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Philip Thompson is a software engineer at DataStax and contributor to Apache Cassandra. The document discusses Apache Cassandra, an open source, distributed database built for scalability and high availability. It describes Cassandra's architecture including data distribution across nodes, replication, consistency levels, and mechanisms for repair and anti-entropy.
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- 1. Apache Cassandra PhilipThompson Software Engineer DataStax ©2014 DataStax. Do not distribute without consent. 1
- 2. Who I am • Philip Thompson • Software Engineer at DataStax • Contributor to Apache Cassandra • A maintainer of CCM, the Cassandra Cluster Manager
- 3. Apache Cassandra™ •ApacheCassandra™ is a massively scalable, open source, NoSQL, distributed database built for modern, mission-critical online applications. •Written in Java and is a hybrid of Amazon Dynamo and Google BigTable •Masterless with no single point of failure •Distributed and data centre aware •100% uptime •Predictable scaling 3
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- 8. Cluster Architecture ©2012DataStax 8
- 9. Data Distribution 75 0 25 50 Murmur3_Hash_Function(Partition Key) >> Token
- 10. Cassandra - Morethan one server • All nodes participate in a cluster • Shared nothing • Add or remove as needed • More capacity? Add a server 10 • Each node owns a number of tokens • Tokens denote a range of keys • 4 nodes? -> Key range/4 • Each node owns 1/4 the data
- 11. Cassandra - LocallyDistributed • Client writes to any node • Node coordinates with others • Data replicated in parallel • Replication factor (RF): How many copies of your data? • RF = 3 here Each node stores 3/4 of clusters total data. 11
- 12. Cassandra - GeographicallyDistributed • Client writes local • Data syncs across WAN • Replication Factor per DC Single coordinator 12
- 13. Cassandra - ReplicationFactor • Replication factor (RF): How many copies of your data? • Replication Factor is set per keyspace • Can be altered by operator 13 RF = 3
- 14. Cassandra - Consistency • Consistency Level (CL) • Client specifies per read or write • ALL = All replicas ack • QUORUM = > 51% of replicas ack • LOCAL_QUORUM = > 51% in local DC ack • ONE = Only one replica acks 14
- 15. Cassandra - Transparentto the application • A single node failure shouldn’t bring failure • Replication Factor + Consistency Level = Success • This example: • RF = 3 • CL = QUORUM >51% Ack so we are good! 15
- 16. Cassandra - Scaling • Take a cluster of four nodes • Where does the fifth node go? • Rebalancing is costly 75 16 0 25 50
- 19. Gossip • Managescluster state • Nodes up/down • Nodes joining/leaving • Decentralized • “Heartbeat” every second • Every node contacts 1-3 other nodes
- 20. Snitch • Responsiblefor determining cluster topology • Datacenter awareness • Tracks node responsiveness • Many snitches provided out of the box • SimpleSnitch • GossipingPropertyFileSnitch (recommended for production) • EC2Snitch and EC2MultiRegionSnitch • For use with AWS • Comparable GCE snitch has just been added • Custom snitches can be added 20
- 21. Anti-Entropy - ReadRepair
- 22. Anti-Entropy - HintedHandoff • Three hour window • Hints are replayed when node is restored • Stored in system.hints table on coordinator • Cassandra does not copy Dynamo’s “sloppy quorum” 22
- 23. Anti-Entropy - Repair • Nodetool repair • Uses merkle trees for data comparison • Should be run weekly. • Cassandra 2.1 has drastically improved repair times, thanks to incremental repair 23
- 24. Node Architecture ©2012DataStax 24
- 25. Write Path commitlog Memtable SSTable Write Memory Disk
- 26. Write Path •By default data is fsynced every 10s • This can be configured in cassandra.yaml commit log Memtable SSTable Write
- 27. Read Path Memtable SSTable Read SSTable Memory Disk
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- 31. Debugging your datamodel • Tracing cqlsh> tracing on; Now tracing requests. cqlsh:foo> INSERT INTO test (a, b) VALUES (1, 'example'); Tracing session: 4ad36250-1eb4-11e2-0000-fe8ebeead9f9 activity | timestamp | source | source_elapsed -------------------------------------+--------------+-----------+---------------- execute_cql3_query | 00:02:37,015 | 127.0.0.1 | 0 Parsing statement | 00:02:37,015 | 127.0.0.1 | 81 Preparing statement | 00:02:37,015 | 127.0.0.1 | 273 Determining replicas for mutation | 00:02:37,015 | 127.0.0.1 | 540 Sending message to /127.0.0.2 | 00:02:37,015 | 127.0.0.1 | 779 Messsage received from /127.0.0.1 | 00:02:37,016 | 127.0.0.2 | 63 Applying mutation | 00:02:37,016 | 127.0.0.2 | 220 Acquiring switchLock | 00:02:37,016 | 127.0.0.2 | 250 Appending to commitlog | 00:02:37,016 | 127.0.0.2 | 277 Adding to memtable | 00:02:37,016 | 127.0.0.2 | 378 Enqueuing response to /127.0.0.1 | 00:02:37,016 | 127.0.0.2 | 710 Sending message to /127.0.0.1 | 00:02:37,016 | 127.0.0.2 | 888 Messsage received from /127.0.0.2 | 00:02:37,017 | 127.0.0.1 | 2334 Processing response from /127.0.0.2 | 00:02:37,017 | 127.0.0.1 | 2550 Request complete | 00:02:37,017 | 127.0.0.1 | 2581
- 32. Nodetool • Commandline interface for monitoring Cassandra and performing routine database operations • Commands for viewing detailed metrics for tables, server metrics, and compaction statistics: • cfstats: statistics for each table and keyspace • cfhistograms: statistics about a table, including read/write latency, row size, column count, and number of SSTables • netstats: statistics about network operations and connections • tpstats: statistics about the number of active, pending, and completed tasks for each stage of Cassandra operations by thread pool 32
- 33. Try it out ©2012 DataStax 33
- 34. Cassandra • Downloadfrom source: • git clone git://git.apache.org/cassandra.git • Packaged install and tarballs available: • http://www.datastax.com/documentation/cassandra/2.1/cassandra/install/install_cassan draTOC.html
- 35. CCM • CCM- Cassandra Cluster Manager • https://github.com/pcmanus/ccm •Warning: not lightweight • Example: • ccm create test -v 2.0.1 • ccm populate -n 3 • ccm start
- 36. Clients • Cqlsh • Bundled with Cassandra • Drivers • java: https://github.com/datastax/java-driver • python: https://github.com/datastax/python-driver • .net: https://github.com/datastax/csharp-driver • and more: http://www.datastax.com/download/clientdrivers • Ruby, C/C++, NodeJS
- 37. Get Help •IRC: #cassandra on freenode • Mailing Lists • Subscribe at cassandra.apache.org • Stack Overflow • DataStax Docs • http://www.datastax.com/docs 37
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- 39. ©2014 DataStax Confidential.Do not distribute without consent. 39