Defining R/3 System
The Simplest R/3 System is "one database".In one R/3 System,there is only one database.To give the correct explaination about SAP R/3 System is considered to be attached to that one database.One R/3 system is composed of one database server accessing single database,one or more application servers,and one or more presentation servers.
The term R/3 system landscape denotes a description of the number of systems within an SAP installation and how they are designated,such as development,test or production.
Defining an R/3 Instance
When you hear someone say the word instance,most of the time,that person will be referring to an application server.The term instance is synonymous with application server.
The term central instance refers to the database server.If an application server and database server both reside on the same machine,the term central instance refers to the computer on which both reside.
In most general terms,an instance is a server.It is a set of R/3 processes providing services to the R/3 System.
Application Server Architecture
All requests that come in from presentation servers are directed first to the dispatcher.The dispatcher writes them first to the dispatcher queue.The dispatcher pulls the requests from the queue on a fist-in,first-out basis.Each request is then allocated to the first available work process.A work process handles one request at a time.
To perform any processing for a user's request,a work process needs to address two special memory areas:the user context and the program roll area.The user context is a memory area that contains information about the user,and the roll area is a memory that contains information about the program execution