This document provides an overview of Cassandra, a NoSQL database management system. It describes Cassandra as a decentralized, fault-tolerant, and elastic key-value store. The document outlines Cassandra's data model including keyspaces, column families, and columns. It also covers topics like consistency levels, secondary indexing, the Cassandra Query Language (CQL), and examples of data modeling in Cassandra.

1. Robert Koletka

2. What is Cassandra
● Basically a key value store
● With some stuff.
● It is a NoSQL database that is
● Decentralized : No single point of failure
● Elastic : Linear Scalability
● Fault Tolerant : Replication
● Optimized for writes, reads don't do badly at all
though.

3. What is Cassandra
● Based on two papers
● Bigtable: Google
● Dynamo: Amazon
● Dynamo partitioning and replication
● Bigtable data model
● CAP Theorem
● Consistent NO
● Available YES
● Partition Tolerant YES

4. What is Cassandra
● Uses consistent hashing

5. Data Model
● Cluster
● Keyspace : like a DB
● Column Families : like a Table
● Super Columns (optional)
● Columns
● Values

6. Data Model
● Keyspace groups column families together
● Column Family groups data together
● Example :
● User Keyspace has
– UserProfiles Column Family
– Friends Column Family

7. Data Model
● Cassandra doesn't require schema's like
traditional DB's
● UserProfiles Example
● Rk = {Name:Robert, Surname:Koletka,
Gender:Male}
● Js = {Name:John, Surname:Smith, Location:WC}
● Rk & Js both valid entries in UserProfiles
Column Family even though different columns.

8. Data Model
● Think about QUERIES not de-normalizing data.
● Use Case: “I want to get friends name's and
surname's for a given UserID”
● Name & Surname needs to be in the friend
column family.
● Js = {Ac:{Name:Alice,Surname:Cook}, Bb:
{Name:Betty,Surname:Blah}} (Super)
● Js = {Ac:”Alice Bob”, Bb:”Betty Blah”}

9. Data Model
● Column
● Rowkey = {ColumnName:Value,CN=V,CN=V}
● Super Column
● Rowkey = {SuperColumnName:{CN=V,CN=V},
SCN:{CN=V,CN=V}}
● Super Columns group columns together
● Cannot index on a Sub column.

10. Define Keyspace
● create keyspace <keyspace> with <att1>=<value1> and
<att2>=<value2> ...;
● create keyspace UserKeyspace with placement_strategy
= 'org.apache.cassandra.locator.SimpleStrategy' and
strategy_options = {replication_factor:2};
● Simple Strategy – place replica on next node
● NetworkTopologyStrategy – for multiple data centers
● OldNetworkToplogyStrategy – different data centers and
different racks

11. Define Column Family
● create column family <name> with
<att1>=<value1> and <att2>=<value2>...;
● create column family UserProfiles with
comparator=UTF8Type and
default_validation_class=UTF8Type and
key_validation_class=UTF8Type and
column_metadata=[{column_name:Location,
validation_class:UTF8Type,
Index_Type:KEYS}];

12. Define Column Family
● Comparator = Column Name validator and
compare column names
● default_validation_class = Validation for values
in columns which are not listed in
column_metadata
● key_validation_class = Validate key
● Default is BytesType

13. Define Column Family
● Other Available Types
● AsciiType
● BytesType
● CounterColumnType (distributed counter column, a CF
either contains counters or non at all)
● Int32Type
● IntegerType (a generic variable-length integer type)
● LexicalUUIDType
● LongType
● UTF8Type

14. Define Column Family
● Many more options
● bloom_filter_fp_chance : false positives
● gc_grace : garbage collection
● keys_cached
● row_cache_save_period
● max_compaction_threshold
● ...

15. Read and Writes
● Cassandra is optimized for writes
● First written to a commitlog
● Then to an in-memory table (memtable)
● Then periodically written to disk (SStable)
● Reads
● Read from all SStables and memtables
● Bloom filters used to speed up Sstable lookups
● Compaction
● Periodically Cassandra merges SStables

16. Indexes
● Row Keys
● Cassandra keeps and index of its Row keys
● Column Indexes
● Known as Secondary Indexes, build an index on
column values.
● Indexes existing data in the background
● Query by using equality predicates
– Then additional filters

17. Indexes
● Get userprofiles where location = 'WC'
● Get userprofiles where location = 'WC' and age
> 18
● NOT
● Get userprofiles where age > 18

18. Consistency
● Allows for configurable consistency settings
● Read
– One, Quorum((Replication Factor / 2) +1), Local/Each_Quorum
(Data Centers), All.
● Write
– Any, One, Quorum, Local/Each_Quorum (Data Centers), All.
● Any means that data can be written to co-ordinator if
replica's are down till replica's come back up.
● Quorum allows for some consistency and tolerating
some failures.
● All replica's must be up.

19. Consistency
● Read
● At least one node needs to be up to read data from,
obvious.
● Reads from a number of replicas returning the
latest data, based on timestamp.
● Read repair ensures data remains consistent,
updates out of date nodes with latest data. Runs in
background.

20. Cassandra Query Language
● Allows for
● Select
– SELECT [FIRST N] [REVERSED] <SELECT EXPR> FROM <COLUMN FAMILY> [USING
<CONSISTENCY>] [WHERE <CLAUSE>] [LIMIT N];
– SELECT [FIRST N] [REVERSED] name1..nameN FROM
– Unlike SQL, no guarantee that columns will be returned
– SELECT ... WHERE KEY >= startkey and KEY =< endkey AND name1 = value1
● Insert
● Delete
● Update
● Batch
● Truncate
● Create Keyspace
● Create Column Family
● Create Index
● Drop

21. Other Stuff
● Cassandra stores columns in sorted order
● Allows you to get the first or last X number of columns
● Potentially store historical user data
● Single column cannot hold more than 2gb
● Max number of columns per row is 2 billion
● Key and Column Names must be <64kb
● Most Languages have client libraries (Python,
Java, Scala, Node.js, PHP, C++...)
● Try not to use raw thrift.

22. Last Example
● User Statuses
● Columns stored in sorted order... use timestamp as column
name
● Rk = {1:'Good morning all',2:'lunch was good',3:'time to get
drunk',4:'so many regrets from last night'}
● Create column family UserStatuses with comparator =
LongType and Key_validation_class=UTF8Type and
default_validation_class=UTF8Type
● Get last X number of Columns, Get first X number of
columns