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Why are some apps fast and rock-solid while others crawl and crash? Good system design. Let us explore the core principles that guide every decision.

First, scalability: when your app goes viral, it must grow without crashing. Think adding lanes to a highway as traffic climbs, not widening the old ones.

Scalability matters because it lets you absorb more users and data while staying fast and responsive — so success never turns into an outage.

Next, reliability: your system does the right thing, consistently and predictably, with minimal downtime — and recovers fast even when parts fail.

Reliability rests on fault tolerance (keep running when components fail), data integrity (no corruption), and high availability (almost always reachable).

Maintainability is how easily you manage the system over its life. Software is never finished — a maintainable one is easy to understand, change, and extend.

You get maintainability through readability, modularity (independent LEGO-like parts), and clear documentation — so changes are safe and onboarding is fast.

Finally, efficiency: doing the job with the least CPU, memory, network, and storage. It is about getting the most out of your hardware without waste.

Efficiency pays off in lower infra bills, faster response times, and a greener footprint — though you often trade a little of it for maintainability or scale.

Now that you've learned about the core principles, let's see if you can identify them!

You learned the four pillars of system design: scalability for growth, reliability for correctness, maintainability for change, and efficiency for resources.