <yarin@spotify.com> #CassandraEU October 18, 2013
- 2. Intro About me • Jimmy Mårdell • Software Engineer • 3 years at Spotify #CassandraEU 2
- 3. Intro About Spotify • 24 million active users – 6 million paying subscribers • 4 000 servers in 4 data centers • Over 1 billion playlists created #CassandraEU 3
- 4. #CassandraEU Intro Contents •Why version control? •Playlists at Spotify •Cassandra data model •Lessons learned 4
- 5. Why version control? #CassandraEU What is version control? • “Version control is the management of changes to documents” (Wikipedia) • Stand-alone (most common) – GIT, Subversion etc • Embedded – Google Docs 5
- 6. Why version control? Embedded usage • Collaborative editing • Undo functionality • Performance • Business logic depends on document history #CassandraEU 6
- 9. Playlists at Spotify Playlist challenges • More than 1 billion playlists • >40 000 requests/second at peak • Offline mode • Concurrent changes #CassandraEU 9
- 10. Playlists at Spotify Playlist client-server • Every playlist is a version controlled object • All playlists are synced on login – Fetch all new changes #CassandraEU 10
- 11. Playlists at Spotify Playlist client-server • Local queue of playlist modifications – Clients optimistically accept changes - fast UI • Queue flushed to server when possible – Offline changes – Fault tolerant #CassandraEU 11
- 12. #CassandraEU Playlists at Spotify 12 Playlist version control 3,038f...: REM(from=2, len=1) A C 2,19ca...: MOV(from=2, to=1, len=1) A C B 1,4ed2...: ADD(ix=0, track=A,B,C) A B C 0,ROOT Representation of a playlist in the backend
- 13. #CassandraEU Playlists at Spotify Playlist branching • Concurrent changes – Offline A B 13
- 14. #CassandraEU Playlists at Spotify Playlist branching merge • Concurrent changes – Offline • Conflict resolution – Operational Transformation • Clients oblivious of branches B’ A’ A B 14
- 15. Cassandra data model Cassandra data model #CassandraEU 15
- 16. Cassandra data model Cassandra at Spotify • Playlist first system to use Cassandra – Now we use it a lot... • Started with Cassandra 0.7 • Using limited set of Cassandra features – No super columns – No CQL #CassandraEU 16
- 17. Cassandra data model Planning a data model • Start with the queries! • Three common playlist queries – SYNC: Get all changes since a particular revision – GET: Get the most recent snapshot – APPEND: Add/move/delete tracks #CassandraEU 17
- 18. #CassandraEU Cassandra data model Playlist data model CF playlist_change Row key spotify:user:spotify:playlist: 3ZgmfR6lsnCwdffZUan8EA 1,4ed2... parent=0,ROOT op=ADD(ix=0, track=A,B,C) 2,19ca... parent=1,4ed2... op=MOV(from=2, to=1, len=1) 3,038f... parent=2,19ca op=REM(from=2, len=1) 18
- 19. #CassandraEU Cassandra data model 19 Playlist data model CF playlist_change Row key spotify:user:spotify:playlist: 3ZgmfR6lsnCwdffZUan8EA Row key 1,4ed2... 2,19ca... parent=0,ROOT op=ADD(ix=0, track=A,B,C) parent=1,4ed2... op=MOV(from=2, to=1, len=1) 1,8a20... 2,dd07... spotify:user:yarin:playlist: prnt=0,ROOT 4Pj4dCOEEYWDixfYyJwxEf op=... 2,b783... prnt=1,8a20... op=... prnt=1,8a20... op=... 3,39ef... 3,038f... parent=2,19ca op=REM(from=2, len=1) 3,5a9c... prnt=2,dd07... prnt=2,b783... op=... op=... 4,03fc... prnt=2,39ef... prnt=3,5a9c...
- 20. Cassandra data model Playlists in Cassandra • Which revision is the latest? – Changes with no children • Multiple heads possible! – Heads may appear anywhere within the row #CassandraEU 20
- 21. #CassandraEU Cassandra data model Playlist data model CF playlist_change Row key spotify:user:spotify:playlist: 3ZgmfR6lsnCwdffZUan8EA 1,4ed2... prnt=0,ROOT op=... CF playlist_head 2,19ca... prnt=1,4ed2... op=... 3,038f... prnt=2,19ca op=... Row key spotify:user:spotify:playlist: 3ZgmfR6lsnCwdffZUan8EA 3,038f... 21
- 22. #CassandraEU Cassandra data model 22 Playlist data model CF playlist_change Row key spotify:user:spotify:playlist: 3ZgmfR6lsnCwdffZUan8EA Row key spotify:user:yarin:playlist: 4Pj4dCOEEYWDixfYyJwxEf 1,4ed2... prnt=0,ROOT op=... 1,8a20... prnt=0,ROOT op=... CF playlist_head 2,19ca... prnt=1,4ed2... op=... 2,b783... prnt=1,8a20... op=... 3,038f... prnt=2,19ca op=... 2,dd07... prnt=1,8a20... op=... Row key 3,038f... spotify:user:spotify:playlist: 3ZgmfR6lsnCwdffZUan8EA Row key spotify:user:yarin:playlist: 4Pj4dCOEEYWDixfYyJwxEf 2,b783... 2,dd07...
- 23. #CassandraEU Cassandra data model Playlist data model CF playlist_change Row key spotify:user:spotify:playlist: 3ZgmfR6lsnCwdffZUan8EA Row key CF playlist_head 1,4ed2... prnt=0,ROOT op=... 2,19ca... prnt=1,4ed2... op=... 3,038f... prnt=2,19ca op=... 1,8a20. 2,b783. 2,dd07. 3,39ef. 3,5a9c. 4,03fc. spotify:user:yarin:p laylist:4Pj4dCOEE prt=0,ROOT YWDixfYyJwxEf op=... prnt=1,8a20 op=... prnt=1,8a20 op=... prnt=2,dd07 op=... prnt=2,b783 op=... prnt=2,39ef prnt=3,5a9c Row key 3,038f... spotify:user:spotify:playlist: 3ZgmfR6lsnCwdffZUan8EA Row key spotify:user:yarin:playlist: 4Pj4dCOEEYWDixfYyJwxEf 4,03fc... 23
- 24. Cassandra data model Playlist heads • playlist_head is a small CF – Fits in RAM • 95% of playlist request only read from playlist_head – Most playlists are already up-to-date #CassandraEU 24
- 25. Cassandra data model Playlist snapshots • playlist_change works well when syncing playlists • Not so well for fetching new playlists – Snapshot cache #CassandraEU 25
- 26. #CassandraEU Cassandra data model Playlist data model CF playlist_change Row key spotify:user:spotify:playlist: 3ZgmfR6lsnCwdffZUan8EA Row key spotify:user:yarin:playlist: 4Pj4dCOEEYWDixfYyJwxEf 1,4ed2... prnt=0,ROOT op=... 1,8a20... prnt=0,ROOT op=... CF playlist_snapshot 2,19ca... prnt=1,4ed2... op=... 2,b783... prnt=1,8a20... op=... 3,038f... prnt=2,19ca op=... 2,dd07... prnt=1,8a20... op=... Row key spotify:user:spotify:playlist: 3ZgmfR6lsnCwdffZUan8EA cache version=3,038f... contents=A,C Row key cache spotify:user:yarin:playlist: 4Pj4dCOEEYWDixfYyJwxEf version=2,b783... contents=... 26
- 27. Cassandra data model Updating playlists • Validate change – Locate snapshot – Client may append to old version • Update all tables – playlist_head last #CassandraEU 27
- 28. Cassandra data model Cassandra consistency levels • Replication factor 3 • All writes using CL_QUORUM • Reads from playlist_head – CL_QUORUM • Reads from playlist_change and playlist_snapshot – CL_ONE but may fallback to CL_QUORUM #CassandraEU 28
- 30. Lessons learned Optimizations • Leveled compaction – Improved performance a lot • Compression – Not as impressive – CRC checks #CassandraEU 30
- 31. Lessons learned Optimizations • Trusted Linux page cache to ensure playlist_head kept in RAM – Didn’t work • Tried Cassandra row cache – NO! • mlock to the rescue #CassandraEU 31
- 32. Lessons learned #CassandraEU An enterprise ready solution bash# while true; do vmtouch -m 10000000000 -l *head* & sleep 10m kill %vmtouch done 32
- 33. Lessons learned No moving parts • Flash disks are awesome • Reduced size of cluster from 60 to 30 nodes – Thanks FusionIO! • IOPS no longer the bottleneck #CassandraEU 33
- 34. Lessons learned Tombstone hell • Noticed requests to playlist_head took several seconds – Huh? • Every change causes a value to be deleted in playlist_head • playlist_head is essentially a queue – Well-known anti-pattern #CassandraEU 34
- 35. Lessons learned Tombstone hell • We had rows with >500,000 tombstones • Solution: major compaction – Relatively fast since playlist_head is in RAM #CassandraEU 35
- 36. Lessons learned And more... • Large rows in playlist_change – Modify version graph • Reduce amount of requests – Group playlists by owner Sounds interesting? We’re hiring! #CassandraEU 36
- 37. Questions?