The 5-Step System Design Interview Framework

A system design interview isn't a test of whether you know the perfect architecture; it's a test of how you navigate ambiguity, communicate trade-offs, and structure your thoughts. Based on the best strategies shared on Reddit and Blind, here is a proven 5-step framework.

Step 1: Requirements Clarification (5 mins)

Never jump straight into drawing boxes.

Functional Requirements: What must the system do? (e.g., "Users can upload photos", "Users can follow others").

Non-Functional Requirements: What are the system qualities? (e.g., "High availability", "P99 latency < 200ms", "Eventual consistency is okay").

Step 2: Back-of-the-Envelope Estimation (5 mins)

Prove you understand the scale.

Calculate Write QPS and Read QPS.

Estimate storage needs for 5 years.

Estimate bandwidth requirements.

Step 3: API Design (5 mins)

Define the contract between the client and server.

Example: `POST /api/v1/tweets` -> Payload: `{ user_id, text, media_urls }`.

Step 4: High-Level Architecture (15 mins)

Draw the big picture.

Start with Client -> Load Balancer -> Web Servers -> Database.

Identify the read path and the write path.

Step 5: Deep Dive & Bottlenecks (15 mins)

This is where you earn the senior title. Find the single points of failure.

*Interviewer*: "What if a celebrity tweets and they have 50M followers?"

*You*: "Our standard fan-out-on-write will crash. We need to implement a hybrid approach: fan-out-on-read for celebrities."

Stick to this framework, manage your time, and communicate your trade-offs clearly.