Building a Cassandra based application from scratch

Patrick McFadinCassandra Summit 2012

#cassandra12

This is me

• Chief Architect at Hobsons– Hobsons is an education services company. More

here: www.hobsons.com• Cassandra user since .7• Follow me here: @PatrickMcFadin

Goals

• Take a new concept• What’s the data model?!?!• Some sample code• You get homework! (If you want)

Here’s the plan

• Conceptualize a new application• Identify the entity tables• Identify query tables• Code. Rinse. Repeat.• Deploy• …• Profit!

* I’ll be using the term Tables which is equivalent to Column Families

Start with a concept

*Cat drawing by goodrob13 on Flickr

Video Sharing Website

Break down the features

• Post a video*• View a video• Add a comment• Rate a video• Tag a video

* Not talking about transcoding! Check out zencoder.com, it’s pretty sweet.

Create Entity Tables

Basic storage unit

Users

• Similar to a RDBMS table. Fairly fixed columns • Username is unique• Use secondary indexes on firstname and lastname for lookup• Adding columns with Cassandra is super easy

CREATE TABLE users ( username varchar PRIMARY KEY, firstname varchar, lastname varchar, password varchar);

Users: The set codestatic void setUser(User user, Keyspace keyspace) { // Create a mutator that allows you to talk to casssandra Mutator<String> mutator = HFactory.createMutator(keyspace, stringSerializer);

try {

// Use the mutator to insert data into our table mutator.addInsertion(user.getUsername(), "users", HFactory.createStringColumn("firstname", user.getFirstname())); mutator.addInsertion(user.getUsername(), "users”, HFactory.createStringColumn("lastname", user.getLastname())); mutator.addInsertion(user.getUsername(), "users", HFactory.createStringColumn("password", user.getPassword()));

// Once the mutator is ready, execute on cassandra mutator.execute();

} catch (HectorException he) { he.printStackTrace(); }}

You can implement the get…

Videos

• Use a UUID as a row key for uniqueness• Allows for same video names• Tags should be stored in some sort of delimited format• Index on username may not be the best plan

CREATE TABLE videos ( videoid uuid PRIMARY KEY, videoname varchar, username varchar, description varchar, tags varchar);

Videos: The get codestatic Video getVideoByUUID(UUID videoId, Keyspace keyspace){ Video video = new Video(); //Create a slice query. We'll be getting specific column names SliceQuery<UUID, String, String> sliceQuery = HFactory.createSliceQuery(keyspace, uuidSerializer, stringSerializer, stringSerializer); sliceQuery.setColumnFamily("videos"); sliceQuery.setKey(videoId); sliceQuery.setColumnNames("videoname","username","description","tags");

// Execute the query and get the list of columns ColumnSlice<String,String> result = sliceQuery.execute().get(); // Get each column by name and add them to our video object video.setVideoName(result.getColumnByName("videoname").getValue()); video.setUsername(result.getColumnByName("username").getValue()); video.setDescription(result.getColumnByName("description").getValue()); video.setTags(result.getColumnByName("tags").getValue().split(",")); return video;}

You can implement the set…

Comments

Time Order

• Videos have many comments• Use Composite Columns to store user and time• Value of each column is the text of the comment• Order is as inserted• Use getSlice() to pull some or all of the comments

CREATE TABLE comments ( videoid uuid PRIMARY KEY comment varchar);

Rating a video

CREATE TABLE video_rating ( videoid uuid PRIMARY KEY, rating_counter counter, rating_total counter);*

* Only valid in CQL 3+

• Use counter for single call update• rating_count is how many ratings were given• rating_total is the sum of rating• Ex: rating_count = 5, rating_total = 23, avg rating = 23/5 = 4.6

Video Event

CREATE TABLE video_event ( videoid_username varchar PRIMARY KEY, event varchar);

• Track viewing events• Combine Video ID and Username for a unique row• Stop time can be used to pick up where they left off• Great for usage analytics later

Time Order

Create Query Tables

Indexes to support fast lookups

Lookup Video By Username

CREATE TABLE username_video_index ( username varchar PRIMARY KEY, videoid_timestamp varchar);

• Username is unique• One column for each new video uploaded• Column slice for time span. From x to y• VideoId is added the same time a Video record is added

Videos by Tag

CREATE TABLE tag_index ( tag varchar PRIMARY KEY, videoid varchar);

• Tag is unique regardless of video• Great for “List videos with X tag”• Tags have to be updated in Video and Tag at the same time• Index integrity is maintained in app logic

Deployment strategies

• Measure your risk– Replication factor?– Multi-datacenter?– Cost?

• Performance– Today != tomorrow. Scale when needed– Have a expansion plan ready

Wrap up

• Similar data model process to RDBMS… to start

• Query -> Index table• Don’t be afraid to write in multiple tables at

once• Bonus points: Hadoop and Solr!

Go play!

• Go to: http://github.com/pmcfadin• Look for projects with cassandra12• Clone or fork my examples• Implement stubbed methods• Send me your solutions: pmcfadin@gmail.com• Follow me for updates: @PatrickMcFadin

Thank You!

Connect with me at @PatrickMcFadinOr linkedIn

Conference tag #cassandra12