Architecture and Functions of the SAP Web Dispatcher 

SAP Web Dispatcher as an Infrastructure Component

The SAP Web dispatcher is located between the Web client (browser) and your SAP system that is running the Web application.

It forwards the incoming requests (HTTP, HTTPS) in turn to the SAP Web AS of the SAP system. The number of requests that are sent to a Web AS ABAP depends on its capacity, which depends on the number of configured dialog work processes. The capacity of a Web AS Java depends on the number of Server Processes. If the application is stateful, the SAP Web dispatcher ensures that with the next request, the user is forwarded to the server that is processing his or her application. To do this, it uses the session cookie with HTTP connections, and the client IP address with end-to-end SSL (see also SAP Web Dispatcher and SSL).

The procedure for this is described in detail in Server Selection and Load Balancing Using the SAP Web Dispatcher.

Furthermore, the SAP Web dispatcher decides whether the incoming request should be forwarded to an ABAP or Java server. See also SAP Web Application Server with ABAP and Java.

Unlike HTTP Load Balancing Using the SAP Message Server, redirects are not executed when the SAP Web dispatcher is used. This avoids the associated disadvantages (several IP addresses must be known, book marking is not possible, authentication after changing the application server).

Structure of the SAP Web Dispatcher

The architecture is the same as the Internet Communication Manager, see the Internet Communication Manager (ICM). It uses the same HTTP framework and is likewise structured modularly from subhandlers. But unlike the ICM, the SAP Web Dispatcher does not directly pass incoming requests to a work process (or to the Java Dispatcher), – it passes them to the ICM of the application server. The response of the application server to a request returns to the client using the same network connection via the Web dispatcher.

If the application server acting as the client opens connections to external HTTP servers, these connections go direct to the server (or possibly via a configured proxy) and not via the Web dispatcher.

The Web dispatcher has the function of a "reverse proxy", and not that of a proxy.

Information Tables

The SAP Web dispatcher gets information about the SAP system that it needs for load distribution from the message server and application server via HTTP.

With Java-only installations of the Web AS the message server of the Central Services supplies the information. With ABAP installations the information comes from the message server of the Web AS ABAP. The SAP Web dispatcher knows which message server is used from the parameters rdisp/mshost and ms/http_port.

You could also use HTTPS here (see Setting Up Metadata Exchange via SSL). The Web dispatcher keeps the information in the following tables:

Table	Information		Source of Information
Server table	All SAP Web Application Servers that process the HTTP(S) requests.		Message server of the SAP system with the server list.
Group table	Groups of HTTPS-enabled ABAP servers. Here there are known logon groups that are maintained in the system, as well as the following internal groups:		You maintain logon groups in transaction SMLG in the system. The SAP Web dispatcher can get the information from any SAP Web AS. Each application server belongs to at least two of the internal groups. Each server can also offer ABAP and Java and hence belong to all groups (compare SAP Web Application Server with ABAP and Java).
	!ALL	Group of all connected application servers.
	!J2EE	Group of those application servers that contain the J2EE engine.
	!DIAG	Group of ABAP servers that provide the dialog work processes.
	!J2EES	Group of HTTPS-enabled application servers that contain the J2EE engine.
	!DIAGS	Group of HTTP-enabled servers that provide the dialog work processes.
URL mapping table	Mapping of the path part that is specified in the browser to the information about the server (groups). You can determine, for example, that all requests that contain the path prefix /sap/bc are only processed by the servers that process logon group GROUP_1.		You maintain the URL path in transaction SICF as a service in the HTTP service tree. The SAP Web dispatcher can get the information from any SAP Web AS.

Additional Information

For more information on architecture and operation of the SAP Web dispatcher see:

Server Selection and Load Balancing Using the SAP Web Dispatcher

SAP Web Dispatcher and SSL

High Availability of the SAP Web Dispatcher

Processing Administration Requests

Memory Requirement of the SAP Web Dispatcher