SAP NetWeaver Instances

The SAP NetWeaver system consists of the following instances in any deployment mode.

Dialog instance

Dialog instance is optional and installed on separate hosts to ensure high availability of dialog processes. Dialog instance includes:

Dispatcher
Work Processes (Dialog, Batch, Spool, and Update)
Gateway
Internet Communication Manager (ICM)
Internet Graphics Service (IGS)

In a cluster scenario, Dialog instance can be added to the cluster. After Dialog instance is added to the cluster, Central instance updates the Dialog instance information. Each cluster requires a load balancing solution, such as SAP Web Dispatcher. Table 1 lists the components contained in Dialog instance.

**Table 1** Components contained in Dialog instance
Instance	Component	Function
Dialog instance	Work Processes (Dialog, Batch, Spool, and Update)	Work processes are used to execute applications and contain the Dialog, Batch, Spool, and Update processes. Work processes are components that are able to execute an application (that is, each dialog step). Each work process is linked to a memory area containing the context of the application being run. The context contains the current data for the application program. This needs to be available in each dialog step.
	Dispatcher	The dispatcher is the link between the work processes and the users logged in to the ABAP application server (that is, the SAP GUIs of these users). Its task is to receive requests for dialog steps from the SAP GUI and direct them to a free work process. In the same way, it directs screen output resulting from the dialog step back to the appropriate user.
	Gateway	This is the interface for the communication protocols of NetWeaver AS ABAP (RFC, CPI/C). It can communicate with other ABAP application servers of the same NW AS, with other SAP Systems, or with external non-SAP systems.
	Internet Communication Manager (ICM)	The Internet Communication Manager ensures that communication between the SAP system (SAP NetWeaver Application Server) and the outside world via HTTP, HTTPS and SMTP protocols works properly. In its role as a server, the ICM can process requests from the Internet that arrive as URLs with the server/port combination that the ICM can listen to. The ICM then calls the relevant local handler for the URL in question.
	Internet Graphics Service (IGS)	IGS is used to generate Web-based graphics from the SAP Web AS.

ASCS instance

ASCS instance includes the Enqueue and Message servers. An SAP NetWeaver system contains only one ASCS instance. Therefore, ASCS instance must have no single point of failure (SPOF) in the PRD system. To ensure high availability of ASCS instance, you need to install ERS instance.

Message Server
A Message server is used for communication among Dialog instances and provides information for Dispatcher for load balancing.
Enqueue Server
An Enqueue server contains a lock table which contains the database lock and the basic lock information configured by Java Server Process. The Enqueue server can also synchronize data among Java clusters. In an AS ABAP case, an Enqueue server processes the locks of data objects.

ERS instance

An ERS instance ensures high availability for ASCS instance and protects Message Server and Enqueue Server on ASCS instance.

Table 2 lists the components contained in ASCS instance.

**Table 2** ASCS instance and its components
Instance	Service	Function
ASCS instance	Message server	Message server is an independent process and contained in ASCS instance. One SAP system contains only one Message server. Message Server provides the following functions: It is the core communication channel between the separated application servers. It allocates login users using the login group function of SAP GUI and RFC. It functions as the information center of Web Dispatcher and application servers.
	Enqueue Server	Enqueue Server is a special lock mechanism in the SAP system, which prevents a line of data from being simultaneously changed by two processes and ensures consistency in updating the database.
	Enqueue Replication Server (ERS)	ERS is used to protect Message and Enqueue servers contained in ASCS instance.

Web Dispatcher
An SAP Web dispatcher is a link connecting Internet to SAP NetWeaver and locates in the demilitarized zone (DMZ). All HTTP requests pass Web Dispatcher, which ensures connection security. Then Web Dispatcher allocates the requests to multiple SAP NetWeaver application servers.
Database instance
The Database instance is mandatory in an SAP NetWeaver system and is used to create an ABAP schema for the SAP system in the database.