Functional and Non-Functional requirements

Every system is built around two fundamental types of requirements:

• Functional Requirements — Define what the system does (features)
• Non-Functional Requirements — Define how well the system performs (quality attributes)

Both are equally critical. A system that has all features but crashes under load is just as unusable as a system that performs well but lacks essential functionality.

Functional Requirements

Functional requirements describe the specific features and capabilities an application must provide. They define what actions users can perform and what the system should do in response.

Example — E-Commerce Application

Every feature that a user interacts with in an application is a functional requirement.

Why Functional Requirements are Important?

They help developers understand:

• what to build,
• which APIs are needed,
• what database operations are required,
• and how users will interact with the system.

Non-Functional Requirements

Non-functional requirements define how the application performs under various conditions. They specify the quality attributes, constraints, and benchmarks that the system must meet.

Example — E-Commerce Application

Key Non-Functional Attributes in System Design

1. Scalability

The system's ability to handle increasing load without degradation.

• Vertical Scaling — Upgrading existing server resources (CPU, RAM, storage)
• Horizontal Scaling — Adding more server instances to distribute load

2. Availability

The percentage of time the system remains operational and accessible.

• Measured through SLA (Service Level Agreement) and SLO (Service Level Objective)
• Expressed as uptime percentage (e.g., 99.9% = ~8.7 hours downtime/year)

3. Reliability

Ensures no data loss — the system consistently produces correct results and preserves data integrity even during failures.

4. Performance (Latency)

How fast the system responds to user requests. Measured using percentile-based metrics:

P99 captures the worst-case experience for the majority of users and is critical for identifying tail latency issues.

5. Security

Protecting the system and its data from unauthorized access and breaches.

6. Maintainability

How easily the system can be modified, updated, and extended over time.

• Achieved through code modularity — clean separation of concerns
• Well-structured code reduces bugs and simplifies onboarding of new developers

7. Observability

The ability to monitor and understand the internal state of a system from its external outputs.

• Monitoring — Real-time tracking of system health and metrics
• Logging — Recording events, errors, and transactions for debugging and auditing

Functional vs Non-Functional

Summary

• Functional requirements define the core features and business operations of a system, such as login, payments, messaging, search, and ordering.
• Non-functional requirements define the system’s quality attributes, including scalability, performance, security, availability, reliability, and maintainability.
• A successful system must balance both functionality and performance to deliver a reliable and efficient user experience.
• Non-functional requirements are critical for building scalable and production-ready systems capable of handling real-world traffic and operational challenges.
• Understanding and clearly defining both requirement types is the foundation of effective System Design and scalable software architecture.