Apache Cassandra

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

• ANY

  : Fastest, least specific.

• QUORUM

  : Majority must acknowledge. Balanced.

• ALL

  : Slowest, safest.

Vector Search (5.0+)

Best Practices (2025)

• 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 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