Cassandra writes-reads-compaction
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Slides from my talk at the NYC Cassandra meetup on 4/29/15. Abstract: You might think that knowledge of how your database performs writes, reads, and other internal operations is arcane knowledge for ops witches, but no! This knowledge can give you tools to dramatically improve the performance of your application. For example, at Venmo, we halved response times on our slowest endpoint by better understanding the implications of our read and write patterns and tuning our Cassandra cluster accordingly. In this talk, we'll clarify what is meant by "in Cassandra, writes are fast, but reads are slow." We'll cover how a node performs writes, reads, and compaction, and we'll share some lessons learned about choosing the right compaction strategy for your query patterns.
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Cassandra writes-reads-compaction
- 1. Cassandra writes, reads, and compaction Amy Hanlon
- 3. “In Cassandra, writes are fast, but reads are slow.”
- 4. DESCRIBE contact_book; +-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐+ | Field | Type | Null | Key | Default | Extra | +-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐+ | user_id | int(11) | NO | PRI | NULL | | | phone_number | varchar(15) | NO | PRI | NULL | | | name | varchar(60) | YES | | NULL | | +-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐+
- 5. SELECT * FROM contact_book; +-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+ | user_id | phone_number | name | +-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+ | 1 | 3453456789 | NULL | | 1 | 6786786789 | Amy Hanlon | +-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+
- 6. UPDATE contact_book SET name='Cassidy Williams' WHERE user_id=1 AND phone_number='3453456789';
- 7. MySQL updates • append to log • locate row(s) • modify index and row(s) in memory • acknowledge • update index and row on disk
- 8. “In Cassandra, writes are fast, but reads are slow.”
- 9. DESCRIBE TABLE contact_book; ! CREATE TABLE contact_book ( user_id int, phone_number text, name text, PRIMARY KEY ((user_id), phone_number) )
- 10. select * from contact_book; ! user_id | phone_number | name -‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ 1 | 3453456789 | null 1 | 6786786789 | Amy Hanlon
- 11. select * from contact_book; ! user_id 1: ! 3453456789:name | 6786786789:name -‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐+-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐-‐ null | Amy Hanlon
- 12. “In Cassandra, writes are fast, but reads are slow.”
- 13. Cassandra updates • append to log • write update to memtable • acknowledge • (eventually) flush memtable to disk into sstable
- 14. INSERT INTO contact_book (user_id, phone_number, name) VALUES (1, '3453456789', 'cassidy'); ! # flush memtable ! UPDATE contact_book SET name='Cassidy Williams' WHERE user_id=1 AND phone_number='3453456789';
- 15. “In Cassandra, writes are fast, but reads are slow.”
- 16. Cassandra reads • read row(s) from sstables • get updates to row(s) from memtable • reduce any multiple values for columns • respond to coordinator
- 17. Compaction!
- 20. Leveled Compaction level 1 level 2
- 22. Faster reads • use the right compaction strategy • row cache • key cache
- 23. Links • http://planetcassandra.org/blog/composite-keys-in-apache-cassandra/ • http://docs.datastax.com/en/cassandra/2.0/cassandra/dml/ dml_write_path_c.html • http://thelastpickle.com/blog/2011/04/28/Forces-of-Write-and-Read.html • http://docs.datastax.com/en/cassandra/2.0/cassandra/dml/ dml_about_reads_c.html • http://docs.datastax.com/en/cassandra/2.0/cassandra/operations/ ops_how_cache_works_c.html • http://www.datastax.com/dev/blog/when-to-use-leveled-compaction • http://www.datastax.com/dev/blog/leveled-compaction-in-apache-cassandra