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    January 4, 2026•16 min read•Technical Guide

    System Design Cheat Sheet: The Framework That Got Me Through 5 Onsites

    System design felt impossible until I found a framework. Here's the exact template I use for every interview.

    System design interview cheat sheet and framework

    "Design Twitter." "Design Uber." "Design a URL shortener." These open-ended questions used to terrify me. Where do you even start? What do they actually want?

    After failing a few system design interviews and reading everything I could find, I developed a framework that works. It's not about knowing every possible architecture—it's about having a structured approach that guides the conversation.

    The 5-Step Framework

    Follow This Every Time

    1
    Requirements (5 min)

    Clarify functional & non-functional requirements. Ask questions.

    2
    Estimations (5 min)

    Back-of-envelope math. Users, QPS, storage, bandwidth.

    3
    High-Level Design (10 min)

    Draw the main components. API design. Data flow.

    4
    Deep Dive (15 min)

    Dive into critical components. Database schema. Caching. Scaling.

    5
    Wrap-up (5 min)

    Discuss trade-offs. Bottlenecks. Future improvements.

    Step 1: Requirements Gathering

    Questions to Ask

    Functional

    • • What are the core features?
    • • Who are the users?
    • • What are the main use cases?
    • • Any features we should skip?

    Non-Functional

    • • How many users/DAU?
    • • Availability requirements?
    • • Latency expectations?
    • • Consistency vs. availability?

    Step 2: Back-of-Envelope Estimations

    Key Numbers to Calculate

    QPS (Queries Per Second)

    DAU × actions per day ÷ 86,400 seconds

    Example: 100M DAU × 10 actions = 1B/day ≈ 12,000 QPS

    Storage

    Object size × objects per day × retention period

    Example: 1KB × 1B/day × 365 days = 365 TB/year

    Bandwidth

    QPS × object size

    Example: 12,000 QPS × 1KB = 12 MB/s

    Useful Conversions

    • • 1 day = 86,400 seconds ≈ 100K seconds
    • • 1 million/day ≈ 12/second
    • • 1 billion/day ≈ 12,000/second

    Essential Components Cheat Sheet

    Load Balancer

    Distributes traffic across servers. Use for horizontal scaling.

    When: Multiple servers, high availability needs

    Types: Round robin, least connections, IP hash, weighted

    Caching

    Reduce latency and database load by storing frequent data in memory.

    Tools: Redis, Memcached

    Strategies: Cache-aside, write-through, write-behind

    Invalidation: TTL, on update, LRU eviction

    Database Selection

    SQL (PostgreSQL, MySQL)

    • • Structured data, relationships
    • • ACID transactions needed
    • • Complex queries

    NoSQL (MongoDB, Cassandra)

    • • Flexible schema
    • • High write throughput
    • • Horizontal scaling priority

    CDN (Content Delivery Network)

    Serve static content from edge locations close to users.

    When: Static assets (images, videos, JS/CSS), global users

    Examples: CloudFront, Cloudflare, Akamai

    Database Scaling Patterns

    Replication

    Master-slave. Writes to master, reads from replicas. Good for read-heavy workloads.

    Sharding

    Split data across multiple databases. Shard by user_id, geographic region, etc.

    Denormalization

    Duplicate data to avoid joins. Trade storage for read performance.

    Common Design Patterns

    Message Queue

    Decouple services, handle async tasks. Kafka, RabbitMQ, SQS.

    Pub/Sub

    One-to-many messaging. Real-time updates, notifications.

    Rate Limiting

    Token bucket, leaky bucket, sliding window. Protect against abuse.

    Consistent Hashing

    Distribute data across nodes. Minimize redistribution when nodes change.

    Circuit Breaker

    Prevent cascade failures. Stop calling failing services temporarily.

    Example: Design a URL Shortener

    Quick Solution Outline

    Requirements

    • • Shorten URLs, redirect to original
    • • 100M URLs created/day, 1B redirects/day
    • • Low latency (<100ms), high availability

    Estimations

    • • Write QPS: 100M/day ≈ 1,200/s
    • • Read QPS: 1B/day ≈ 12,000/s (10:1 read:write)
    • • Storage: 500 bytes × 100M × 365 × 5 years ≈ 100TB

    Key Design Decisions

    • • Short code: Base62 encoding, 7 chars = 3.5T combinations
    • • Database: NoSQL (DynamoDB/Cassandra) for high writes
    • • Caching: Redis for hot URLs
    • • ID generation: Twitter Snowflake or pre-generated IDs

    Practice Articulating Your Design

    System design is about communication, not just architecture. LastRound AI helps you practice explaining your designs clearly.

    The Bottom Line

    System design interviews are learnable. Follow the framework, practice common problems, and focus on trade-offs. You don't need to know every possible technology—you need to reason about systems effectively.

    Print this cheat sheet. Review before interviews. And remember: the interviewer wants to see how you think, not a perfect answer.

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