Beyond the Mnemonic: The 3 Triangles of System Design

If you’re preparing for a System Design interview or leading a new project, stop trying to build the "perfect" system. It doesn't exist. Instead, focus on these three tension points where one "win" usually means a "loss" somewhere else. 1. The Data Triangle (CAP Theorem) You’ve heard of CAP, but here is how to explain it simply: You can only pick two at any given time during a network failure. Consistency: Everyone sees the same data at the same time. Availability: The system is always up, even if the data is slightly old. Partition Tolerance: The system keeps working even when communication between nodes breaks. The Real-World Choice: Is your app a Bank (Consistency is non-negotiable) or a Social Feed (Availability matters more than seeing a "like" 2 seconds late)? 2. The Performance Triangle (L-T-P) When a system feels "slow," you need to know which lever to pull: Latency: how long a single request takes (The "Speed" lever). Throughput: how many requests you can handle per second (Th