What is the difference between modifiability and flexibility as quality attributes of software architecture?

Modifiability and flexibility are both important quality attributes of software architecture, but they refer to different aspects of the system.

1. Modifiability: This refers to the ease with which a system can be changed to correct faults, improve performance or other attributes, or adapt to a changed environment. Modifiability is about making sure that the software can be updated or altered without causing problems or requiring massive amounts of work. It's about designing the software in such a way that changes can be made easily and safely.

2. Flexibility: This refers to the ability of a system to adapt to varying environments and requirements without significant changes to the system's architecture. Flexibility is about designing software to be versatile and able to handle a wide range of situations or tasks. It's about making sure the software can adapt to new requirements or conditions without needing a complete overhaul.

In summary, while both modifiability and flexibility deal with the ability of a system to change, modifiability is more about the ease of making changes, while flexibility is about the system's ability to adapt to a wide range of situations without significant changes.