Using Apache Cassandra: What is this thing, and how do I use it?

Using Apache Cassandra for Big Data
What is this thing, and how do I use it?

Jeremiah Jordan
Lead Software Engineer/Support
@zanson

©2013 DataStax. Do not distribute without consent.
Monday, October 14, 13

1

Who I am
• Jeremiah Jordan
• Lead Software Engineer in Support at DataStax
• Previously Senior Architect at Morningstar, Inc.
• Using Cassandra since 0.6
• Before that, wrote code for the F22


Cassandra - An introduction


Cassandra - Intro
• Based on Amazon Dynamo and Google BigTable papers
• Shared nothing
• Distributed
• Data safe as possible
• Predictable scaling
Dynamo

BigTable
4

Cassandra - More than one server
• All nodes participate in a cluster
• Shared nothing
• Add or remove as needed
• More capacity? Add a server

• 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
5

Cassandra - Locally Distributed

• 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.
6

Cassandra - Geographically Distributed
• Client writes local
• Data syncs across WAN
• Replication Factor per DC

Single coordinator

7

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
8

Cassandra - Transparent to 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!
9

Application Example - Layout
• Active-Active
• Service based DNS routing

Cassandra Replication

10

Application Example - Uptime
• Normal server maintenance
• Application is unaware

Cassandra Replication

11

Application Example - Failure
• Data center failure

Another happy user!

• Data is safe. Route traffic.

12
33

Five Years of Cassandra

0.1
Jul-08

...

0.3
Jul-09

0.6
May-10

0.7
Feb-11

1.0
Dec-11

DSE


1.2
Oct-12

2.0
Jul-13

Cassandra 2.0 - Big new features


Lightweight transactions: the problem
Session 1

Session 2

SELECT * FROM users
WHERE username = ’jbellis’

SELECT * FROM users

[empty resultset]

[empty resultset]

It’s a Race!
INSERT INTO users
(username,password)
VALUES (’jbellis’,‘xdg44hh’)

Who wins?

INSERT INTO users
(userName,password)
VALUES (’jbellis’,‘8dhh43k’)

LWT: details
• 4 round trips vs 1 for normal updates
• Paxos - Paxos made easy
• Immediate consistency with no leader election or failover
• For reads, ConsistencyLevel.SERIAL
• http://www.datastax.com/dev/blog/lightweight-transactions-incassandra-2-0


Using LWT
• Don’t overwrite an existing record
INSERT INTO USERS (username, email, ...)
VALUES (‘jbellis’, ‘jbellis@datastax.com’, ... )
IF NOT EXISTS;

• Only update record if condition is met
UPDATE USERS
SET email = ’jonathan@datastax.com’, ...
IF email = ’jbellis@datastax.com’;


LWT: Use with caution
• Great for 1% of your application
• Eventual consistency is your friend
• http://www.slideshare.net/planetcassandra/c-summit-2013-eventual-consistencyhopeful-consistency-by-christos-kalantzis


Installing Cassandra


Download Cassandra


Extract Cassandra


Setup Data and Log Directories


Start Cassandra


Installing Cassandra Python Driver


Python Cassandra Driver


Install Python Cassandra Driver


Connect and Create a Keyspace
from cassandra.cluster import Cluster
cluster = Cluster(['127.0.0.1'])
session = cluster.connect()
log.info("creating keyspace...")
KEYSPACE = "testkeyspace"
session.execute("""
CREATE KEYSPACE IF NOT EXISTS %s
WITH replication = { 'class': 'SimpleStrategy',
'replication_factor': '1' }
""" % KEYSPACE)


Create a Table
log.info("setting keyspace...")
session.set_keyspace(KEYSPACE)
log.info("creating table...")
session.execute("""
CREATE TABLE IF NOT EXISTS mytable (
thekey text,
col1 text,
col2 text,
PRIMARY KEY (thekey, col1)
)
""")


Insert a Row
query = SimpleStatement("""
INSERT INTO mytable (thekey, col1, col2)
VALUES ('key1', 'a', 'b')
""", consistency_level=ConsistencyLevel.ONE)
log.info("inserting row")
session.execute(query)


Insert Rows (Prepared Statement)
prepared = session.prepare("""
INSERT INTO mytable (thekey, col1, col2)
VALUES (?, ?, ?)
""")
for i in range(10):
log.info("inserting row %d" % i)
bound = prepared.bind(("key%d" % i,
"b%d" % i,
"c%d" % i))
session.execute(bound)


Query Results
future = session.execute_async("""
SELECT * FROM mytable WHERE thekey='key1'
""")
rows = future.result()
log.info("keytcol1tcol2")
log.info("---t----t----")
for row in rows:
log.info("t".join(row))


Run It


Cassandra Applications - Drivers
• DataStax Drivers for Cassandra
• Java
• C#
• Python
• more on the way

36

Find Out More
Cassandra: http://cassandra.apache.org
DataStax Drivers: https://github.com/datastax
Documentation: http://www.datastax.com/docs
Getting Started: http://www.datastax.com/documentation/gettingstarted/index.html
Developer Blog: http://www.datastax.com/dev/blog
Cassandra Community Site: http://planetcassandra.org
Download: http://planetcassandra.org/Download/DataStaxCommunityEdition
Webinars: http://planetcassandra.org/Learn/CassandraCommunityWebinars
Cassandra Summit Talks: http://planetcassandra.org/Learn/CassandraSummit


Using Apache Cassandra: What is this thing, and how do I use it?

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