SAP Web Dispatcher 

Integration des SAP J2EE Application Servers

Uses

The SAP Web dispatcher is used as a "software Web switch" between the Internet and your SAP system, which consists of one or more Web Application Servers. You therefore have only one point of access for HTTP(S) requests in your system. Furthermore, the SAP Web dispatcher balances the load, so that the request is always sent to the server with the greatest capacity.

Introductory Comments

The SAP Web dispatcher is recommended when you use an SAP system with several SAP Web Application Servers for Web applications.

The SAP Web dispatcher is a program that you can run on the machine that is connected directly to the Internet. It is very easy to configure. The SAP Web dispatcher’s profile file only needs to know the port on which it should accept HTTP(S) requests (parameter icm/server_port_<xx>), where it can find the SAP message server (parameter rdisp/mshost) and on which port the message server accepts HTTP requests (parameter ms/http_port).

If you should be able to call the Web application externally, for example using the URL www.shop.acme.com, this host name must be mapped internally to the SAP Web dispatcher. This then forwards the HTTP(S) request to a suitable SAP Web AS.

The prerequisites for operating the SAP Web dispatcher are described in Operating the SAP Web Dispatcher.

SAP Web Dispatcher describes the SAP Web dispatcher’s profile parameters. The settings should be suitable for normal operation. You can also find the parameters in transaction RZ11.  You can find an example of the SAP Web dispatcher’s profile file in Example: Profile File of a SAP Web Dispatcher.

Integration

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 depends on its capacity, which is linked to the number of dialog work processes that are configured. 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 J2EE server. For more information, see SAP Web Application Server with ABAP and J2EE.

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).

Functions

The SAP Web dispatcher keeps information about the SAP system that it needs for load distribution 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 J2EE and hence belong to all groups (compare SAP Web Application Server with ABAP and J2EE).
	!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-Tabelle	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 Profile Parameters

SAP Web Dispatcher Profile Parameters describes the profile parameters for configuring the SAP Web dispatcher.

When you set the maximum values, note that unnecessarily large values can increase the SAP Web dispatcher’s memory consumption.

You can find an example of a SAP Web dispatcher’s profile file in Example: Profile File of a SAP Web Dispatcher.

Starting and Stopping the SAP Web Dispatcher

Operating the SAP Web Dispatcher describes how you start and stop the SAP Web dispatcher.

Handling HTTPS Requests

The SAP Web dispatcher can handle HTTPS requests in different ways:

·        End-to-End SSL

·        Decrypt them and forwards them unencrypted

·        Decrypt and encrypt them again

These options are described in SAP Web Dispatcher and SSL.

Tracing the SAP Web Dispatcher

If you get traces from the SAP Web dispatcher, for more information see Monitoring the SAP Web Dispatcher.

Rule of Thumb for the SAP Web Dispatcher’s Memory Requirement

Of course, the SAP Web dispatcher must reserve as much memory as possible to store large tables.

The following formula estimates roughly how much memory is required:

Memory requirement  = (S × N_S + G × N_G + U × N_U) × 2 Bytes

The letters have the following meaning:

Variable	Parameter Value
S	wdisp/max_servers
NS	wdisp/max_server_name_len
G	wdisp/max_server_groups
NG	wdisp/max_server_group_name_len
U	wdisp/max_url_map_entries
NU	wdisp/max_url_map_path_len

Additional SAP Web dispatchers are described in the following sections:

·        Server Selection and Load Balancing Using the SAP Web Dispatcher

·        SAP Web Dispatcher as a URL Filter

Restrictions

The SAP Web dispatcher is only useful in the Web environment. In the classic SAP system, load is balanced by the message server.

The SAP Web dispatcher forwards only incoming HTTP(S) requests to the Web application server and the response is then returned to the client.

Outgoing requests (such as requests to a different SAP Web Application Server) are not sent via the SAP Web dispatcher. They are sent via the proxy that uses the appropriate intranet.