Architecture Concerns

1. Scalability: how many users or transactions can it process concurrently?
2. Performance: how fast does it respond? what is the max throughput?
3. Recoverability: how does the application recover from a fault?
4. Security: can it sustain attacks? is data protected?
5. Maintainability: how easy is it to maintain?
6. Operability: how easy is it to use and managed? how is it operated?
7. Availability: is it available 24x7? and is it 99.9% or 99.99%?

Scalability

• Horizontal and Vertical Scalability
• Vertical: Add resources to a single computer (more CPU/RAM). vertical scaling is cheaper
• Horizontal: Add more boxes of similar memory & CPU

Performance

• Parallel Processing, Asynchronous Processing, Caching, multithreading

Recoverability

• Session replication - ability to replicate the user sessions to a secondary server in the cluster
• Backup
• Business Continuity or Disaster Recovery Infrastructure Requirements  

Security

•  Authentication & Authorization (LDAP, single sign-on), encryption

Maintainability 

• Application complexity has direct correlation to maintainability. It is relatively easy to maintain a system with less dependencies.
• Logging
• Robust Error Handling - will keep the system's availability & uptime high
• No direct access to server
• Layered web application is easy to maintain. but multi-threaded application is hard to maintain
• KISS (simple design)

Operability

• Understands the purpose of the system
• Ask users what they really want and design accordingly
• Consistency - standard look & feel

Availability

• Is the amount of time that the system is up and running
• Robust Error Handling - will keep the system's availability & uptime high
• Backups & recovery
• Fault recovery

Backward error recovery (implementing transactions)
System is restored to a previous known valid state

• Exception Handling