What Are Functional and Non-Functional Requirements in System Design?

In system design, requirements describe what a system must do and how well it must perform. They are divided into two main categories: functional requirements (FRs) and non-functional requirements (NFRs). Functional requirements define the system's features and behavior, while non-functional requirements define its quality attributes, constraints, and operational expectations.

1. Functional Requirements (FRs): These specify the capabilities and behaviors the system must provide to users or other systems. They answer the question: What should the system do?

2. Non-Functional Requirements (NFRs): These specify measurable quality attributes, performance expectations, and constraints. They answer the question: How well should the system perform, and under what conditions?

Good NFRs should be measurable and testable. Instead of saying 'the system should be fast,' specify a latency target, percentile, workload, and measurement conditions. For example: 'Under 5,000 requests per second, 95% of product-search requests should complete within 200 ms.'

Example: For a LeetCode-like platform, functional requirements include browsing problems, submitting code, running test cases, and viewing results. Non-functional requirements include low submission latency, high availability, secure execution of untrusted code, and the ability to handle submission spikes.

In an interview, clarify the most important functional requirements first, then establish the key NFRs and their targets. Functional requirements determine the capabilities and main components; NFRs heavily influence architectural choices such as caching, replication, queues, database selection, isolation, and deployment topology.