---
title: CRDTs and Eventual Consistency Anti-Patterns on AWS
description: Last-write-wins is not a CRDT—it is how Global Tables lose cart merges. When to use counters, OR-Sets, and conflict-free merges vs when to keep a single Aurora writer.
url: https://www.factualminds.com/blog/crdts-eventual-consistency-anti-patterns-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, aws, dynamodb
---

# CRDTs and Eventual Consistency Anti-Patterns on AWS

> Last-write-wins is not a CRDT—it is how Global Tables lose cart merges. When to use counters, OR-Sets, and conflict-free merges vs when to keep a single Aurora writer.

**June 2026**: Multi-Region active-active without a merge strategy is how **eventual consistency anti-patterns** ship—users see flickering state, not “high availability.”

## Anti-patterns

1. **RWM on same record across Regions** without merge → lost updates
2. **Cache-aside as source of truth** across Regions
3. **SQS at-least-once** without idempotent handlers (looks like consistency bug)

## CRDTs (when they help)

Conflict-free replicated data types (counters, OR-Sets, LWW-Element-Set with care) suit **collaboration** and **device sync**. On AWS, implement in app layer or use services with documented merge (DynamoDB counters are not full CRDTs).

**Opinionated take:** Prefer **single-writer Aurora Global** for financial aggregates; use CRDT-style merges only when product accepts semantic merge rules in writing.

## AWS map

| Pattern             | Service                                    |
| ------------------- | ------------------------------------------ |
| Active-active items | DynamoDB Global Tables + version attribute |
| Session sync        | ElastiCache + sticky + short TTL           |
| Ordered workflow    | SQS FIFO + dedup                           |

## What to do this week

1. Document merge function for each Global Table entity.
2. Add integration test simulating concurrent cross-Region writes.
3. Metric: conflict resolution count per hour.

## What this guide doesn't cover

Distributed locks—next guide in track.

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*Source: https://www.factualminds.com/blog/crdts-eventual-consistency-anti-patterns-aws/*