Apache Cassandra

Harshit DagaSoftware ConsultantKnoldus Software LLP

Agenda

What is Cassandra

Gossip communication protocol

Cassandra- Data Model

Cassandra- Architecture

Reading/Writing a node

Data consistency

Cassandra

Cassandra is massively scalable schemaless database.

Open source database, licensed under Apache.

Originally, developed by Facebok for inbox search.

Data model based upon Googles BigTable.

Distributed design is based upon Amazon Dynamo.

Promoted massively by Datastax.

Gossip Communication Protocol

Peer to peer communication protocol.

Nodes are arranged in ring format.

Data is replicated to multiple nodes.

Nodes periodically exchange info. they have.

Nodes also exchange their own info.

Each message has its associated version.

No master-slave concept, and hence no single point of failure.

Cassandra- Data Model

Column data is stored as in key/value pair.

Collection of column makes a Row.

Column family is then becomes as collection of all rows.

In RDBMS, each column must have some value else NULL, but not in case of cassandra database.

Cassandra- Data Model

Consider following example,

Now inserting a new row:

Above insertion would not fail.

Cassandra- Data Model

It means, data are stored as multi-dimensional sparse array.

Cassandra- Architecture

A ring has several nodes.

Each node is assigned a Partition value.

Data processing is based on the Partition Key.

When a client makes a request to a node, it becomes the coordinator for that request.

The coordinator determines which node in the ring should process upon that request.

Cassandra- Architecture

Virtual Nodes (Vnodes)Responsible for assigning the partition token range.

Tokens are automatically calculated & assigned to each node.

Cluster re-balancing is done automatically.

Cassandra- Architecture

Which node gets what data is based on the partition key.

Cassandra assigns a hash value to each partition key.

And data gets to a node as per the hash value

Cassandra- Architecture

How write request gets fulfilled:-

Data Replication

Data replicationSimple StrategyUsed for only one cluster

Network Topology StrategyUsed for multiple clusters in multiple data centers.

Writing data in a Node

Write an entry in the commit log

Write data to memtable.

When memtable is full, Store data on disk in SSTables.

SSTables are immutable data structure.

Also has a support for TTL.

Cassandra is the fastest db in concern with the write operation

Reading data from a Node

First, checks the memtable using Bloom filter.

If found, then data is sent as response.

Else, fetch the data from the SSTables.

Cassandra may write many versions of the same row, then how to identify the latest one?

Update/Delete data from Node

Data is not immediately deleted.

It is marked to be deleted/updated in memtables.

This process is called tombstone.

Tombstone, runs at configured interval of time.

During each interval, it collects all the SSTables and updates the marked record and discards the old SSTables.

Data Consistency

Data is not necessarily on every node all the time.

For maintaining consistency, no. of replicas should respond:ONE

QUORUM

ALL

Consistency has major impact on performance.

For strong consistency:R + W > N

References

Oreilly- Cassandra Definitive Guide

https://cassandra.apache.org/doc/latest/

http://docs.datastax.com/en/cassandra/3.0/

Thank You !!