This presentation introduces people to Cassandra and Column Family Datastores in general. I will discuss what Cassandra is, how and when it is useful, and how it integrates with Rails. I will also go in to lessons learned during our 3-month project, and the useful patterns that emerged. The discussion will be very technical, but targeted at developers who are not familiar with, or have not done a project with Cassandra.

1. Cassandra and Rails
What we learned on our first project
Mike Wynholds
Carbon Five
@mwynholds

2. Agenda
Introduction
What is Cassandra?
Cassandra and Rails
Emergent data patterns
Deployment

3. What is Cassandra?
Column Family database
Distributed design - “eventually consistent”
Open sourced by Facebook, now Apache
Used by Facebook, Twitter, Digg, Rackspace...
Largest cluster: 100 TB, 150 nodes
Still very immature - at version 0.6.2

4. Relational vs Column Family
Schema-ful Schema-less (mostly)
Row-based Column-based
Robust SQL queries No query language
Transactional Eventually consistent *
ODBC/JDBC Thrift *
Fast - 300/350ms Blazing - .12/15ms **
* Cassandra-specific
** MySQL vs Cassandra, > 50GB data, write/read

5. Relational Column Based
Database Keyspace
Column Family
Table 1
col1 col2 col3 col4 col5 key1: col1:val col3:val
key2: col1:val1 col2:val col3:val
key3: col3:val
Table 2 Table 3 Super Column Family
col1 col2 col1 col2 col3
col1: col1:
key1:
col1:val col1:val col2:val
key2::

6. What about ORM?
Cassandra is NOT a relational database
No ActiveRecord support (currently)
No Hibernate support (currently)
OCFM? Lots of room for jars/gems

7. An example in Rails
bitchroom - a place to bitch and whine
Twitter-like features - user post timelines
Digg-like features - up/down, fav, reply

8. Custom Rails config
-- /config/cassandra.yml --
development:
servers: "127.0.0.1:9160"
keyspace: "bitchroom_development"
timeout: 3
retries: 2
-- /initializers/initialize_app.rb --
require 'cassandra'
env = ENV['RAILS_ENV'] || 'development'
cfg = YAML.load_file('#{RAILS_ROOT}/config/cassandra.yml')[env]
thrift_options = { :timeout => cfg['timeout'], :retries => cfg['retries'] }
$cassandra = Cassandra.new(cfg['keyspace'], cfg['servers'], thrift_options)
$cassandra.disable_node_auto_discovery!

9. Cassandra API
get(keyspace, key, column_path, level)
get_slice(keyspace, key, column_parent, predicate, level)
multiget_slice(keyspace, keys, column_parent, predicate, level)
get_count(keyspace, key, column_parent, level)
get_range_slices(keyspace, column_parent, predicate, range, level)
insert(keyspace, ey, column_path, value, timestamp, level)
batch_mutate(keyspace, mutation_map, level)
remove(keyspace, key, column_path, timestamp, level)
describe_*(...)
Version 0.6.x only

10. Sample save
def save
uuid = SimpleUUID::UUID.new(Time.now)
@id = uuid.to_guid
post_hash = self.to_cassandra_hash
cassandra.insert(:Posts, @id, post_hash)
pointer = {uuid => @id}
cassandra.insert(:Timelines, "main", pointer)
cassandra.insert(:Timelines, "user-#{@author_id}")
return self
end

11. Emergent data patterns
Simple object map
Object relationship map
Timeline <-- this one is the key

12. Simple Object Map
Row id = object id (primary key)
Attribute column names
String column values
ids = [ 1, 2, 3 ]
posts = cassandra.multi_get(:Posts, ids, { })

13. Object Relationship Map
Row id = object id (primary key)
Relationship attribute column names
External ID super-column values
ids = [ 1, 2, 3 ]
prs = cassandra.multi_get(:PostRelationships, ids, { })

14. Timeline
TimeUUID column names
External ID column values
result = []
options = { :count => 20, :reverse => true }
timelines = [ 'user-1', 'user-2', 'user-3' ]
multi_get_result = cassandra.multi_get(:Timelines, timelines, options)
multi_get_result.values().each do |timeline|
timeline.each { |uuid, id| result << [uuid, id] }
end
result = result.sort { |a, b| b[0] <=> a[0] }.slice(0, options[:count])

15. Deployment
Load balancing + failover
:80
haproxy
:81 :9160 cassandra
nginx
:5000 :9160
:9160
mongrel cassandra

16. Resources
http://cassandra.apache.org/
http://wiki.apache.org/cassandra/API
http://github.com/fauna/cassandra
http://github.com/ryanking/simple_uuid