Requirements

Function Requirements

These are your use-cases. For example, for a e-commerce site, you may need to offer functionality like: search inventory, place order and review past orders.

Non-Functional Requirements

How should the system behave.

These are requirements like:

• usability
• reliability
• efficiency
• scalability
• maintainability

Restrictions

Restrictions limit your options.

Restrictions include: 

• compliance
• costs
• standards
• time to market

Priorities

We need to prioritize functional and non-functional requirements based on restrictions.

For example: may not prioritize portability if we control where the solution is to be deployed.

Implementation

Key Concepts

• Start with one thing at a time.
• Keep it simple. (KISS)
• Don't over engineer. You ain't gonna need it. (YAGNI)

Software Architecture Patterns

See O'Reillys - Software Architecture Patterns

References