When a user logs on, a connection is established from the SAP GUI to a dialog instance. This is either the central instance (CI) or an additional application server (AS), which is running several dialog work processes internally. The user’s SAP GUI normally remains connected to the same SAP NetWeaver ASinstance as long as the session lasts. If the SAP NetWeaver AS dialog instance (CI or AS) fails, the user is disconnected, loses all open transactions, and has to log on manually to another SAP NetWeaver AS instance running dialog work processes.
We use SAP GUI as a synonym for SAP GUI for Windows. There are additional SAP GUI products: SAP GUI for Java and SAP GUI for HTML, which use the same load-balancing mechanisms, based on logon groups and the message server. However, SAP GUI for HTML needs the Internet Transaction Server (ITS), which you must also consider for high availability.
For more information about high availability for SAP ITS see the following documentation on SAP Service Marketplace:
service.sap.com/ti ® SAP NetWeaver ® SAP Internet Transaction Server – Technical Infrastructure
Users can log on either directly to a specified host machine or with group logon to a group of machines using logon load balancing. For more information about using SAPGUI for logon, see SAP Logon.
This is possible when the local host name of the SAP NetWeaver ASinstance and the number of the application server are known. In SAPGUI, you can specify the required application server for logon.
If the mapping of virtual host names to IP addresses is correctly configured in the network, users can connect the SAP GUI to a <virtual host name>, which makes the logon more transparent in the event of a switchover. In this case the LAN that serves the front end (SAP GUI) connects must be able to access virtual host names directly or using appropriate routing entries. The network setups that we recommend for switchover environments, single-LAN and two-LAN networks, both meet these requirements.
If you have correctly configured virtual host names, you can enter the following command to log on again after switchover:
sapgui <virtual host name> <number of application server>
<virtual host name>is the host name configured for a switchable SAP NetWeaver AS instance (normally the CI) running on the switchover cluster.
For the SAP NetWeaver ASlogon process (saplogon) on front-end machines, you can define groups of SAP NetWeaver ASinstances (group logon) that together provide the dialog service to users. Users do not log on to a specified machine, but log on to a group. The system connects the front-end session to the preferred host machine, so enabling dialog workload balancing among the different instances.
To log on, the user selects a particular group for the desired SAP NetWeaver ASsystem. The front end contacts the message service of this system by looking up the message server’s location (CI) on the local front end in the SAPLOGON.INI file. The message service determines the instance (CI or AS) that the GUI connects to in order to achieve a balanced logon load. It chooses the instance from a list that contains either the SAPLOCALHOST names (if this parameter is set on the host machine) or the local host names of instances (if SAPLOCALHOST is not set on the particular host). The decision to connect to a particular instance is made by the message server based on actual response time, number of users, and a “favorite factor.”
The details of this process depend on the network topology. Therefore, proper configuration of the network and the internal address parameters of the SAP NetWeaver ASbecomes a critical issue:
? For a simple network topology, where only one LAN is used for the front-end and server traffic of the SAP NetWeaver AS system, no problems result from this approach.
? If two separate LANs are used to separate the front-end from the server network traffic, you must pay careful attention to network configuration. The network card and its corresponding host name or ip address, which is presented to the business users, has to me published to the message server. The host name can be entered during logon group configuration using transaction SMLG. For more information see Configure Dynamic Logon Load Distribution for RFC Calls.
The consequences of host failure also depend on the service that is running on the failing host:
? If the front-end GUI was connected to an AS that failed, the situation remains transparent. Either you log on directly to another specified instance or you use load balancing again. The message service running on the CI is still accessible at the same network address for the saplogon program. The user can log on to the same logon group as usual and is automatically connected to a different SAP NetWeaver ASinstance by the message service, provided that there is another machine available within the group.
Therefore, AS host failure is not critical to the availability of the dialog service as a whole, as long as the CI host machine on the cluster is functioning. This is also one of the reasons why it is not essential to make AS machines part of the switchover cluster and to protect them directly with switchover software.
? If the front-end GUI was connected to the CI, but the CI has failed and has been switched over to a different host machine, the situation is more complex. The message service that is handling the group logon has moved to a different host machine. A fixed addressing scheme that is not using identity takeover or virtual addressing is no longer sufficient for the front-end connects.
For these cases virtual addressing is necessary and you have to set it up with special care, taking account of the specific network topology.
For more information see Logon Load Balancing which is written from a standard viewpoint, not from a high-availability viewpoint.
You can also use virtual addressing for the front-end connections if there is one common LAN for both the following:
? Server traffic – SAP NetWeaver ASinstances accessing each other, between SAP NetWeaver ASinstances and the database
? Front-end connections – front end to application servers or CI
Using virtual addressing for the front-end connections makes a group logon transparent in switchover situations because access to the switched CI is transparent when a virtual address is available for the CI.
To enable this you simply use virtual addresses in the front-end SAPLOGON.INI file that is used to look up the location of the message service. If the CI fails, the switchover product migrates the SAP NetWeaver ASinstance and the virtual IP address on the front-end LAN to another node. The user is logged off, but is able to log on again transparently by double-clicking on the same group icon.
If the user is logged off, it is possible to log on again by double-clicking on the same group icon. Two virtual addresses are used per host, one on the server and one on the front-end LAN. Both migrate simultaneously with their dedicated SAP NetWeaver AScomponent (CI or DB). Logon groups use the virtual front-end address in the CI front-end file SAPLOGON.INI that is used to look up the location of the message service.
The message service returns the front-end addresses (virtual for the CI, normal addresses for AS), which can be accessed directly on the front-end LAN. The path for performing the user logon is not affected by the routing that is only needed to separate the server traffic from the front-end traffic at a later stage.