1. Codebase 2011 Getting to know the codebase Gary Dusbabek @gdusbabek

2. Questions?

3. Outline How to contribute Internals Some thoughts

4. How to Contribute

5. How to Contribute http://wiki.apache.org/cassandra/HowToContribute JIRA: “lhf” label (Low hanging fruit) Scratch your itch

6. How to Contribute Run the tests ant test nosetests test/system/test_thrift_server.py

7. How to Contribute http://wiki.apache.org/cassandra/CodeStyle Avoid: Reformatting white space Renaming things everywhere Unrelated changes

8. How to Contribute Use git Attach patches git format-patch as jira attachments. Group them sensibly

9. How to Contribute Someone will review your code Usually a committer Persistence helps Don’t get your feelings hurt It usually takes a few rounds

10. How to Contribute Participate! #cassandra-dev on freenode dev@cassandra.apache.org

11. Internals

12. Services Ring Operations (StorageService) Storage Operations (StorageProxy)

13. Startup Sequence bin/cassandra Finds cassandra.in.sh $CLASSPATH (mandatory) $CASSANDRA_HOME $CASSANDRA_CONF (mandatory) Executes $CASSANDRA_CONF/cassandra-env.sh Sets heap sizes (gc tuning goes here!)

14. o.a.c.thrift.CassandraDaemon

15. AbstractCassandraDaemon ACD.setup(): Reads configuration: DatabaseDescriptor Loads schema: DD.loadSchemas() Scrub directories Initialize storage (keyspaces + CFs) Commit log recovery: CL.recover() StorageService.initServer() -> StorageService.joinTokenRing()

16. Attn Tinkerers! Abstracted initialization of transport. Handy if you’re experimenting with transports/RPC Just extend AbstractCassandraDaemon and make sure that class is started up via bin/cassandra.

17. o.a.c.thrift.CassandraServer Implements thrift interface methods (the API). Start here when trying to understand the read/write path and RPC.

18. Configuration DatabaseDescriptor Side-effect of ACD.setup() Reads config settings from yaml Defines system tables Changes regularly I hate this code. Please fix it.

19. Main Singletons StorageService StorageProxy MessagingService CompactionManager StageManager MigrationManager

20. Did you just say ‘Singletons?’

21. Main Singletons StorageService StorageProxy MessagingService CompactionManager StageManager MigrationManager

22. JMX MBeans Tooling supplied by Mbeans Anything that does measureable/configurable work is tooled Thread pools Compaction Hinted handoff Streaming Storage Commit log

23. StorageService initServer() -> joinTokenRing() Starts gossip Starts MessagingService Negotiates bootstrap Many ring operations live here. Repository of ring topology TokenMetadata (quasi-singleton via SS.tokenMetadata_) Partitioner instance is also here

24. MessagingService Verb handlers live here (initialized from SS). Main event handlers, haven’t changed much. Socket listener 2 threads per ring node Message gateway emitted from MessageProducerimpls MS.sendRR() MS.sendOneWay() MS.receive() Messages are versioned now (0.8) IncomingTCPConnection

25. StorageProxy Top level of all read/write operations Called from o.a.c.thrift.CassandraServer Write path changed because of counters Notion of WritePerformer Eventually to Table and ColumnFamilyStore Further, to SSTable and related classes.

26. StageManager Fancy java ThreadPoolExecutor SEDA: http://www.eecs.harvard.edu/~mdw/papers/seda-sosp01.pdf consumes callables from a queue. Manages concurrency. Hasn’t changed much.

27. Adding API Methods Define method+structures in IDL interface/cassandra.thrift Regenerate files ant gen-thrift-java gen-thrift-py Implement stubs: o.a.c.thrift.CassandraServer Create a system test tests/system/test_thrift_server.py

28. Reading Socket->CassandraServer Permissions Request validation Marshalling ReadCommands created in CS.multigetSliceInternal, passed to StorageProxy 1 per key

29. Reading StorageProxy.read(), fetchRows() For each ReadCommand Determine endpoints Local & remote branches

30. Reading StorageProxy local READ stage executes a LocalReadRunnable True read vs digest Table, ColumnFamilyStore CFS.getTopLevelColumns Make QueryFilter Query Memtables Query SSTables Coalesce in iterators

31. Reading StorageProxy remote read command Response handler Send to remote nodes Read repair happens in SP.fetchRows().

32. Writing CS.doInsert() Marshalling, creates RMs StorageProxy local/remote branch SP.sendToHintedEndpoints() RowMutation one Key per (several CFs) ColumnFamily Collection of column modifications

33. Writing RM.apply->Table.apply Write to CL Iterate over RM CFs CFS.apply() Overwrites results on pre-existing column families

34. Writing RM is serialized into a Message and sent to other nodes Waits for ACKs depending on CL

35. Challenges

36. Challenges To have an in-depth understanding of everything. Hard for hobbyist/part-timers Outside of Datastax, little support for full-timers Still changing fast Keeping up

37. Challenge: Lines of Code 0.4 (Sep 2009) 52 kloc 0.5 (Jan 2010) 59 kloc 0.6 (Apr 2010) 73 kloc 0.7 (Jan 2011) 122 kloc 0.8 (Jun 2011) 146 kloc Trunk (yesterday) 149 kloc Average: 4,500 lines per month

38. Challenges Codewise Growing pains Software maturity Decisions made early on