Cassandra

Theory

Apache Cassandra is a distributed wide-column store (partition key + clustering columns). Data is partitioned with consistent hashing across nodes.

Tunable consistency per query: ONE, QUORUM, ALL — trade latency vs correctness. Lightweight transactions (LWT) use Paxos for compare-and-set.

Replication Factor (RF) copies data across nodes; NetworkTopologyStrategy places replicas per datacenter for local reads.

Writes go to commit log then memtable; SSTables flush to disk with periodic compaction (size-tiered, leveled).

CQL resembles SQL but multi-table JOINs are absent — model denormalized tables per query pattern (one table per query).

Netflix and Apple use Cassandra for time-series-ish, high-ingest workloads where Postgres would struggle to scale writes horizontally.

Operations: nodetool repair, rebuild, decommission; watch compaction lag, pending hints, and p99 read latency.

Architecture Diagram

  Client (CQL)
      |
  Coordinator node
      |
  +---+---+---+
  v   v   v   v
  N1  N2  N3  N4   (ring)
  RF=3 replicas per DC

Examples

CREATE TABLE orders_by_user (
  user_id uuid,
  order_id timeuuid,
  total decimal,
  PRIMARY KEY (user_id, order_id)
) WITH CLUSTERING ORDER BY (order_id DESC);

SELECT * FROM orders_by_user WHERE user_id = ? LIMIT 20;

INSERT INTO events (bucket, ts, data) VALUES (?, ?, ?) USING QUORUM;

Key Concepts

Interview Questions

Best Practices

• Version-control all Cassandra configuration and review changes like application code.
• Automate validation (lint, policy checks) in CI before production apply.
• Document ownership, on-call runbooks, and rollback for every production change.
• Use least privilege for credentials and rotate secrets regularly.
• Measure before/after: error rate, deploy time, cost, or toil hours saved.

Common Mistakes

• Adopting Cassandra without a written problem statement or success metric.
• Skipping staging that mirrors production networking and data volume.
• No rollback path — forward-only changes during incidents.
• Alert noise without actionable runbooks — on-call ignores pages.

Cheat Sheet

Practical Exercises