Cassandra Tutorial

Apache Cassandra in Action

Jonathan Ellis
jbellis@datastax.com / @spyced

Why Cassandra?
•
Relational databases are not designed to
scale
•
B-trees are slow
–
and require read-before-write

(“The eBay Architecture,” Randy Shoup and Dan Pritchett)

Reader
Memtable
Writer

Commitlog

The Log-Structured Merge-Tree,
Bigtable: A Distributed Storage
System for Structured Data

Dynamo, 2007
Bigtable, 2006

OSS, 2008

Incubator, 2009 TLP, 2010

Cassandra in production
•
Digital Reasoning: NLP + entity analytics
•
OpenWave: enterprise messaging
•
OpenX: largest publisher-side ad network in the
world
•
Cloudkick: performance data & aggregation
•
SimpleGEO: location-as-API
•
Ooyala: video analytics and business intelligence
•
ngmoco: massively multiplayer game worlds

FUD?
•
“Cassandra is only appropriate for
unimportant data.”

Durabilty
•
Write to commitlog
–
fsync is cheap since it’s append-only
•
Write to memtable
•
[amortized] flush memtable to sstable

SSTable format, briefly

<key 127>
<key 255> <row data 0>
... <row data 1>
...
<row data 127>
...
<row data 255>
...

Sorted [clustered] by row key

W A

(A-F] F

T
(F-L] L

Key “C”
W A

F

T
L

Reliability
•
No single points of failure
•
Multiple datacenters
•
Monitorable

Some headlines
•
“Resyncing Broken MySQL Replication”
•
“How To Repair MySQL Replication”
•
“Fixing Broken MySQL Database Replication”
•
“Replication on Linux broken after db restore”
•
“MySQL :: Repairing broken replication”

Good architecture solves multiple
problems at once
•
Availability in single datacenter
•
Availability in multiple datacenters

Y
Key “C”
A
W

U
F

T
L
P

Y
Key “C”
A
W

U
F

X
T hint
L
P

Tuneable consistency
•
ONE, QUORUM, ALL
•
R+W>N
•
Choose availability vs consistency (and latency)

When do you need Cassandra?
•
Ian Eure: “If you’re deploying memcache on top of your
database, you’re inventing your own ad-hoc, difficult to
maintain NoSQL data store”

Not Only SQL
•
Curt Monash: “ACID-compliant transaction integrity
commonly costs more in terms of DBMS licenses and many other
components of TCO (Total Cost of Ownership) than [scalable
NoSQL]. Worse, it can actually hurt application uptime,
by forcing your system to pull in its horns and stop functioning in the
face of failures that a non-transactional system might smoothly work
around. Other flavors of “complexity can be a bad thing” apply as
well. Thus, transaction integrity can be more trouble
than it’s worth.” [Curt’s emphasis]

Keyspaces & ColumnFamilies
•
Conceptually, like “schemas” and “tables”

Inside CFs, columns are dynamic
•
Twitter: “Fifteen months ago, it took two
weeks to perform ALTER TABLE on the
statuses [tweets] table.”

ColumnFamilies
•
Static
–
Object data
•
Dynamic
–
Precalculated query results

“static” columnfamilies

Users
zznate Password: * Name: Nate

driftx Password: * Name: Brandon

thobbs Password: * Name: Tyler

jbellis Password: * Name: Jonathan Site: riptano.com

“dynamic” columnfamilies

Following
zznate driftx: thobbs:

driftx

thobbs zznate:

jbellis driftx: mdennis: pcmanus thobbs: xedin: zznate

Inserting
•
Really “insert or update”
•
Not a key/value store – update as much of
the row as you want

Example: twissandra
•
http://twissandra.com

CREATE TABLE users (
id INTEGER PRIMARY KEY,
username VARCHAR(64),
password VARCHAR(64)
);

CREATE TABLE following (
user INTEGER REFERENCES user(id),
followed INTEGER REFERENCES user(id)
);

CREATE TABLE tweets (
id INTEGER,
user INTEGER REFERENCES user(id),
body VARCHAR(140),
timestamp TIMESTAMP
);

Cassandrified
create column family users with comparator = UTF8Type
and column_metadata = [{column_name: password,
validation_class: UTF8Type}]

create column family tweets with comparator = UTF8Type
and column_metadata = [{column_name: body, validation_class:
UTF8Type}, {column_name: username, validation_class:
UTF8Type}]

create column family friends with comparator = UTF8Type
create column family followers with comparator = UTF8Type

create column family userline with comparator = LongType and
default_validation_class = UUIDType
create column family timeline with comparator = LongType and
default_validation_class = UUIDType

Connecting
CLIENT = pycassa.connect_thread_local('Twissandra')

USER = pycassa.ColumnFamily(CLIENT, 'User')

User
RowKey: ericflo
=> (column=password, value=****,
timestamp=1289446382541473)

-------------------
RowKey: jbellis
=> (column=password, value=****,
timestamp=1289446438490709)

uname = 'jericevans'
password = '**********'

columns = {'password': password}

USER.insert(uname, columns)

Friends and Followers
RowKey: ericflo

=> (column=jbellis, value=1289446467611029,
timestamp=1289446467611064)

=> (column=b6n, value=1289446467611031,
timestamp=1289446467611080)

to_uname = 'ericflo'

FRIENDS.insert(uname, {to_uname: time.time()})
FOLLOWERS.insert(to_uname, {uname: time.time()})

zznate driftx: thobbs:

driftx

thobbs zznate:

jbellis driftx: mdenni pcmanu thobbs: xedin: zznat
s: s: e:

Tweets
RowKey: 92dbeb50-ed45-11df-a6d0-000c29864c4f

=> (column=body, value=Four score and seven years ago,
timestamp=1289446891681799)

=> (column=username, value=alincoln,
timestamp=1289446891681799)

-------------------
RowKey: d418a66e-edc5-11df-ae6c-000c29864c4f

=> (column=body, value=Do geese see God?,
timestamp=1289501976713199)

=> (column=username, value=pdrome,
timestamp=1289501976713199)

Userline
RowKey: ericflo

=> (column=1289446393708810, value=6a0b4834-ed44-11df-
bc31-000c29864c4f, timestamp=1289446393710212)

=> (column=1289446397693831, value=6c6b5916-ed44-11df-
bc31-000c29864c4f, timestamp=1289446397694646)

=> (column=1289446891681780, value=92dbeb50-ed45-11df-
a6d0-000c29864c4f, timestamp=1289446891685065)

=> (column=1289446897315887, value=96379f92-ed45-11df-
a6d0-000c29864c4f, timestamp=1289446897317676)

Userline

zznate 1289847840615: 3f19757a-c89d... 1289847887086: a20fcf52-595c...

driftx

thobbs 1289847887086: a20fcf52-595c...

jbellis 1289847840615: 3f19757a-c89d... 128984784425: 844e75e2-b546...

Timeline
RowKey: ericflo

=> (column=1289446393708810, value=6a0b4834-ed44-11df-
bc31-000c29864c4f, timestamp=1289446393710212)

=> (column=1289446397693831, value=6c6b5916-ed44-11df-
bc31-000c29864c4f, timestamp=1289446397694646)

=> (column=1289446891681780, value=92dbeb50-ed45-11df-
a6d0-000c29864c4f, timestamp=1289446891685065)

=> (column=1289446897315887, value=96379f92-ed45-11df-
a6d0-000c29864c4f, timestamp=1289446897317676)

Adding a tweet
tweet_id = str(uuid())
body = '@ericflo thanks for Twissandra, it helps!'
timestamp = long(time.time() * 1e6)

columns = {'uname': useruuid, 'body': body}
TWEET.insert(tweet_id, columns)

columns = {ts: tweet_id}
USERLINE.insert(uname, columns)

TIMELINE.insert(uname, columns)
for follower_uname in FOLLOWERS.get(uname, 5000):
TIMELINE.insert(follower_uname, columns)

Reads
timeline = USERLINE.get(uname, column_reversed=True)
tweets = TWEET.multiget(timeline.values())

start = request.GET.get('start')
limit = NUM_PER_PAGE

timeline = TIMELINE.get(uname, column_start=start,
column_count=limit, column_reversed=True)
tweets = TWEET.multiget(timeline.values())

Programatically
•
Don't use thrift directly
•
Higher level clients have a lot of features you
want
–
Knowledge about data types
–
Connection pooling
–
Automatic retries
–
Logging

Raw thrift API: Connecting
def get_client(host='127.0.0.1', port=9170):
socket = TSocket.TSocket(host, port)
transport = TTransport.TBufferedTransport(socket)
transport.open()
protocol =
TBinaryProtocol.TBinaryProtocolAccelerated(transport)
client = Cassandra.Client(protocol)
return client

Raw thrift API: Inserting
data = {'id': useruuid, ...}
columns = [Column(k, v, time.time())
for (k, v) in data.items()]
mutations = [Mutation(ColumnOrSuperColumn(column=c))
for c in columns]
rows = {useruuid: {'User': mutations}}

client.batch_mutate('Twissandra', rows,
ConsistencyLevel.ONE)

API layers
•
libpq •
Thrift
•
JDBC •
Hector
•
JPA •
Hector object-
mapper

Running twissandra
•
Login: notroot/notroot
–
(root/riptano)

•
cd twissandra
•
python manage.py runserver &
•
Navigate to http://127.0.0.1:8000
•
Login as jim/jim, tom/tom, or create your own

One more thing
•
!PUBLIC! userline

Exercise 1
•
$ cassandra-cli --host localhost
•
] use twissandra;
] help;
] help list;
] help get;
] help del;
•
Delete the most recent tweet
–
How would you find this w/o looking at the UI?

Exercise 2
•
User jim is following user tom, but
twissandra doesn't populate Timeline with
tweets from before the follow action.
•
Insert a tweet from tom before the follow
action into jim's timeline

Exercise 3
•
Add a state column to the Tweet column
family definition, with an index (index_type
KEYS).
–
Hint: a no-op update column family on Tweet would be
update column family Tweet with
column_metadata=[{column_name:body,
validation_class:UTF8Type}, {column_name:username,
validation_class:UTF8Type}]
•
Set the state column on several tweets to TX.
Select them using get … where.

Language support
•
Python
–
pycassa
–
telephus
•
Ruby
–
Speed is a negative
•
Java
–
Hector
•
PHP
–
phpcassa

Done yet?
•
Still doing 1+N queries per page
•
Solution: Supercolumns

Applying SuperColumns to Twissandra

jbellis 1289847840615
1289847844275 1289847844275 1289847887086
1289847844275
Id:
Id: Id:
Id: Id:
Id:
3f19757a-c89d...
3f19757a- 844e75e2-b546...
3f19757a- a20fcf52-595c...
3f19757a-
c89d... c89d... c89d...
uname:
uname: uname:
uname: uname:
uname:
zznate
zznate driftx
zznate zznate
zznate

body:
body: body:
body: body:
body:
O Do geese see
stone be not so Rise geese see
Do to vote sir Do Igeese see
prefer pi
... ... ...

Supercolumns: limitations
•
Requires reading an entire SC (not the entire
row) from disk even if you just want one
subcolumn

UUIDs
•
Column names should be uuids, not longs,
to avoid collisions
•
Version 1 UUIDs can be sorted by time
(“TimeUUID”)
•
Any UUID can be sorted by its raw bytes
(“LexicalUUID”)
–
Usually Version 4
–
Slightly less overhead

Lucandra
•
What documents contain term X?
–
… and term Y?
–
… or start with Z?

Fields and Terms

<doc>
<field name=”title”>apache talk</field>
<field name=”date”>20110201</field>
</doc>

feld term freq position
title apache 1 0
title talk 1 1
date 20110201 1 0

Lucandra ColumnFamilies
create column family documents with comparator = BytesType;

Create column family terminfo with column_type = Super and
comparator = BytesType and subcomparator = BytesType;

Lucandra data
Document Key col name value
"documentId" => { fieldName , value }

Term Key col name value
"field/term" => { documentId , position vector }

Lucandra queries
•
get_slice
•
get_range_slices
•
No silver bullet

FAQ: counting
•
UUIDs + batch process
•
column-per-app-server
•
counter API (after 1.0 is out)

Locking
•
Zookeeper
•
Cages: http://code.google.com/p/cages/
•
Not suitable for multi-DC

UUIDs

counter1 672e34a2-ba33... b681a0b1-58f2...

counter2 3f19757a-c89d... 844e75e2-b546... a20fcf52-595c...

counter1 aggregated: 27

counter2 aggregated: 42

Column per appserver

counter1 672e34a2-ba33: 12 b681a0b1-58f2: 4 1872c1c2-38f1: 9

counter2 3f19757a-c89d: 7 844e75e2-b546: 11

Counter API

key counter1: (14, 13, 9) counter2: (11, 15, 17)

General Tips
●
Start with queries, work backwards
●
Avoid storing extra “timestamp” columns
●
Insert instead of check-then-insert
●
Use client-side clock to your advantage
●
use TTL
●
Learn to love wide rows

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