System Design Interview Prep: A Senior+ Engineer's Guide

System design interviews aren't testing your ability to design Twitter. They're testing whether you can frame an open problem and trade off cleanly.

What's actually being tested

• Scoping: can you turn a vague prompt into a concrete spec?
• Estimation: can you size traffic, storage, throughput within an order of magnitude?
• Tradeoffs: can you justify caching strategy, consistency model, sharding key — out loud?
• Communication: can the interviewer follow your diagram without you re-explaining?

The reusable template

1. Clarify scope (3–4 questions, 2 minutes).
2. Functional + non-functional requirements (1 minute).
3. Back-of-envelope estimation (2 minutes).
4. API contract (3 minutes).
5. Data model + storage choice (5 minutes).
6. High-level diagram (5 minutes).
7. Deep-dive on the riskiest component (10–15 minutes).
8. Tradeoffs + followups (3 minutes).

6 prompts to drill

• URL shortener (write-heavy, cache, hash collision).
• Twitter feed (fanout-on-write vs read).
• Chat / messaging (long-poll vs WebSocket, ordering).
• Ride-matching (geo-index, dispatch).
• Rate limiter (token bucket, sliding window).
• Search autocomplete (trie, top-k, freshness).

Generate variants of these on demand with the system design interview prep tool.

Tradeoff vocabulary

• CAP / PACELC, eventual vs strong consistency.
• SQL vs document vs wide-column vs key-value.
• Cache strategies (write-through / write-back / cache-aside).
• Sharding by user vs by key range vs by hash.
• Sync vs async / message queue vs pub/sub.

A 3-week study plan

• Week 1: template + 2 prompts, watch ByteByteGo for each.
• Week 2: 4 more prompts, 2 mock sessions with a senior peer.
• Week 3: mixed-prompt reps, 2 deep-dives per prompt, 1 ML system design if relevant.

Pair with the SWE interview prep playbook.