Practice: Design Notification System

Multi-channel notifications platform that ingests events and delivers messages via email, SMS, push, and in-app with retries, preferences, and observability.

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

  • Ingest notification events via API and webhook with authentication and per-tenant quotas.
  • Multi-channel delivery: email, SMS, push, and in-app; routing by preferences and channel availability.
  • Templating with variables, locale support, and per-tenant overrides.
  • Per-user preferences: opt-in/out per channel, quiet hours, and topic-level subscriptions.
  • Idempotent send semantics using client or generated dedupe keys.
  • Retries with exponential backoff; DLQ for poison messages and manual replays.
  • Delivery status tracking: accepted, sent, delivered, opened/clicked, bounced/failed.
  • Provider abstraction to switch email/SMS/push vendors without code changes.
  • Scheduling (send-at) and priority queues for critical vs marketing traffic.
  • Admin dashboards: templates, preferences, provider health, and analytics.

Non-functional requirements

  • Availability: 99.95%+; single AZ failure must not drop ingest or delivery.
  • Performance: ingest API p99 < 150 ms; enqueue-to-send median < 2 s; OTP send p95 < 1 s.
  • Scalability: independently scale ingest, workers, and provider adapters; sustain bursty spikes.
  • Durability: delivery logs and idempotency keys persisted with backups and retention policies.
  • Consistency: strong for idempotency keys and preference updates; eventual for analytics aggregates (< 1 min).
  • Security: TLS 1.2+; secrets in KMS; signed webhooks and provider callbacks; PII encrypted at rest.
  • Resilience: timeouts/circuit breakers; retries with backoff and DLQ; circuit health-based provider failover.
  • Observability: traces/logs/metrics for queue depth, send latency, success rates, and provider health.
  • Cost: batch and aggregate analytics; tune retries and TTLs; multi-provider cost-based routing.
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Round 1 of 4 · Architecture Design · 60:00 remaining
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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
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System Design Interview

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

Questions

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Design Decisions

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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.

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