Most SAP services have the ability to reconnect dynamically – that is, automatically – if the corresponding process is restarted. The reconnect feature offers automatic recovery from many problems that affect only individual processes.
Most SAP processes are restarted automatically by the dispatcher after process failure: this includes all the work processes (dialog, update, enqueue, background, spool) and the gateway process. Only the processes of the frontend, message and dispatcher services are not restarted automatically. The graphic below shows how the connections between SAP processes are recovered after failure:
Recovery of Connections Between SAP System Services
· Connections shown with thick line
These can only be recovered manually by restarting the frontend. The user context is preserved on the application host.
· Connections shown with medium line
These are recovered by the automatic reconnect of the dispatcher to the message service. If the message service process fails, the process has to be restarted manually. For an automatic restart of this process, consider using Switchover Software.
· Connections shown with thin line
These are automatically recovered if the dispatcher restarts one of the processes – that is, dialog, update, enqueue, background, spool, or gateway – after a failure.
SAP logon load balancing allows application host machines in an SAP system environment to be used more efficiently. Users can be dynamically distributed at logon time over several application hosts.
You can make a particular group of application hosts available for certain workgroups or certain tasks. An appropriate host is then automatically selected when a user logs on. A user is automatically logged on to the host with the best performance and the fewest users.
Logon groups are installed and maintained centrally in the SAP system. You can define a maximum response time for each application host, and a maximum number of users that can log on to that host. This means that you can set up logon groups with faster response times for important workgroups requiring time-critical transactions.
Logon load balancing also protects the user from SAP system failure involving dialog services by preventing logon to a host on which the dialog service has failed.
HTTP load balancing with the SAP NetWeaver dispatcher or message server in Web-Server Networks and DMZs