You can implement Internet Application Components (IACs) in the flexible SAP system architecture very rapidly. The application and processing logic of the IACs is implemented using the standard ABAP development environment. This means that the Internet applications run within the SAP system. To link the SAP system to the web, an additional software component called the SAP Internet Transaction Server (ITS) is required, as shown in the graphic below.
SAP system to Web Connection with the Internet Transaction Server
The ITS consists of the following components:
· Web gateway (WGate)
This is a gateway program that guarantees the integrity of the data flow between the HTTP server and the AGate component using TCP/IP. The WGate modularizes the interface to the HTTP server and is implemented as an exchangeable dynamic link library (DLL).
· Application gateway (AGate)
This is a gateway program that guarantees the connection to the SAP system across the DIAG interface and the GUIRFC interface as follows:
Ў Incoming data from the WGate is converted into a DIAG datastream, which is then passed to the SAP system using the SAPGUI protocol.
Ў Output data from the SAP system is dynamically converted into HTML documents, which are then passed to the WGate.
Ў Additionally, data can be exchanged between AGate and the SAP system across the GUIRFC Interface.
The AGate is implemented as an executable file and uses multithreading to process multiple requests.
· Mapping Manager (not shown on the graphic)
The mapping manager is implemented as a Microsoft Windows NT service and is therefore administered via the NT service manager. For each new session, the mapping manager assigns an AGate process to the WGate. After the TCP/IP channel between the WGate and the AGate has been established, these communicate directly with one another. The mapping manager supervises the AGate processes, checking on workload and current status. To enable this supervision, a permanent TCP/IP connection between the mapping manager and the AGate processes is established.
For more information on setting up the ITS for high availability, see Parallel Configuration of the Internet Transaction Server.
The components in the above diagram (that is, web browser, WGate/HTTP server, AGate, mapping manager, and SAP system) can run on physically different hosts, so you need to consider each independently in terms of high availability:
· Web browser
This component is connected to the HTTP server using a WAN connection across the Internet. Since the web browser can run on any host in the WAN, it cannot be localized and you need therefore give it no further consideration.
· Connection web browser to HTTP server
This connection needs to be considered in the same way as any wide-area network (WAN) connection. See “Wide Area Networks (WAN)” in Access Network. Internet connections are provided by a variety of hosts (acting as routers), which are supplied by a number of Internet service providers. These service providers are responsible for providing a corresponding level of availability for the connections
· Web server (HTTP server and WGate)
Web servers currently run under Windows NT 4.0 on hosts with Intel processors. If such a host fails, the connection to the web browser also fails. Since the Internet user can no longer communicate with the IAC, the AGate times out and the session is canceled (that is, the session is automatically logged out from the SAP system). In the event of failure in the web server, the corresponding web service must be started again with the HTTP server manager. To be sure of high availability for the web server, you must make sure that a pre-configured standby host is ready to take over in the event of host failure. See Switchover Software for High Availability.
· Mapping manager
The mapping manager supervises the status of all AGate processes. It runs together with the AGate processes on a single host machine. When the host is started, the mapping manager service is automatically started (depending on the setting in the NT service manager). The mapping manager then automatically starts the corresponding AGate processes, depending on the settings in the NT registry (see parameter MaxAGates, which specifies the number of AGate processes).
If an AGate process fails, its current sessions are aborted. The mapping manager detects this and automatically restarts the AGate processes. Other AGate processes continue normally since they are not affected. If the mapping manager itself fails due to any reason (operating system, hardware or software failure), all AGate processes also fail. You must then restart the mapping manager (either manually via the NT service manager or by re-booting the host machine). The mapping manager therefore represents a single point of failure, analogous to the message service in the SAP system.
The AGate runs as multiple processes on a single NT host machine. It is fault tolerant in that failure of a single AGate process does not impact the remaining AGate processes or their associated sessions. For more information on fault tolerance of the AGate, see Parallel Configuration of the Internet Transaction Server.
· SAP system
For general information about high availability in your SAP system, see SAP NetWeaver Application Server: High Availability.