Cassandra History

● Created at Facebook● Open-sourced since 2008 ● Current version = 3.3● Column-oriented ☞ distributed table

NoSQL database Cluster Layer● Amazon DynamoDB paper● Masterless architecture

Data-store layer● Google Big Table paper● Columns/columns family

But what is Cassandra ?● Fast Distributed Database/Table● High Availability● Linear Scalability● Predictable Performance● No SPOF● Multi-DC● Commodity Hardware● Easy to manage operationally

How is Cassandra different ?● No master / slave / replica sets● No config servers, zookeeper● Data is partitioned around ring● Data is replicated to RF=N servers● All nodes hold data and can answer

queries (both reads & writes)● Location of data on ring is determined by

partition key

● Cassandra chooses Availability & Partition Tolerance over Consistency

● Queries have tunable consistency level● ALL, QUORUM, ONE● Hinted Handoff to deal with failed nodes

CAP Theorem

Cassandra under the hood

Cassandra Writes● Writes are written to any node in the cluster

(coordinator)● Writes are written to commit log, then to

memtable● Every write includes a timestamp● Memtable flushed to disk periodically (sstable)● Deletes are actually a special write case, called

a “tombstone”● Partition is spread across multiple SSTables● Same column can be in multiple SSTables● Merged through compaction, only latest

timestamp is kept

Cassandra Reads● Any node may be queried, it acts as

coordinator● On each node, data is pulled from

Memtable + SSTables and merged● Each SSTable has a BloomFilter

Cassandra is not...

● An In-Memory database

● A Key-Value storage (it has schema)

● A magical unicorn database that farts rainbow

Cassandra Data Modeling

Relational Cassandra

CQL vs SQL

● no Sequences○ Natural Keys - email etc…○ Surrogate Keys - UUID

● no Order By - during select

Clustering Key// Comments for a given videoCREATE TABLE comments_by_video ( videoid uuid, comment_time timestamp, userid uuid, comment text, PRIMARY KEY (videoid, comment_time)) WITH CLUSTERING ORDER BY (comment_time DESC);

SELECT * FROM comments_by_video

videoid commendid userid comment

Video1 Dec 13, 11:00 user1 comment3

Video1 Dec 12, 15:00 user2 comment2

Video1 Dec 12, 12:00 user1 comment1

... ... ... ...

more comments more columns ...Dec 13, 11:00

user1, comment3Dec 12, 15:00user2, comment2

Dec 12, 12:00user1, comment3

Video1 Up to 2 Billion

New Data

Example: KillrVideo.comCREATE TABLE user_videos (

userid uuid,added_date timestamp,videoid uuid,name text,priview_image_location text,PRIMARY KEY (userid, added_date, video_id)

) WITH CLUSTERING ORDER BY (added_date DESC, video_id ASC);

// Index for tag keywordCREATE TABLE videos_by_tag (

tag text,videoid uuid,added_date timestamp,name text,priview_image_location text,tagged_date timestampPRIMARY KEY (tag, video_id)

);

// Index for tag keywordCREATE TABLE tags_by_letter (

first_letter text,tag text,PRIMARY KEY (first_letter, tag)

);

Secondary Indexes

Country Continent

Germany Europe

China Asia

France EuropeNODE 1

Germany: Europe

NODE 2

China: Asia

NODE 2

France: Europe

SELECT * FROM … WHERE Continent = ‘Europe’

SELECT * FROM … WHERE Country = ‘France’

● Continent is Secondary Index.