What are requirements in system design?

In system design, requirements are a crucial part of the process because they define what the system is supposed to achieve and how it should perform. Requirements are typically divided into two categories: functional and non-functional requirements. Understanding these requirements helps guide the architecture and design decisions to meet the intended goals of the system.

1. Functional Requirements

These define what the system should do. Functional requirements outline the core features and capabilities that the system must provide to fulfill the user's needs. They focus on input, processing, and output operations.

• Examples:
  • In a messaging system, functional requirements might include sending and receiving messages, storing message history, and supporting group chats.
  • For an e-commerce platform, functional requirements could include product search, user authentication, order placement, and payment processing.

Functional requirements answer questions like:

• What functions should the system perform?
• How should the system respond to different inputs?
• What workflows or business processes does the system support?

2. Non-Functional Requirements (NFRs)

Non-functional requirements define how the system should perform. These requirements are often related to system quality attributes and influence architecture and performance.

• Types of Non-Functional Requirements:
  • Scalability: How well the system can handle an increasing number of users or data.
  • Performance: Expected response times, latency, and throughput (e.g., the system should handle 10,000 requests per second).
  • Availability: The system's uptime and how reliably it operates (e.g., 99.99% uptime).
  • Security: How the system protects data and prevents unauthorized access (e.g., data encryption, role-based access control).
  • Maintainability: How easy it is to update, debug, or scale the system in the future.
  • Usability: How intuitive and easy to use the system is for end users.
  • Compliance: Adherence to laws, regulations, or industry standards (e.g., GDPR for data privacy).

Non-functional requirements help define the quality of the system and provide constraints that the architecture must meet. They typically address questions like:

• How fast does the system need to respond to requests?
• How will the system handle failures?
• How secure should the system be?

3. Scalability Requirements

Scalability requirements ensure that the system can handle increased loads without significant degradation in performance. This may involve horizontal scaling (adding more servers) or vertical scaling (increasing the power of existing servers). Scalability can affect choices such as:

• Database partitioning or sharding.
• Use of caching mechanisms like Redis or Memcached.
• Load balancing strategies to distribute traffic across multiple servers.

4. Availability and Reliability Requirements

Systems with high availability ensure that they are operational almost all the time, even in case of hardware or software failures. This often requires:

• Replication of data across multiple locations.
• Failover mechanisms to switch to backup systems if the primary system fails.
• The system’s reliability is often measured by its ability to handle errors and recover from them with minimal impact on the user experience.

5. Security and Privacy Requirements

Security requirements address how the system will protect data and prevent unauthorized access. This can involve:

• Authentication mechanisms (e.g., multi-factor authentication, OAuth).
• Authorization (e.g., role-based access controls).
• Data encryption (both in transit and at rest).
• Compliance with privacy laws like GDPR or HIPAA.

6. Performance Requirements

Performance requirements define how fast the system should respond under normal and peak loads. This includes:

• Response time: The maximum allowable time for the system to respond to a request.
• Throughput: The number of requests the system can process per second.
• Latency: How quickly the system processes a request end-to-end.

7. Trade-offs and Constraints

Often, there are trade-offs between different types of requirements. For example:

• Consistency vs. availability (as per the CAP theorem in distributed systems).
• Cost vs. performance (higher performance often requires more resources, which increases cost).
• Simplicity vs. flexibility (simpler designs are easier to maintain, but might be less flexible for future changes).

Final Thoughts

Requirements in system design guide the architecture, ensuring that the system meets its intended purpose while operating efficiently and reliably. A good system design balances functional and non-functional requirements, taking into account scalability, performance, security, and reliability to meet user needs and business goals.

For more detailed insights into handling requirements in system design, resources like Grokking the System Design Interview can provide practical examples and help prepare for real-world scenarios.