SAP : 3-Tier Architecture:

Application Server/Layer/Tier:- 
It is introduced between client and server to address the above Issues (Two Tier Architecture)

1. The User submit the request using SAPGUI using application server Host Name and Instance number (Port)
2. The requests are received by the dispatcher and kept in the wait queue.
3. Dispatcher maintains the work processes and identifies the load handled by Processes.
4. The dispatcher allocates a freely available process to the user request based on FIFO(First In First Out)
5. The work process contains inbuilt task handlers to interpret the request. The user context rolled into WP(Work process) task handler.
6. The screen in the user request is interpreted by screen handler (interpreter)
7. The Programming language in the user request is Interpreted by ABAP Handler (Interpreter)
8. The SQL Scripts (Commands) in the user request are interpreted by SQL Handler (Interpreter)
9. The request checks whether the content is available in R3Buffer. R3Buffer is used to store the most frequently used content and less frequently modified(updated) content.
10. If the response is not available in R3Buffer then the process goes to the database using DB Specific client and SAP Kernal (OS and DB Specific)
11. Database provides the response to the SAP work process after necessary interpretations.
12. The SAP work process checks whether the response (Content) is eligible for buffering, so that a copy can be kept in R3Buffer for future similar requests.
13. The work process roll out the user related information into user context.                                             User Context: It is a small memory area where the user related information is stored. It contains user authorisations, parameters, common user attributes etc..                                        R3Buffer builds when the instance (application server) is started and R3Buffer is lost when the application server stopped.
14. The response is sent to user.                                                                                                             
• Roll-in: It is the process of copying the user context into WP task handler. When user is logging for the first time the Roll-in is not available.
• Roll-Out: It is the process of copying the user specific information into user context while sending the response to the user by a process. 
• Kernal: Set of executable, binaries and libraries to communicate with OS, DB and Hardware.
• DB Client: Set of executable, binaries and libraries to communicate with the database in the native languages.

Port is derived from a two digit instance number where SAP application server is installed.

Instance number is a two digit ID which ranges between 00 - 99 (where 97, 98 & 99 are reserved)

The Host names and ports are resolved either from /etc/hosts and /etc/services. Most of the companies uses DNS Server for name resolution.