Architecture of SAP NetWeaver Application Server Dual Stack 
SAP NetWeaver Application Server is the powerful foundation for the entire SAP software stack. It also provides a platform for other NetWeaver components (for example, SAP NW Portal, SAP NW Process Integration), as well as for ABAP and Java applications (for example, SAP Business Suite).
The design of SAP NetWeaver Application Server is aimed at providing an exceptionally high level of robustness and supportability for the applications running on it. To be able to use the applications as independently as possible of the hardware environment, NetWeaver Application Server Dual Stack consists of Application Server ABAP (AS ABAP) and Application Server Java (AS JAVA).
The architecture of SAP NetWeaver Application Server Dual Stack and basic terminology are explained below. For details, see the component-specific documentation.
Specifications of SAP NetWeaver Application Server
SAP NetWeaver Application Server can be installed in various specifications depending on the SAP product and applications used. A distinction is made between ABAP systems, Java systems, and dual-stack systems.
An AS ABAP system provides the complete technology and infrastructure to run ABAP applications. ABAP applications and selected SAP Java applications can be run on an installed Application Server ABAP. A pure Java system provides a Java™ 2 Enterprise Edition (Java EE) 1.6 compliant-environment for developing and running Java EE programs. With this installation you can run Java EE applications, but no ABAP programs.
A dual-stack system provides both ABAP and Java environments. A dual-stack should only be installed if the application explicitly requires this.
There is one database for each SAP system.
Instances of SAP NetWeaver Application Server
An SAP NetWeaver Application Server has various instances. An instance is an administrative unit containing various components of an SAP system. The components of an instance are parameterized in a shared instance profile. Each instance is identified by a system ID and an instance number.
A distinction is made between application server instances, system central services instances (SCS), and ABAP central services instances (ASCS instances).
There can be any number of application server instances in an SAP NetWeaver application. The instances process incoming user requests.
An ABAP application server instance contains one Internet Communication Manager, one ABAP dispatcher, one gateway, one SAP start service, and any number of work processes. A Java application server instance contains one startup framework, one SAP start service, and any number of server processes.
An application server instance in a dual-stack system contains all the components of AS ABAP and AS Java listed above, with the exception of the separate start service of the Java instance.
As well as multiple application server instances, there is one special instance that contains the message server, the enqueue server, and a separate start service - The SCS (System Central Services) for application server JAVA, or the ASCS instance (ABAP Central Services) for Application Server ABAP. In a dual-stack system there is the SCS for the Java part and the ASCS for the ABAP part. The SCS or ASCS instance cannot process dialog requests - they are used to manage locks, exchange messages, and balance workload in the SAP system.
Overview of the Components of SAP NetWeaver Application Server
SAP NetWeaver Application Server Dual Stack is made up of the following components:
Internet Communication Manager
ABAP dispatcher
Task handler
Any number of ABAP work processes (dialog, update, print, background, etc.)
SAP Gateway
Standalone enqueue server
SAP Message Server
Java server processes
Startup framework
The system can be enhanced with the following optional components:
SAP Web Dispatcher
SAProuter
The following figure shows the components of a dual-stack system. For a better overview, series lines have not been used. Incoming requests are handled directly by the Internet Communication Manager or SAP Web Dispatcher, ABAP Dispatcher, or SAP Gateway, depending on the request type. In each dual-stack system there is one database with an ABAP and a Java schema.
