Determining the Server Group 
The SAP Web dispatcher determines the server group responsible for processing a request in the following way:
First it checks whether the system is configured for J2EE. It gets this information from the message server.
J2EE Not Active
If no J2EE instance is active, in the stateless case the request is assigned to the next SAP Web AS in the queue according to load balancing. The server group which can process the request is the internal group !DIAG or !DIAGS (if HTTPS is forwarded, see SAP Web Dispatcher and SSL). In this case !DIAG/!DIAGS are all application servers that are supplied with the ICM and have configured an HTTP/HTTPS port (see icm/server_port_<xx>).
If it is a stateful connection, the system selects the SAP Web AS that is processing the transaction.
J2EE Active
If the J2EE engine is active, the Web dispatcher looks through the list of ICF prefixes. If the URL begins with a prefix that belongs to an ICF service, the server group !DIAG is used, otherwise the group !J2EE. The groups are described in SAP Web Dispatcher under Functions.
The start page and the root directory are handled in a special way, since when you search through the list of prefixes, you cannot search through the root directory (it would always be found because all URLs begin with /). If the URL is a root URL, the profile parameter is/HTTP/default_root_hdl determines which root directory should be used. Permitted values are abap and j2ee, although the value can also be changed dynamically using Transaction RZ11 or SMMS.
The following flow diagram shows the algorithm that is used.
Special Case: HTTPS Re-encryption
If the SAP Web dispatcher is to forward the request (HTTP or HTTPS, not end-to-end) SSL-encrypted (parameter wdisp/ssl_encrypt, see SAP Web Dispatcher and SSL), it has to select !J2EES or !DIAGS as the internal group. Otherwise the algorithm runs the same way.
SAP Prefix?
The decision whether it is a SAP prefix is described in detail below. With URL http://hostname:port/A/B/C the prefix to be examined is the character string /A/B.
The SAP Web dispatcher must therefore decide whether it is an SAP prefix, which should be sent to the ABAP server (the ICF), or if it is a prefix that is not recognized by the ICF, in which case it is sent to the J2EE engine. To do this, the following process is used.
If you enter the URL http://saphost:8080/index.html, the prefix to be checked is /index.html. This is not found in the URL mapping table and it contains one forward slash. If you now set parameter is/HTTP/default_root_hdl to j2ee(default setting), the system searches the root directory of the SAP J2EE engine for file index.html. This is then returned to the client by the ICM of the application server.
See also: