Apache Cassandra

Cassandra is a wide-column store database designed for scalability and high availability without compromising performance. Linear scalability and proven fault-tolerance on commodity hardware or cloud infrastructure make it the perfect platform for mission-critical data.

When to Use

• High Write Throughput: Ingests millions of writes per second.
• Always On: Zero single points of failure. Updates can happen even if nodes are down (Eventual Consistency).
• Multi-Region: Active-Active multi-region replication is built-in.

Quick Start (CQL)

CREATE TABLE users (
  user_id UUID PRIMARY KEY,
  name text,
  email text
);

INSERT INTO users (user_id, name) VALUES (uuid(), 'Alice');

Core Concepts

Partition Key & Clustering Key

• Partition Key: Determines which node holds the data.
• Clustering Key: Sorts data within the partition on disk.

Tunable Consistency

You choose consistency level per query.

• ANY: Fastest, least specific.
• QUORUM: Majority must acknowledge. Balanced.
• ALL: Slowest, safest.

Vector Search (5.0+)

Native support for Vector Search (ANN) allows using Cassandra as a Vector DB for AI apps.

Best Practices (2025)

Do:

• Query by Partition Key: Always. Scans are prohibited in production.
• Use SAI (Storage Attached Indexes): New in 5.0. Better than old secondary indexes.
• Denormalize: Optimize schema for Reads. It is okay to duplicate data into 3 tables to satisfy 3 different query patterns.

Don’t:

• Don’t use distributed joins: Cassandra doesn’t do joins. Join in the app.
• Don’t use large partitions: Keep partitions under 100MB to avoid compaction issues.

References

• Apache Cassandra Documentation