Cassandra: Open Source Bigtable + Dynamo

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Cassandra is a highly scalable, eventually consistent, distributed, structured columnfamily store with no single points of failure, initially open-sourced by Facebook and now part of the Apache Incubator. These slides are from Jonathan Ellis's OSCON 09 talk: http://en.oreilly.com/oscon2009/public/schedule/detail/7975

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Cassandra: Open Source Bigtable + Dynamo

  1. 1. Cassandra Jonathan Ellis
  2. 2. Motivation ● Scaling reads to a relational database is hard ● Scaling writes to a relational database is virtually impossible ● … and when you do, it usually isn't relational anymore
  3. 3. The new face of data ● Scale out, not up ● Online load balancing, cluster growth ● Flexible schema ● Key-oriented queries ● CAP-aware
  4. 4. CAP theorem ● Pick two of Consistency, Availability, Partition tolerance
  5. 5. Two famous papers ● Bigtable: A distributed storage system for structured data, 2006 ● Dynamo: amazon's highly available key- value store, 2007
  6. 6. Two approaches ● Bigtable: “How can we build a distributed db on top of GFS?” ● Dynamo: “How can we build a distributed hash table appropriate for the data center?”
  7. 7. 10,000 ft summary ● Dynamo partitioning and replication ● Log-structured ColumnFamily data model similar to Bigtable's
  8. 8. Cassandra highlights ● High availability ● Incremental scalability ● Eventually consistent ● Tunable tradeoffs between consistency and latency ● Minimal administration ● No SPF
  9. 9. Dynamo architecture & Lookup
  10. 10. Architecture details ● O(1) node lookup ● Explicit replication ● Eventually consistent
  11. 11. Architecture layers Messaging service Commit log Tombstones Gossip Memtable Hinted handoff Failure detection SSTable Read repair Cluster state Indexes Bootstrap Partitioner Compaction Monitoring Replication Admin tools
  12. 12. Writes ● Any node ● Partitioner ● Commitlog, memtable ● SSTable ● Compaction ● Wait for W responses
  13. 13. Memtable / SSTable Disk Commit log
  14. 14. SSTable format ● Key / data
  15. 15. SSTable Indexes ● Bloom filter ● Key ● Column (Similar to Hadoop MapFile / Tfile)
  16. 16. Compaction ● Merge keys ● Combine columns ● Discard tombstones
  17. 17. Remove ● Deletion marker (tombstone) necessary to suppress data in older SSTables, until compaction ● Read repair complicates things a little ● Eventually consistent complicates things more ● Solution: configurable delay before tombstone GC, after which tombstones are not repaired
  18. 18. Cassandra write properties ● No reads ● No seeks ● Fast ● Atomic within ColumnFamily ● Always writable
  19. 19. Read path ● Any node ● Partitioner ● Wait for R responses ● Wait for N – R responses in the background and perform read repair
  20. 20. Cassandra read properties ● Read multiple SSTables ● Slower than writes (but still fast) ● Seeks can be mitigated with more RAM ● Scales to billions of rows
  21. 21. Consistency in a BASE world ● If W + R > N, you will have consistency ● W=1, R=N ● W=N, R=1 ● W=Q, R=Q where Q = N / 2 + 1
  22. 22. vs MySQL with 50GB of data ● MySQL ● ~300ms write ● ~350ms read ● Cassandra ● ~0.12ms write ● ~15ms read ● Achtung!
  23. 23. Data model ● Rows, ColumnFamilies, Columns
  24. 24. ColumnFamilies keyA column1 column2 column3 keyC column1 column7 column11 Column Byte[] Name Byte[] Value I64 timestamp
  25. 25. Super ColumnFamilies keyF Super1 Super2 column column column column column column keyJ Super1 Super5 column column column column column column
  26. 26. Types of queries ● Single column ● Slice ● Set of names / range of names ● Simple slice -> columns ● Super slice -> supercolumns ● Key range
  27. 27. Range queries ● Add “master” server ● Implement on top of K/V ● Order-preserving partitioning
  28. 28. Modification ● Insert / update ● Remove ● Single column or batch ● Specify W, number of nodes to wait for
  29. 29. Thrift struct Column {    1: binary                        name,    2: binary                        value,    3: i64                           timestamp, } struct SuperColumn {    1: binary                        name,    2: list<Column>                  columns, } Column get_column(table, key, column_path, block_for=1) list<string> get_key_range(table, column_family, start_with="",  stop_at="", max_results=100) void insert(table, key, column_path, value, timestamp,  block_for=0) void remove(tablename, key, column_path_or_parent, timestamp)
  30. 30. Honestly, Thrift kinda sucks
  31. 31. Example: a multiuser blog Two queries - the most recent posts belonging to a given blog, in reverse chronological order - a single post and its comments, in chronological order
  32. 32. First try JBE Cassandra is teh awesome BASE FTW blog post comment comment post comment comment Evan I like kittens And Ruby blog post comment comment post comment comment <ColumnFamily Type="Super" CompareWith="TimeString" CompareSubcolumnsWith="UUID" Name="Blog"/>
  33. 33. Second try JBE blog Cassandra BASE FTW Cassandr comment comment is teh a is teh awesome awesome Evan blog I like kittens And Ruby Base FTW comment comment I like comment comment kittens And Ruby comment comment <ColumnFamily <ColumnFamily CompareWith="UUIDType" CompareWith="UUIDType" Name="Blog"/> Name="Comment"/>
  34. 34. Roadmap
  35. 35. Cassandra 0.3 ● Remove support ● OPP / Range queries ● Test suite ● Workarounds for JDK bugs ● Rudimentary multi-datacenter support
  36. 36. Cassandra 0.4 ● Branched May 18 ● Data file format change to support billions of rows per node instead of millions ● API changes (no more colon delimiters) ● Multi-table (keyspace) support ● LRU key cache ● fsync support ● Bootstrap ● Web interface
  37. 37. Cassandra 0.5 ● Bootstrap ● Load balancing ● Closely related to “bootstrap done right” ● Merkle tree repair ● Millions of columns per row ● This will require another data format change ● Multiget ● Callout support
  38. 38. Users Production: facebook, RocketFuel Production RSN: Digg, Rackspace No date yet: IBM Research, Twitter Evaluating: 50+ in #cassandra on freenode
  39. 39. More ● Eventual consistency: http://www.allthingsdistributed.com/2008/12/ ● Introduction to distributed databases by Todd Lipcon at NoSQL 09: http://www.vimeo.com/5145059 ● Other articles/videos about Cassandra: http://wiki.apache.org/cassandra/ArticlesAndP ● #cassandra on irc.freenode.net
  40. 40. Cassandra

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