Coursera’s Adoption of Cassandra

Biography

Daniel Chia @DanielJHChia

Software Engineer, Infrastructure Team

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1 Introduction

2 Want We Want From Our Database

3 MySQL Limitations

4 Cassandra - What and Why

5 Looking Back

Coursera

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Web iOS Android

Database Wants

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Consistently Fast Latencies

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Availability

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Scalability

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Other Niceties

• Operational ease • Multi-region capability

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Coursera Tech Stack

• 100% AWS • MySQL + Cassandra • Service-oriented

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RDS Challenges

• Normalized data model ⇒ Unpredictable query performance

• Scaling by sharding not ideal

• Single master limitation

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C*• Columnar model • Tunable consistency • Fast • Horizontally scalable • Great community

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Looking Back

Cassandra - Initial Pain Points

• Can’t execute arbitrary queries • Filtering, sorting, etc.

• Can’t be abused as an OLAP database

• Worries about ‘eventual’ consistency

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SQL ⇒ NoSQL Mindset Shift

• Build in-house Cassandra expertise

• Data modeling still important

• Know your queries

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Cassandra ≠ [database XYZ]

“But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is stupid.”

-Albert Einstein

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Enrollment Example

• Learners enroll into a course • learner (many-to-many) course

• Need to track this membership

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MySQL

CREATE TABLE `courses_learners` (

`id` INT(11) NOT NULL auto_increment,

`course_id` INT(11) NOT NULL,

`learner_id` INT(11) NOT NULL,

PRIMARY KEY (`id`),

UNIQUE KEY `c_l` (`learner_id`, `course_id`),

CONSTRAINT `ref1` FOREIGN KEY (`course_id`)

CONSTRAINT `ref2` FOREIGN KEY (`learner_id`)

)

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MySQL

CREATE TABLE `courses_learners` (

`id` INT(11) NOT NULL auto_increment,

`course_id` INT(11) NOT NULL,

`learner_id` INT(11) NOT NULL,

PRIMARY KEY (`id`),

UNIQUE KEY `c_l` (`learner_id`, `course_id`),

CONSTRAINT `ref1` FOREIGN KEY (`course_id`)

CONSTRAINT `ref2` FOREIGN KEY (`learner_id`)

)

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MySQL

CREATE TABLE `courses_learners` (

`id` INT(11) NOT NULL auto_increment,

`course_id` INT(11) NOT NULL,

`learner_id` INT(11) NOT NULL,

PRIMARY KEY (`id`),

UNIQUE KEY `c_l` (`learner_id`, `course_id`),

CONSTRAINT `ref1` FOREIGN KEY (`course_id`)

CONSTRAINT `ref2` FOREIGN KEY (`learner_id`)

)

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MySQL

CREATE TABLE `courses_learners` (

`id` INT(11) NOT NULL auto_increment,

`course_id` INT(11) NOT NULL,

`learner_id` INT(11) NOT NULL,

PRIMARY KEY (`id`),

UNIQUE KEY `c_l` (`learner_id`, `course_id`),

CONSTRAINT `ref1` FOREIGN KEY (`course_id`)

CONSTRAINT `ref2` FOREIGN KEY (`learner_id`)

)

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Cassandra

CREATE TABLE courses_by_learner (

learner_id uuid,

course_id uuid,

PRIMARY KEY (learner_id, course_id)

)

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Helpful Things

• Data modeling consulting

• Monitoring

• Data access layer for common use cases

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Gotchas

• Lots of truly ad-hoc queries is hard • Don’t use C* directly to explore your data. (Spark?)

• Sorting, filtering can be hard • Consider Solr / ElasticSearch • Or even MySQL depending on load / importance

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Thank you