Introduction to Cassandra and CQL for Java developers

Julien Anguenot (@anguenot)!Houston Java User Group!

July 30th, 2014

Agenda

C* overview!C* key features!C* key concepts!Getting started with C*!CQL!DataStax CQL Java driver

C* overview

What is C*?• Open source distributed storage system!• Essentially a partitioned row store!• A cross between Google’s BigTable (data model) and Amazon’s Dynamo

(architecture)!• Runs off commodity hardware!• Optimized for non-relational models!• Cassandra Query Language (CQL)!• Written in Java!• Apache Licence v2.0!• An open source community

History

• Developed by Facebook for its inbox search!• Open sourced in 2008!• Apache Foundation top project in 2009!• 1.0 released in 2011!• 2.0 released in 2013!• 2.1 to be released this year

C* is today

• One of the most popular “NoSQL” database!• Used by many (and large) organizations (Netflix, Instagram,

Twitter, eBay, etc.)!• Contributors include Facebook, IBM, Twitter, Rackspace, etc.!• Cassandra 2.0+ and CQL 3.1!• Drivers and client libs available for various languages:

Python, Java, C++, C#, etc.

When to consider C*?• Performance: write is great, read is good on very large datasets.

(hundreds of TB)!• Application running across multiple data-centers in different

geographic locations!• Application requiring HA w/ no-SPOF (hundreds of nodes)!• Elastic scalability is critical!• Application running off commodity servers in premises or VMs at

your favorite IaaS!• Looking for simplicity over other solutions such as Hadoop /

HBase7

Cassandra vs HBase vs MongoDB

Let’s just get this out of the way

MongoDB to be considered if / when?• (much) smaller datasets!• your application does not need to run across multiple

data centers.!• it is ok for your application to have a SPOF!• you do not need to scale out your application elastically!• write performance decreasing with amount of data is not

a big deal

HBase to be considered if / when?• You do analytics: HBase running off Hadoop is a good

option!• Your application has a very low transaction rate!• Your application does not need to run in multiple data

centers!• You are not scared of moving parts!• Increasing your application overall architecture is fine

C* key features

Scalability

• linearly scales reads and writes with number of nodes. Throughput of application // # of nodes!

• hundreds of nodes supported!• no downtime adding nodes!• no application level interruption!• multi-datacenter native replication support

High Availability

• fault tolerant with tunable consistency (more on this later)!• data replicated to multiple nodes!• continuous availability: no SPOF (vs master / slave)

Performances

• low latency!• write is great!• read is good!• can handles hundreds of TB

Transaction Support!

• commit log: atomicity, isolation and durability of ACID compliance!

• consistency is tunable (more on this later)

Simplicity

• all nodes in cluster are the same!• configuration is simple!• operation is simple

Cassandra Query Language (CQL)

• SQL-like query language!• data are in tables containing rows of columns!• v3 replaces Thrift API and CQL v2

C* key concepts

Tunable consistency!

• RDBMS: consistency and availability => transactions!• NoSQL: partition tolerance over consistency?!• Cassandra tunable consistency: tradeoffs in between

performance or accuracy on a per-query basis!• Write requests: all nodes, quorum of nodes or any available

nodes!• Read requests: all nodes “strong consistency”, quorum of

nodes or any nodes.

Data model!• Flexible data storage: structured, semi-structured,

unstructured!• Change to data structures is dynamic!• strict minimum: essentially a distributed hash map!• low-level: requires application to have extensive knowledge

about the dataset!• Does not support a fully relational model: application

responsibility!• No foreign keys, no JOIN

Partitioned row store!• keyspace (KS) is the primary container of data (like RDBMS database)!• KS contains column families (CF) (like relational tables)!• CF contains rows and rows contain columns!• CF requires a primary key: partition key (PK) is the first part of the primary key. !• PK determines on which nodes the data is stored. !• SELECT must include PK!• remaining columns part of primary key are clustering columns (think ordering)!• INSERT / UPDATE / DELETE OPS on rows w/ same PK for a CF are atomic and

isolated!• partitioning: C* distributes transparently data across multiple nodes (nodes can be

added and removed)!• Secondary indexes possible

Getting started

Where to get started?• http://cassandra.apache.org/

Apache foundation project Web site!• http://planetcassandra.org/

Community Web site!• http://www.datastax.com/

company providing Cassandra support and solutions to enterpriseslots of great documentation

Requirements

• Java >= 1.7 (prefer Oracle JVM)!• Python 2.7 (cqlsh only)

Downloading

• stable releases available from Apache Foundation Web site!• binary distributions!• Debian / Ubuntu packages!

• DataStax provides RPMs!• you can build C* from source (testing patches etc.)

Getting started with tarball distribution$ wget http://www.apache.org/dyn/closer.cgi?path=/cassandra/2.0.9/apache-cassandra-2.0.9-bin.tar.gz !$ sudo mkdir -p /var/log/cassandra $ sudo chown -R `whoami` /var/log/cassandra $ sudo mkdir -p /var/lib/cassandra $ sudo chown -R `whoami` /var/lib/cassandra $ tar -xzf apache-cassandra-2.0.9-bin.tar.gz !$ bin/cassandra -f

Getting started with Debian / Ubuntu (1/2)

$ sudo vim /etc/apt/sources.list.d/java.list deb http://ppa.launchpad.net/webupd8team/java/ubuntu trusty main deb-src http://ppa.launchpad.net/webupd8team/java/ubuntu trusty main

$ sudo apt-get update $ sudo apt-get oracle-java7-installer $ sudo apt-get install oracle-java7-set-default !

Getting started with Debian / Ubuntu (2/2)

$ sudo vim /etc/apt/sources.list.d/cassandra.list deb http://www.apache.org/dist/cassandra/debian 20x main deb-src http://www.apache.org/dist/cassandra/debian 20x main

$ sudo apt-get update $ sudo apt-get install cassandra

Running the CQL shell

$ (bin/)cqlsh Connected to Test Cluster at localhost:9160. [cqlsh 4.1.1 | Cassandra 2.0.9 | CQL spec 3.1.1 | Thrift protocol 19.39.0] Use HELP for help. cqlsh> •

Cassandra Query Language (CQL)

Using CQL

• cqlsh!• DataStax driver!• simpler than Thrift API!• hide C* internal implementation details!• native transport port: 9042

CQL basics

• usual statements!• CREATE / DROP / ALTER!• SELECT!• INSERT and UPDATE are the same (create or replace)

Keyspace (KS)• “like” a RDBMS database but…!• replication strategy!

• SimpleStrategy: simple single DC cluster!• NetworkTopologyStrategy: multi-DC cluster!

• replication factor: total number of replicas across the cluster!• A replication factor of 1 means that there is only one copy of each row in

the DC!• A replication factor of 2 means two copies of each row, where each copy is

on a different node in every DC!• if RF > # nodes: writes rejected and read will depend on consistent level

Creating KS: single node in a single DC

cqlsh> CREATE KEYSPACE HJUG WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 1 };!!

1 node == 1 copy!

Creating KS: 4 nodes cluster in a single DC (1/2)

cqlsh> CREATE KEYSPACE HJUG WITH REPLICATION = { 'class' : 'SimpleStrategy', 'replication_factor' : 3 };!!

3 copies of data across 4 nodes

Creating KS: 4 nodes cluster in a single DC (2/2)

• first replica on a node determined by the partitioner!• Additional replicas placed on the next nodes clockwise in

the ring

Multi-DC (NetworkTopologyStrategy)• cluster deployed across multiple data centers!• specify how many replicas in each data center!• what to consider:!

• local reads with low net latency!• failure!• disk space!

• example:!1. 2 replicas in each DC: 1 node can be down per DC and still allows local reads at

a consistency level of ONE (1).!2. 3 replicas in each DC. 1 node per DC at a strong consistency level of

LOCAL_QUORUM (2) depending on query consistency level

Creating KS: 2 DC of 3 nodes and RF 3

cqlsh> CREATE KEYSPACE HJUG WITH REPLICATION = { 'class' : 'NetworkTopologyStrategy', ‘us-east' : 3, ‘us-west’: 3 };!!

3 copies of data across 3 nodes in each DC (6 totals)

nodetool status <KS>$ bin/nodetool status HJUG!!Datacenter: us-east!===============!Status=Up/Down!|/ State=Normal/Leaving/Joining/Moving!-- Address Load Tokens Owns (effective) Host ID Rack!UN 10.241.206.82 989.91 GB 256 100.0% 1aeb620e-f22d-485b-b755-323f8e20388a 206!UN 10.241.206.80 989.14 GB 256 100.0% aefbe1fc-3436-48ac-a07f-ac664c2b823f 206!UN 10.241.206.81 989.7 GB 256 100.0% acd7b4db-7a3f-4dac-96ef-9389a2f807ba 206!!Datacenter: us-west!===============!Status=Up/Down!|/ State=Normal/Leaving/Joining/Moving!-- Address Load Tokens Owns (effective) Host ID Rack!UN 10.243.206.80 989.7 GB 256 100.0% 3d8ea269-3e59-400c-9f77-727da2bcf8a6 206!UN 10.243.206.81 988.49 GB 256 100.0% 5832b870-fcfc-4046-a2d5-eff65fa53f4c 206!UN 10.243.206.82 987.92 GB 256 100.0% b8d0792a-b5fb-433f-a9f6-ce1110a3420b 206!!

ALTER KEYSPACE <KS>cqlsh> ALTER KEYSPACE HJUG WITH REPLICATION = { 'class' : 'NetworkTopologyStrategy', ‘us-east' : 3, ‘us-west’: 2 };!!

You then need to run a repair

DROP KEYSPACE <KS>

cqlsh> drop keyspace HJUG;!cqlsh> drop keyspace if exists HJUG;!!

Immediate and irreversible removal

Using KS

cqlsh> use HJUG;cqlsh> describe keyspace HJUG;

To go further

• partitioner!• snitch!• rack!• seeds!• nodetool!• read configuration file

Creating table with a single primary keycqlsh:HJUG> CREATE TABLE users ( username varchar,! password varchar,! […], ! PRIMARY KEY (username));

Creating table with a compound primary keycqlsh:HJUG> CREATE TABLE users( username varchar,! location_id int,! […],! PRIMARY KEY (username, location_id));!!

partition key: username!location_id: clustering columns (ordering)

Creating table with a composite primary key

cqlsh:HJUG> CREATE TABLE users( username varchar,! location_id int,! […],! PRIMARY KEY ((username, location_id)));!!

each row will be on a separated partition of its own

ALTER TABLE <T>

cqlsh:HJUG> ALTER TABLE users ADD last_login varchar;!cqlsh:HJUG> ALTER TABLE users ALTER last_login TYPE timestamp;!cqlsh:HJUG> ALTER TABLE users DROP last_login;!!cqlsh:HJUG> ALTER TABLE users with COMPRESSION = {'sstable_compression': ''};!

DESCRIBE TABLE <T>cqlsh> use HJUG;cqlsh:HJUG> DESCRIBE TABLE HJUG;CREATE TABLE users( username varchar,! location_id int,! […],! PRIMARY KEY (username, location_id)!) WITH![…]!compaction={'class': 'SizeTieredCompactionStrategy'} AND!compression={'sstable_compression': 'LZ4Compressor'};!!

INSERT

cqlsh> INSERT INTO HJUG.users (username, location_id) VALUES (‘janguenot’, ‘Houston’); !!cqlsh> use HJUG;!cqlsh:HJUG> INSERT INTO users (username, location_id) VALUES (‘janguenot’, ‘Houston’);

UPDATE

cqlsh:HJUG> UPDATE USERS set X=‘Y’ where username=‘janguenot’ and location_id = ‘Houston’;

SELECTcqlsh:HJUG> SELECT * FROM USERS;!cqlsh:HJUG> SELECT * FROM USERS ORDER BY location_id ASC;!cqlsh:HJUG> SELECT * FROM USERS where username = ‘janguenot’;!!Remember ORDER BY can ONLY be used with columns part of primary

CQL predicates

• on partition keys: =, IN!• on the cluster columns: <,<=,=,>=,>,IN

Performance considerations

• query against single partition are fast!• pk = <whatever>!

• queries spanning multiple partitions are slow!• new disk seek for each partition!

• queries spanning multiple cluster columns are fast

GROUP BY?

• partition key cluster columns for grouping!• no group by statement

DELETE

cqlsh:HJUG> DELETE FROM USERS where username = ‘janguenot’ and location_id = ‘Houston’;!!

Deleted values will be permanently deleted after next compaction

TRUNCATE TABLE <T>

cqlsh:HJUG> truncate table users;

DROP TABLE <T>

cqlsh:HJUG> drop table users;!cqlsh:HJUG> drop table if exists users;

CQL Types

CQL Collectionscqlsh:HJUG> CREATE TABLE users ( username varchar,! password varchar,! emails set<text>, ! PRIMARY KEY (username));!• Set, List and Map are supported!• 1 to many relationship!• they get serialized: keep it small or use extra table!• list, that are ordered, are not performant, use set if possible or consider additional

tables if large collection

Secondary Indexes

• Query against a column outside the primary key!• CREATE INDEX <index_name> ON <T>(<column>);!• SELECT * FROM T where column=‘x’;!

• Performances are good but not great but definitely getting better and better

Final remarks about CQL

• no sequences: you manage UUID at the app level (time UUID types might be used for time series though)!

• remember partition key is not a primary key: beware of UPDATE!

• In doubt, you can write: C* is good at it. Create table and store data (One to One, One To Many)!

• Your application will drive your data model!

To go further

• TTL!• Counters!• Static column!• Lightweight transactions (IF, IF NOT EXISTS)

DataStax native CQL Java driver

Main features• Provides CQL3 access to C* using Java!• Uses C* CQL Native protocol!• Tunable policies (including consistency)!• Load balancing / reconnection / failover / routing of requests!• prepared statements and batches!• Sync and Async queries supported!• tracing query supported (for debug purposes)!• Driver available for Python, C++ and C# as well (similar API)

Driver modules• driver-core: the core layer!• driver-examples: example applications using the other

modules which are only meant for demonstration purposes.

Maven dependency

<dependency> <groupId>com.datastax.cassandra</groupId> <artifactId>cassandra-driver-core</artifactId> <version>2.0.3</version> </dependency>

Optional dependencies for compression

<dependency> <groupId>net.jpountz.lz4</groupId> <artifactId>lz4</artifactId> <version>1.2.0</version> <scope>runtime</scope> </dependency> <dependency> <groupId>org.xerial.snappy</groupId> <artifactId>snappy-java</artifactId> <version>1.0.5</version> <scope>runtime</scope> </dependency>

Driver documentation• Docs

http://www.datastax.com/documentation/developer/java-driver/2.0/index.html!

• APIhttp://www.datastax.com/drivers/java/2.0 !

• Jira https://datastax-oss.atlassian.net/browse/JAVA !

• Mailing listhttps://groups.google.com/a/lists.datastax.com/forum/#!forum/java-driver-user

Open Source• Apache v2 licence!• https://github.com/datastax/java-driver

Examples

Step 1: connection to the cluster

Cluster.Builder clusterBuilder = Cluster.builder(); !// Connect to one (1) node clusterBuilder.addContactPoint(“10.10.10.2”); !// Connect to several nodes clusterBuilder.addContactPoints(“10.10.10.2”, “10.10.10.3”); !// Build the the cluster Cluster cluster = clusterBuilder.build(); !// … do work with the cluster … !// Shutdown the cluster cluster.shutdown();

Step 2: connection to a keyspace

// Creating a session against the keyspace you want to interact with Session session = cluster.connect("HJUG"); !// Close up the session session.shutdown()

Example 1: search queries and result set // TODO catch exceptions !// Execute a query using the cluster and iterate over the results ResultSet result = session.execute("SELECT * from USER;"); !// Option 1: iterate over the results Iterator<Row> iter = result.iterator(); while (iter.hasNext()) { Row row = iter.next(); log.info(String.format("Found user w/ username=%s", row.getString(“username”)); } !// Option 2: get all rows and iterate List<Row> rows = result.all(); for (Row row : rows) { log.info(String.format("Found user w/ username=%s", row.getString(“username”)); }

Example 2: inserting data

// TODO catch exceptions !// INSERT a new user (TODO: escape parameters when used this way) session.execute(String.format("INSERT INTO USER (username, location_id) VALUES (%s, %s);", "Jim", “Houston"));

Example 3: prepared statements// TOTO catch exceptions !// Create prepared statement that can be reused throughout the application. // You only need to create it once PreparedStatement usersByLocationStatement = session.prepare(String.format( "SELECT * FROM %s WHERE %s = ?;", USER, "location_id")); !// Create bound statement and bind query parameters BoundStatement boundStatement = new BoundStatement(usersByLocationStatement); !// You can override the default consistent level defined at the cluster level on a per// query basis boundStatement.setConsistencyLevel(ConsistencyLevel.LOCAL_QUORUM); !// Bind parameters boundStatement.bind(“Houston”); !!// Execute bound statement and get results ResultSet resultSet = session.execute(boundStatement);

Example 4: Batch Statement// TODO catch exceptions !// Create a batch statement // Type logged ensures atomicity BatchStatement batchStatement = new BatchStatement(BatchStatement.Type.LOGGED); !// Create bound statement and bind query parameters BoundStatement boundStatement = new BoundStatement(usersByLocationStatement); boundStatement.bind("Houston"); !// Add the bound statements to the batch batchStatement.add(boundStatement); !// ... you can several bound statements to the batch ... !// execute batch session.execute(batchStatement);

Example 5: Synchronous vs Asynchronous

// TODO catch exceptions !// INSERT synchronously a new user (TODO: escape parameters when used this way) session.execute(String.format("INSERT INTO USER (username, location_id) VALUES (%s, %s);", "Jim", “Houston”)); !// INSERT asynchronously a new user (TODO: escape parameters when used this way) session.executeAsync(String.format("INSERT INTO USER (username, location_id) VALUES (%s, %s);", "Jim", “Houston"));

Example 6: batching result sets// We will get <limit> items at offset <x> // offset = x; // limit = y; !// Create bound statement and bind query parameters BoundStatement boundStatement = new BoundStatement(usersByLocationStatement); boundStatement.setFetchSize(limit); boundStatement.bind("Houston"); !!// Execute bound statement and get results ResultSet resultSet = session.execute(boundStatement); !for (int i = 0; i < (offset / limit); i++) { // Fetch the number of pages needed resultSet.fetchMoreResults(); } !Iterator<Row> iter = resultSet.iterator(); for (int i = 0; i < offset; i++) { // Throw away results from earlier pages if (iter.hasNext()) { iter.next(); } } !final List<Row> rows = new ArrayList<>(); for (int i = 0; i < limit; i++) { // Keep results from desired page if (iter.hasNext()) { rows.add(iter.next()); } }

DataStax CQL driver rule #1

Use one Cluster instance per (physical) cluster (per application lifetime)

Cluster• handles queries, connections and their policies!• share cluster instance at the application level!• must be tuned according to C* nodes / cluster

configuration (timeouts, retries etc.)!• Consistency

Example of a more complex Cluster setup// Initialize cluster like in example 1.// You can customize policies before build()clusterBuilder .withQueryOptions( new QueryOptions().setConsistencyLevel( ConsistencyLevel.LOCAL_QUORUM)) .withCompression(Compression.LZ4) .withSocketOptions( // Setting a value of 0 disables read timeouts: we let Cassandra timeout // before the cluster here. new SocketOptions().setConnectTimeoutMillis(1500) .setReadTimeoutMillis(0)) .withLoadBalancingPolicy(new DCAwareRoundRobinPolicy(“us-east”));

Use at most one Session per keyspace, or use a single Session and explicitly specify the keyspace in your

queries

Session• API centered around query execution!• manages per-node connection pools!• avoid large # of sessions or major impact on server

resources (C* side)!• share session instance at the application level!• one session per keyspace at most!• if large number of keyspace: pre-defined number of sessions

if you execute a statement more than once, consider using a PreparedStatement

Prepared statements• prepare once, bind and execute multiple times.!• parsed and prepared on the Cassandra nodes!• cache prepared statement at the application level!• only bound parameters and query are sent to nodes!• performance gains are significant!• prepared statements should be configured to rarely

receive null values when binding parameters

You can reduce the number of network roundtrips and also have atomic operations by using Batches

Batch operations!• single request!• combines multiple data modification statements into a

single logical operation!• atomic operation: all statements pass or fail!• can use combinations of batch and prepared statements!• keep batch statement below the value specified in conf

file: batch_size_warn_threshold_in_kb (5 kb by default)

Thanks!

Slides available @ http://www.slideshare.net/anguenot/cassandra-cql-javahjug20140730 !

@anguenot / ja@iland.com!!

iland: http://www.iland.com!We are hiring in Houston!!

https://www.linkedin.com/company/iland-internet-solutions/careers !!

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