Like any other projects, begin with asking:

• What are the business goals/objectives?
• What problem are you trying to solve?
• Why are you using RFI, SSO?
• What are the business requirements?

Given the nature of your project, non-functional requirements will also play a very big role:

• What is the expected demand on the system: how many users, how many transactions per day, per hour, etc.?
• Future expected volume.
• Scalability requirements?
• Up-time requirements and redundacy aka what if the system is down? 

You did not provide much details about the problem you are trying to solve.  The more details you can provide the more specific answers you'll get in this forum.

Sincerely,

Adrian