---
title: Exactly-Once, CQRS, and Event Sourcing Replay on AWS
description: Exactly-once is a myth end-to-end—but idempotent consumers plus event stores get you close. CQRS read models on DynamoDB streams, Kinesis, and EventBridge replay semantics.
url: https://www.factualminds.com/blog/exactly-once-cqrs-event-sourcing-replay-aws/
datePublished: 2026-06-12T00:00:00.000Z
dateModified: 2026-06-12T00:00:00.000Z
author: Palaniappan P
category: Cloud Architecture
tags: engineering-guide, distributed-systems, event-sourcing, aws
---

# Exactly-Once, CQRS, and Event Sourcing Replay on AWS

> Exactly-once is a myth end-to-end—but idempotent consumers plus event stores get you close. CQRS read models on DynamoDB streams, Kinesis, and EventBridge replay semantics.

**June 2026 truth:** AWS messaging is **at-least-once** unless you engineer Kafka EOS or FIFO dedup—and **your database write** must still be idempotent.

## Layers of “exactly-once”

1. **Broker** — Kafka transactions / SQS FIFO dedup ID
2. **Consumer** — idempotency key store (DynamoDB)
3. **Projection** — CQRS read model version column

## CQRS + event sourcing on AWS

| Piece        | Service                                    |
| ------------ | ------------------------------------------ |
| Event log    | Kinesis / MSK / EventBridge archive        |
| Command side | Lambda + Aurora                            |
| Read model   | DynamoDB / OpenSearch                      |
| Replay       | New consumer group / replay archive to SQS |

**Replay strategies:** rebuild read model from offset 0; use **version vectors** / optimistic locking on projections.

## What to do this week

1. Add `event_id` unique constraint on projection table.
2. Test replay in staging—measure duplicate suppression.
3. Read [microservices patterns guide](/blog/microservices-design-patterns-aws-production-guide-2026/) for outbox.

## What this guide doesn't cover

Saga orchestration—canonical microservices post.

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*Source: https://www.factualminds.com/blog/exactly-once-cqrs-event-sourcing-replay-aws/*