An interactive study companion for Martin Kleppmann's classic on reliable, scalable, and maintainable systems.
End-to-end guarantees, constraints, and verification — correctness requires deliberate design.
Correctness is not accidental. It requires end-to-end reasoning: the database can enforce constraints, but the application must define invariants. Auditing, idempotency, and deterministic replay help verify that derived systems match the source of truth.
Stripe's API requires Idempotency-Key headers so network retries never double-charge. PostgreSQL UNIQUE and FOREIGN KEY constraints catch bugs at the database layer. In microservices, the SAGA pattern coordinates multi-service transactions with compensating actions (cancel shipment if payment fails). OpenTelemetry traces verify that an event published to Kafka was processed by every downstream consumer.
Every charge API call accepts an Idempotency-Key header — network retries return the original result instead of double-charging. Correctness is enforced end-to-end, not just at the TCP layer.
// Stripe-style idempotency — safe retries over unreliable networks
async function chargeCard(req: Request) {
const idempotencyKey = req.headers.get("Idempotency-Key");
const existing = await redis.get(`idem:${idempotencyKey}`);
if (existing) return JSON.parse(existing);
const result = await paymentService.charge(/* ... */);
await redis.setex(`idem:${idempotencyKey}`, 86400, JSON.stringify(result));
return result;
}