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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Writing an event handler that assumes each event will be delivered and processed exactly once

// Wrong: assumes exactly-once delivery, will eventually double-process
async function onPaymentRequested(event) { await chargeCard(event.amount); }

// Correct: idempotent, safe under duplicate delivery
async function onPaymentRequested(event) {
  if (await alreadyProcessed(event.id)) return;
  await chargeCard(event.amount);
}

The Solution //

Most message brokers provide at-least-once delivery guarantees, not exactly-once, meaning duplicate delivery of the same event is an expected, normal occurrence, not an edge case. A handler performing a non-idempotent action (like charging a payment) on every delivery will eventually produce incorrect results, such as a duplicate charge.

The Error //

Attempting a single ACID transaction spanning multiple services' separate databases

// Not possible: a single transaction can't span separate service databases
// BEGIN TRANSACTION; UPDATE inventory_db...; UPDATE payment_db...; COMMIT;

// Correct: Saga pattern, local transactions + compensating actions
reserveInventory() -> compensate: releaseInventory()

The Solution //

A traditional ACID transaction cannot span multiple independent databases owned by different services — this is a fundamental constraint of a distributed system, not a technical limitation to work around. The Saga pattern (a sequence of local transactions with explicit compensating actions for rollback) is the standard way to coordinate a multi-step process across service boundaries instead.