Practice: Design UPI Payments (PhonePe / Google Pay)

UPI payment app inspired by PhonePe and Google Pay on NPCI's UPI network: a transaction saved as pending under an idempotency key before money moves, switch calls from a queue with timeouts, status served from a cache, UPI ID name lookup, push and SMS notifications, daily reconciliation against NPCI settlement files, and pre-payment risk checks. A teaching design.

Build this system yourself on the canvas below. Add components, connect them, and run traffic simulations to see where your design holds up and where it breaks.

Functional requirements

  • Users can pay a UPI ID or a QR code from a linked bank account.
  • Users see the payee's verified name before confirming.
  • Users can check whether a payment succeeded, failed or is pending.
  • Payer and payee are notified when a payment completes.
  • A retried request never debits twice.
  • Every transaction is reconciled against NPCI's settlement records, and stuck payments are reversed automatically.

Non-functional requirements

  • Pay request latency: p99 < 500ms to accept; result within a few seconds.
  • Status check latency: p99 < 100ms.
  • Scale: 200M monthly users, 6K requests/s at peak, with 5x spikes during festival sales.
  • Availability: 99.95%; no payment is ever lost or duplicated.
  • Every transaction is reconciled within one day.
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Phase 01: Thinking in Systems
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Components

User
CDN
Load Balancer
Server
Cache
Database
Blob Storage
Search Index
Queue
Worker
Rate Limiter
Service
API Gateway
Reverse Proxy
WebSocket Server
Third-party API

Inspector

Notes
Use clear names so your design intent is easy to understand.
Good
Name by business meaning
"Order API", "Restaurant Service"
Avoid
Generic names = zero signal
"Server 1", "API", "Queue"
A short description for each component makes feedback much better.
100%
Start by identifying:
  • Users & entry points
  • APIs & services
  • Databases & storage
  • Traffic flow & scale

Round 1 of 4 Architecture Design

Run a simulation to see results.

Time Remaining
60:00
System Design Interview

What are the core functional requirements?
What are the key non-functional constraints?

Questions

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EVALUATE MODE

Test yourself like it's the real thing.

A structured 4-module evaluation that mirrors how top companies assess system design candidates.

Architecture Design
Draw your system on the canvas. Define components, connections, and data flow.
FR & NFR Requirements
Answer functional and non-functional requirement questions about your design.
MCQ Round
Multiple choice questions testing your depth on the chosen system.
Tradeoff Analysis
Justify your design decisions and defend your architectural tradeoffs.
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Round 2 of 4
MCQ Round
Answer multiple-choice questions based on your design.

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My Evaluations

Your past evaluation sessions

Here’s a simple request flow that follows the expected layer order.

External User
→
Edge CDN → API Gateway → Load Balancer
→
Compute App Servers / Services
→
DataAccess Cache
→
Storage Database / Search Index
→
Async Queue → Worker

Tip: keep arrows moving forward through layers (Edge → Compute → Storage). Avoid sending storage back to compute.

Evaluation Instructions

Read the rules carefully before starting. The test auto-submits on refresh.

Before you start

  • Build your architecture on the canvas. The timer starts when you click Start Evaluation.
  • Don't forget to answer Functional Requirement and Non Functional Requirements.
  • When satisfied with your design, click Next to lock it and view the questions.
  • Please answer final step questions to complete the evaluation.

Dos

  • Do read each question carefully before answering.
  • Do include required components to maximize component coverage.
  • Do save a copy of your design if you want to keep it before submission.

Don'ts

  • Don't refresh or close the tab during an active evaluation — this will auto-submit your answers.
  • Don't switch app modes or open another tab while the evaluation is running.
  • Don't attempt to edit the design after clicking Next; the workspace will be locked.

All the best!!

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