Cassandra Core Concepts andDesign Internals

Cassandra Core Concepts and Design Internals

at

New Delhi Cassandra Users Meetup – November

2014

By: Salil Kalia

We’re going to talk about:

1. What is Cassandra?

2. High Level Architecture

3. Data Modeling

4. Write Path

5. Read Path

6. Tools

7. Q/A

A Database:

✓ Highly available

✓ Fully distributed, with no single point of failure

✓ Free & open source, with deep developer support

✓ Highly performing with near-linearhorizontal scaling

✓ Replicated & durable

What is Cassandra ?

Elastic Scalability

Distributed

Decentralized

FaultToleran

tColumn Oriented

TunableConsistenc

y

Highly available

KEY FEATURES

Open Source

Cassandra – Features

Google Big Table

Amazon Dynamo DB

[Facebook] Cassandra

Cassandra Evolution

✓ Ring based data distribution

✓ Only one type of Server

✓ Highly distributed

✓ All nodes hold data

✓ All nodes answer queries

✓ All nodes are replicas

✓ In-built Multi DC

✓ In-built Snitch feature

High Level Architecture

✓ Nodes and Virtual nodes

✓ Primary & Secondary range

✓ Partition Key (Hash)

✓ Partitioner

✓ Client & Coordinator

✓ Replication Factor (RF)

✓ Consistency Level (CL)

Few Common Terms

Magic Formula

Write CL + Read CL >RF

Immediate Consistency

Keyspace

Table

Partition

Row

Column

Data Modeling

✓ Like an RDBMS, Cassandra uses a Table to store data

✓ Partitions within tables

✓ Rows within partitions (or a single row)

✓ CQL to create tables & query data

✓ Partition keys determine where a partition is found

✓ Clustering keys determine ordering of rows within a partition

Data Modeling

name age occupation

Salil 32 Tech Lead

Vishal 25 Software Engineer

Akshay 45 Actor

Sheri 29 Singer

cqlsh:demo> create table user (name text primary key,age int, occupation text);

cqlsh:demo> select * from user WHERE name = ’Vishal'

Example: Single Row Partition

✓ User identified by name (PK)

✓ Single row per partition✓ RDBMS like structure

Video_id Comment_id Comment

5 1 Nice pic

5 2 Which place?

5 3 lol

6 4 Great!

cqlsh:demo> create tablecomment (video_id int, comment_id int, comment text, primary key ( video_id, comment_id));

cqlsh:demo> select * from comment WHERE Video_id=5;

Example: Multiple Rows Partition

• Video_id - partition key• comment_id – cluster key

* In real world, use UUIDs instead of int for PK

Query before data modeling

Denormalize the

data Create multiple views into your data

Cassandra is built for faster

writes Better – as few reads as possible

Data Modeling – Best practices

CommitLog – append only logs

Memtables – In memory table

SSTables – created after the data flushes to disk

Compaction – process to merge SSTables

Key components of the Write Path

✓ Memtables – In memory table✓ Row Cache – In memory cache stores recent read

rows✓ Bloom Filters – reports if a partition key may found in its corresponding SSTable

✓ Key Caches – in memory (on heap)

✓✓

Partition Summaries – in memory (on heap)

Partition Indexes – on disk

✓ SSTables – on disk

Key components of the Read Path

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