What key characteristics should an effective architectural pattern have for building reliable systems?

An effective architectural pattern for reliable systems should ensure:

• Modularity and separation of concerns — each component is responsible for its part of the logic, which simplifies maintenance and testing.
• Fault tolerance — exception handling and the ability to recover after failures.
• Scalability — the ability to easily expand the system without a complete overhaul.
• Testability — the architecture should allow easy writing of unit and integration tests.
• Clear state management — minimizing side effects and predictable behavior.

For example, in iOS, the MVVM pattern is often used, which separates presentation logic from business logic, increasing reliability and ease of maintenance.