Your e-commerce platform just crossed 2M daily active users. 70% are in the US, 30% in Europe.
Latency complaints are piling up from European users — 380ms average round-trip to your US-East region. Support tickets are up 40%. Black Friday is in 6 weeks.
Your infrastructure: single AWS us-east-1 region, RDS PostgreSQL (primary), Redis cache, 12 microservices behind an API Gateway.
You need to get European latency under 80ms. The engineering team is debating four approaches.
Here’s your constraint: you cannot afford a full database rewrite, and you need this shipped before Black Friday.
A) Active-Active multi-region — deploy the full stack in eu-west-1, use a distributed database (CockroachDB or Aurora Global), route users to the nearest region. Writes go to both regions simultaneously.
B) Active-Passive with read replicas — keep us-east-1 as primary, spin up eu-west-1 as a hot standby with read replicas. European reads go local, writes still go to US. Failover in minutes if US goes down.
C) CDN + Edge caching — keep the single region, push static assets and cacheable API responses to CloudFront edge nodes in Europe. No database changes.
D) Active-Active with eventual consistency — deploy full stack in both regions, allow each region to own its writes, sync asynchronously. Accept that a European user might see a US write 200ms late.
Three of these are real patterns production teams use. Only one actually solves the problem you have — under your constraints, before Black Friday.
Pick one — A, B, C, or D — and tell me why. Full breakdown in the comments.
If your team has been in this exact debate, share this with them. The tradeoffs are what matter.
Drop your answer 👇
#30DaysOfSystemDesign #SystemDesign #DistributedSystems #SoftwareArchitecture

Joud Awad

A system design challenge presents a real-world scenario: an e-commerce platform with 2M daily active users experiencing 380ms latency for European users needs to get under 80ms before Black Friday, with no time for a full database rewrite. Four architectural approaches are presented for debate: Active-Active multi-region with distributed DB, Active-Passive with read replicas, CDN+Edge caching only, and Active-Active with eventual consistency. Readers are asked to pick the best solution given the constraints and explain their reasoning.

31/60 Days System Design Questions!

Why B wins:
The constraint is the answer. You can’t rewrite the database, and you have 6 weeks.
European users are complaining about read latency — browsing products, checking order status, loading their profile. Those are reads. An RDS read replica in eu-west-1 costs you 1–2 days of infra work and immediately moves European reads to ~15ms instead of 380ms. Writes still go to us-east-1 (acceptable — users tolerate slightly higher write latency for checkout). Replication lag is typically under 100ms.
Active-passive also gives you a real failover story: US goes down → promote EU replica in minutes.
<img src="https://media.daily.dev/image/upload/s--cAPi_azE--/f_auto/v1780767284/ugc/content_32bef9e1-4efe-40fa-af69-0c54609acbd8?_a=BAMAMiWQ0" alt="ChatGPT Image Jun 6, 2026, 08_33_28 PM.png">

The architecture lesson:
Multi-region is a spectrum, not a single decision:
<pre><code>| Pattern | Complexity | What it solves

| ----------------------------- | ------------- | -------------------------------- |

| Read replicas | Low | Read latency in other regions

| Active-passive | Medium | Read latency + failover story

| Active-active (consistent) | Very high | Write latency + fault isolation

| Active-active (eventual) | Highest | Max scale, needs conflict resolution
</code></pre>
Most teams move left → right over years. The mistake is jumping to the right side before you’ve mastered the left.
Name your actual constraint. The architecture follows.

Why D is dangerous (Active-Active + eventual consistency):
Most honest version of multi-region — and the most dangerous to deploy without prep. Eventual consistency across regions means a European user places an order and your US region sees a different inventory count for 200ms. For limited stock = oversell. For checkout = duplicate charges. Conflict resolution is a product problem disguised as an infra problem.

Why C is wrong (CDN + Edge caching):
CDN solves static asset latency — images, JS, CSS. It does nothing for dynamic reads (user cart, order history, account data) and zero for write latency. It’s a layer you add on top of a real solution, not instead of one. Partial credit, not the answer.

Why A is the trap (Active-Active + distributed DB):
This is the “correct at scale” answer that kills teams before Black Friday. Migrating from RDS PostgreSQL to CockroachDB or Aurora Global under a 6-week deadline is a multi-quarter project — distributed transactions, global clock skew, new failure modes to test. Right architecture for the long term. Wrong answer for the problem as stated.