Event-Driven Architecture

Event-driven architecture (EDA) treats state changes as first-class events. When something happens — an order is placed, a payment fails, a user signs up — the service publishes an event describing what occurred. Other services subscribe and update their own state or trigger side effects. No service calls another directly; they react to facts. This creates temporal decoupling: the publisher does not know or care who listens. New consumers can be added without changing the producer. The cost is eventual consistency — you cannot assume all services see the new state immediately. Event sourcing takes EDA further by storing events as the source of truth and rebuilding state by replaying them.

// Order service knows every downstream step
async function placeOrder(data) {
  const order = await db.create(data);
  await inventory.reserve(order);
  await payment.charge(order);
  await analytics.track(order);
  return order;
}

// Order service publishes a fact and stops
async function placeOrder(data) {
  const order = await db.create(data);
  await eventBus.publish('OrderPlaced', {
    orderId: order.id,
    items: order.items,
    total: order.total,
  });
  return order;
}
// Inventory, Payment, Analytics each subscribe on their own