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How Cassandra Deletes Data (Alain Rodriguez, The Last Pickle) | Cassandra Summit 2016
- 1. Licensed under a Creative Commons Attribution-NonCommercial 3.0 New Zealand License HOW CASSANDRA DELETES DATA Alain Rodriguez
- 2. • Tombstone issues • Why tombstones • Tombstone removal
- 3. Introduction
- 4. About The Last Pickle
- 5. About The Last Pickle and Alain Rodriguez
- 6. About The Last Pickle and Alain Rodriguez
- 7. About deletes in Cassandra Deleted data in Cassandra do not just disappear,
- 8. Deleted data in Cassandra do not just disappear, instead a tombstone is added. About deletes in Cassandra
- 9. Ok so what’s the matter, why this talk ? Tombstone are needed in Cassandra, not an issue…
- 10. Ok so what’s the matter, why this talk ? Tombstone are needed in Cassandra, not an issue… …until an SSTables or a result to a query look like this…
- 11. Then we can see that in the user mailing list or other community tools Ok so what’s the matter, why this talk ?
- 12. Then we can see that in the user mailing list or other community tools So I thought I could share, about this topic. thelastpickle.com/blog/2016/07/27/about-deletes-and-tombstones.html Ok so what’s the matter, why this talk ?
- 13. Tombstone issues
- 14. Tombstone issues: impacts The read path: Reading tombstones induces Latencies, Timeouts or Exceptions
- 15. Tombstone issues: impacts The read path: Reading tombstones induces Latencies, Timeouts or Exceptions The disk space: tombstones can fill up the disk 100%
- 16. Tombstone issues: impacts The read path: Reading tombstones induces Latencies, Timeouts or Exceptions The disk space: tombstones can fill up the disk I am facing one of these issues, is it caused by tombstones? 100%
- 17. Tombstone issues: Read Path grep -i -e "ERROR" -e "WARN" /var/log/cassandra/system.log
- 18. Tombstone issues: Read Path grep -i -e "ERROR" -e "WARN" /var/log/cassandra/system.log WARN [SharedPool-Worker-7] 2016-07-16 16:31:09,048 SliceQueryFilter.java:319 - Read 276 live and 1104 tombstone cells in mykeyspace.mytable for key: ItV9kZC8mFNiSvYM8AwufBU8tTtJkW5dUH5MNcq1H18 (see tombstone_warn_threshold). 500 columns were requested, slices=[-]
- 19. Tombstone issues: Read Path grep -i -e "ERROR" -e "WARN" /var/log/cassandra/system.log WARN [SharedPool-Worker-7] 2016-07-16 16:31:09,048 SliceQueryFilter.java:319 - Read 276 live and 1104 tombstone cells in mykeyspace.mytable for key: ItV9kZC8mFNiSvYM8AwufBU8tTtJkW5dUH5MNcq1H18 (see tombstone_warn_threshold). 500 columns were requested, slices=[-] ERROR [ReadStage:290729] 2016-07-16 17:00:18,708 SliceQueryFilter.java (line 206) Scanned over 100000 tombstones in mykeyspace.mytable; query aborted (see tombstone_failure_threshold) ERROR [ReadStage:290729] 2016-04-22 17:00:18,709 CassandraDaemon.java (line 258) Exception in threadThread[ReadStage:290729,5,main] java.lang.RuntimeException: org.apache.cassandra.db.filter.TombstoneOverwhelmingException
- 20. Tombstone issues: Read Path tombstoneScannedHistogram metric Through nodetool cfstats, JMX…
- 21. Tombstone issues: Read Path tombstoneScannedHistogram metric Through or a plugged monitoring tool such as Datadog, Grafana, SPM, OpsCenter… Commercial Free
- 22. Tombstone issues: Disk space DroppableTombstoneRatio metric provide interesting info.
- 23. Tombstone issues: Disk space DroppableTombstoneRatio metric provide interesting info. Through sstablemetadata tool, JMX and plugged monitoring tool such as Datadog, Grafana, SPM, OpsCenter, etc. Possible to write a script to check biggest SSTables ratio for example
- 24. Why tombstones? I want to remove data !
- 25. WhyTombstones: Cassandra write path Write path Client write Memory Disk Memtable Commit Log SSTable SSTable SSTable SSTable Cassandra node Flush Immutable
- 26. WhyTombstones: Cassandra write path Write path Client write Memory Disk Memtable Commit Log SSTable SSTable SSTable SSTable Cassandra node Immutable Client read Flush
- 27. WhyTombstones: Distributed system Cassandra is a distributed system Distributed deletes are tricky !
- 28. WhyTombstones: Cassandra consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency
- 29. WhyTombstones: Cassandra consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency
- 30. WhyTombstones: Cassandra consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency
- 31. WhyTombstones: Cassandra consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency
- 32. WhyTombstones: Cassandra consistency & availability Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A Down Client write “A” Client read “A” Ack Ack High availability
- 33. WhyTombstones: Distributed deletes Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A
- 34. WhyTombstones: Distributed deletes Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A”
- 35. WhyTombstones: Distributed deletes Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A” WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Ack Ack
- 36. WhyTombstones: Distributed deletes Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A” WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Ack Ack WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Client read “A” Ack Ack Wrong
- 37. WhyTombstones: Distributed deletes Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A” WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Ack Ack WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Client read “empty” Ack Ack Correct
- 38. WhyTombstones: Distributed deletes WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A” WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Ack Ack WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Client read “A” Ack Ack Wrong
- 39. WhyTombstones: Distributed deletes WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A” Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A” WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Ack Ack WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Client read “A” Ack Ack Wrong
- 40. WhyTombstones: Distributed deletes WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A A A Client delete “A” WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Ack Ack Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A” WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Ack Ack WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Client read “A” Ack Ack Wrong
- 41. WhyTombstones: Distributed deletes WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A A A Client delete “A” WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Ack Ack WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Client read “A*” meaning “empty” Ack Ack Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A” WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Ack Ack WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Client read “A” Ack Ack Wrong Correct
- 42. Cool story, but I really want to remove the data ! Tombstone removal!
- 43. When are tombstones removed? When should tombstones be removed? • Once the tombstone is fully replicated • When deleted data has been removed
- 44. When are tombstones removed? When should tombstones be removed? • Once the tombstone is fully replicated • When deleted data has been removed When are tombstones actually removed? • After gc_grace_seconds • During compactions IF all the deleted data and the tombstone itself are involved
- 45. How tombstones are removed: Compaction! Write path Client write Memory Disk Memtable Commit Log SSTable SSTable SSTable SSTable Cassandra node Immutable Client read Flush
- 46. How tombstones are removed: Compaction! Write path Client write Memory Disk Memtable Commit Log SSTable SSTable SSTable SSTable Cassandra node Immutable Client read Compacting 4 SSTables Flush
- 47. How tombstones are removed: Compaction! Write path Client write Memory Disk Memtable Commit Log SSTable Cassandra node Immutable Client read Flush
- 48. Implications in the real world • No compaction = no eviction • + TTLs or deletes, tombstone stack (up to 100%)
- 49. Implications in the real world • No compaction = no eviction • + TTLs or deletes, tombstone stack (up to 100%) • Overlapping SSTable = no eviction • Fragmented data = eviction unlikely • LCS: tombstone level ≠ than data level = no eviction
- 50. Implications in the real world • No compaction = no eviction • + TTLs or deletes, tombstone stack (up to 100%) • Overlapping SSTable = no eviction • Fragmented data = eviction unlikely • LCS: tombstone level ≠ than data level = no eviction • TTL << gc_grace_seconds = high % of useless data
- 51. Some tuning ! Good news: Cassandra community and Committers are Awesome!
- 52. Some tuning ! Issue: No compaction = No eviction CASSANDRA-3442: tombstone_threshold (C* 1.2.b1) Compaction option, default: tombstone_threshold = 0.2 (ratio = 20% has been deleted) Single SSTable compaction triggered based on an estimate! Low risk: worst case —> No-op
- 53. Some tuning ! Issue: Tombstone compaction loop! CASSANDRA-4022: Check for key overlaps (C* 1.2.b1) Internals improvement, not an option: Estimated droppable tombstone improved Now considering key overlapping with other SSTable
- 54. Some tuning ! Issue: Tombstone compaction loop! CASSANDRA-4781: tombstone_compaction_interval (C* 1.2.b2) Compaction option, default: tombstone_compaction_interval = 86400 (in seconds = 1 day) Definitely prevents loops
- 55. Some tuning ! Issue: Compacting to remove tombstone is expensive CASSANDRA-5228: Expired SSTables (C*2.0.b1) Internals improvement, not an option Effective with Time series, DTCS / TWCS and TTLs !
- 56. Some tuning ! Issue: Tombstone compactions not triggering CASSANDRA-6563: unchecked_tombstone_compaction (C* 2.0.9) Compaction option, default: unchecked_tombstone_compaction = false CASSANDRA-4022 becomes an option
- 57. Some tuning ! Issue: Overlapping preventing efficient tombstone compactions CASSANDRA-7019: provide_overlapping_tombstones (C* 3.10) Compaction option, default: provide_overlapping_tombstones = NONE (CELL / ROW / NONE) Risky: • Not yet released, so not really tested • Heavier tombstones compactions
- 58. Some tuning -Tombstone distribution ! WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A A A Client delete “A” WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Ack Ack WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Client read “A*” meaning “empty” Ack Ack Correct Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A” WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Ack Ack Tombstones not replicated Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Client read “A*” Ack Ack Correct
- 59. Some tuning -Tombstone distribution ! Case were node fail + no repair = Case without tombstone WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A A A Client delete “A” WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Ack Ack WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Client read “A*” meaning “empty” Ack Ack Correct Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A” WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Ack Ack Tombstones not replicated Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* AClient read “A” Wrong A* removed
- 60. Some tuning -Tombstone distribution ! Case were node fail + no repair = Case without tombstone = Zombie data ! WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A A A Client delete “A” WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Ack Ack WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Client read “A*” meaning “empty” Ack Ack Correct Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 ? ? ? Client write “A” Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Ack Ack Strong Consistency Consistency Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A ? Client write “A” Client read “A ” Ack Ack Strong Consistency WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A Client delete “A” WITHOUT Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A Client delete “A” Ack Ack Tombstones not replicated Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A AClient read “A” Wrong A* removed
- 61. Some tuning -Tombstone distribution ! CASSANDRA-6434 (C*3.0.b1): only_purge_repaired_tombstones (Default: False) WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A A A Client delete “A” WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Ack Ack Tombstones not replicated Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A A* not removed Client read “A*” meaning “empty” Correct
- 62. Some tuning -Tombstone distribution ! CASSANDRA-6434 (C*3.0.b1): only_purge_repaired_tombstones (Default: False) Limitation Repair failing or no repair = permanent tombstone WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A A A WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A A A Client delete “A” WITH Tombstones Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A Client delete “A” Ack Ack Tombstones not replicated Cassandra Cluster 4 nodes RF = 3 Write CL = Quorum = 2 Read CL = Quorum = 2 A* =Tombstone on A A* A* A A* not removed Client read “A*” meaning “empty” Correct
- 63. Conclusion
- 64. Things we know about tombstones • Tombstones due to deletes and TTLs • Tombstone fits with Cassandra write path • Tombstones ensure consistency • Reading tombstones is expensive and can produce failures • Tombstones take space on disk and might be tricky to remove • Tombstones need to be distributed before being removed
- 65. Takeaways • Model data and workflow to avoid to reading many tombstones • Deleted data = repair table within gc_grace_seconds • Monitor tombstones, keep control! (Set some alerts ?) • Use compaction options to tackle problems, there is always a way. • Is there no way? Ask, or create a Jira and keep improving Cassandra!
- 66. Thank you Questions ? thelastpickle.com/blog/2016/07/27/about-deletes-and-tombstones.html