Question 1

What is system design? Give the one-line definition.

Accepted Answer

System design is the process of defining the architecture, components and interactions of a system so that it satisfies its requirements. Compressed: System = Components + Common Goal. Client, server, database, cache, queue each has one job, and all of them work toward one objective: serve user requests and data reliably, at scale.

Question 2

Walk through the AlienBank analogy. Which problem maps to which system design concept?

Accepted Answer

A bank with one cash counter (customer, counter, cashier, receipt system, database) works fine for a few customers. As usage grows, five problems appear, each with a mapping:

- Each customer takes ~10 min: Fix at the bank - Train the cashier to count and type faster (~5 min). System concept - Better code quality, Low-Level Design (LLD).
- Customer count keeps rising: Fix at the bank - Bigger desk, cash-counting machine, pre-filled forms (~3 min). System concept - Vertical scaling (more CPU, RAM, storage on one server).
- Waiting time high at peak: Fix at the bank - Add more counters and cashiers. System concept - Horizontal scaling (more server instances).
- Counter 1 and Counter 2 have different data: Fix at the bank - One shared record book for all counters. System concept - Centralized database, synchronization.
- Some counters overloaded, others idle: Fix at the bank - A middleman checks load and directs customers. System concept - Load balancer.

Question 3

Vertical scaling vs horizontal scaling. When do you move from one to the other?

Accepted Answer

Vertical: upgrade the existing machine (CPU, RAM, storage). Simple, no application changes, but expensive and it hits a physical ceiling.

Horizontal: add more machines and distribute the work. Near-unlimited scale, better fault tolerance and availability, but now you need a load balancer, shared or synchronized data, and joins and transactions across machines get harder.

SAY THIS:
Start vertical because it is cheap in effort; go horizontal when cost or the hardware ceiling bites, and design for it (stateless servers, central data) before you need it.

Question 4

Why does horizontal scaling immediately create a data-consistency problem, and what fixes it?

Accepted Answer

Two counters each keeping their own ledger will drift: a withdrawal at one is unknown to the other, so balances disagree. The fix is a centralized database that every instance reads from and writes to, a single source of truth. Application servers become stateless; state lives in the database (and later in caches and replicas that are kept in sync).

Question 5

Where does the load balancer enter the story, and what is its job in one sentence?

Accepted Answer

When some counters are overloaded while others sit idle. The middleman who checks each counter's load and directs customers is the load balancer: it distributes incoming requests across servers so no one server is overwhelmed and resources are used evenly.

Question 6

What qualities should a well-designed system have?

Accepted Answer

Scalable, reliable, efficient, maintainable, fault-tolerant and highly available. The course's framing: good system design comes from understanding trade-offs, not memorizing diagrams. Every later topic (caching, replication, CAP, queues) is a trade-off between these qualities.

Introduction

Introduction