Components of R/3 Architecture

Database Server
The database server is the most powerful server in an R/3 system. R/3 uses the database management system as central storage for all R/3 data and R/3 metadata information. This central storage, the basis for the tight integration of all R/3 application modules, guarantees consistent data storage.
Three important factors govern the decision-making process for selecting data storage options.
· Adequate protection from data loss
· Appropriate speed of access
· Sufficient storage capacity to accommodate growth
The application server
While SAP uses the generic term application server to define a computer that receives connections from SAP clients, the actual connections are managed by SAP dialog servers.
A dialog instance is a software program that is running the SAP kernel (similar to an Oracle instance), and it is the job of the dialog instance to execute the ABAP programs and manage the requests for data and services. While there is generally a one-to-one mapping between an application server and a dialog instance, it is possible to have more than one dialog instance on an application server.

Instance
An instance is a group of R/3 services that are started and stopped together
The central instance
The central instance is a concept that is unique to SAP. The central instance is a combination of hardware and software. It contains a physical server (the application server) and numerous software components, including a message server, a database gateway (a pre-established connection between SAP and Oracle--or another database), and various update, enqueue, dialog, and spool facility software. In most generic SAP architectures, there are numerous application servers but only a single central instance. However, in addition to managing the SAP interfaces, the central instance can also serve as an application server.
Work Process
A work process is where individual dialog steps are actually processed and the work is done. Each work process handles one type of request.
The work process types are:
Dialog, for executing dialog programs(processes only one request at a time)
Update, for changing database entries( primary or secondary )
Background, for executing background jobs( started at a specified time )
Spool, for print formatting (generated online or during back ground processing For printing ) and
Enqueue, for executing lock operations

Dispatcher
Central process on an instance. It is responsible for starting work processes and distributing transaction load across work processes.
Message Server

Independent program that maintains a list of all instances in a SAP system. The message server determines which instance a user logs on to and organizes communication between instances

Gateway
Interface that converts one communication protocol into another.



Explain work processes
A work process is where individual dialog steps are actually processed and the work is done. Each work process handles one type of request.
The work process types are:
Dialog, for executing dialog programs(processes only one request at a time)
Update, for changing database entries( primary or secondary )
Background, for executing background jobs( started at a specified time )
Spool, for print formatting (generated online or during back ground processing For printing ) and
Enqueue, for executing lock operations
.