# Ordering, Retries and Dead Letters — Event-Driven Architecture & CQRS

Source: https://www.skillbyai.com/en/event-driven-architecture/m-ordering

> Keep related events in order and handle messages that keep failing.

## Order per key, not globally

Global ordering across a whole system does not scale, and you rarely need it. What you usually need is **ordering per entity**: all events for order `o-1001` in sequence. Log brokers give ordering **within a partition**, so use the entity ID as the **partition key**; queues offer features like Service Bus sessions or SQS FIFO message groups. Retries can break ordering: if event 5 fails and is retried later while event 6 succeeds, the consumer sees 6 before 5. Options include blocking the partition while retrying (simple, but one bad message stalls a key), carrying a **version number** so consumers ignore stale updates, or making handlers order-independent. A **poison message** that always fails (bad data, a bug) must not block forever: after N attempts with backoff, move it to a **dead-letter queue (DLQ)**, alert someone, fix the cause and replay it.

## Ignoring stale updates with a version

If an older event arrives late, the condition fails and nothing changes.

```sql
-- each OrderStatusChanged event carries the aggregate version that produced it
UPDATE order_view
SET    status  = :status,
       version = :event_version
WHERE  order_id = :order_id
  AND  version  < :event_version;   -- stale or duplicate events update 0 rows
```

## A DLQ nobody watches is a black hole

Dead-lettered messages are business transactions that did not happen. Alert on DLQ depth, record why each message failed, and build a tool to replay messages after a fix.

**Quiz:** How do log-based brokers usually guarantee that events for one order are processed in sequence?

- [ ] They order every event globally
- [ ] They retry until order is correct
- [x] Events with the same key go to the same partition, which is ordered
- [ ] Consumers sort events by timestamp

*Answer:* Events with the same key go to the same partition, which is ordered. Partitioning by entity key keeps that entity's events in one ordered partition.