High Availability of the SAP Web Dispatcher (SAP Library - Architecture of the SAP Web Application Server)

High Availability of the SAP Web Dispatcher

You have different options to make the SAP Web Dispatcher highly available.

Monitoring the SAP Web Dispatcher with an External HA Tool

The tool monitors the SAP Web Dispatcher and restarts it if the SAP Web Dispatcher crashes. See also the description of option -shm_attach_mode in Operating the SAP Web Dispatcher.

Using Option -auto_restart

The following procedure applies to UNIX platforms only. This process is not possible on Windows and iSeries since they do not support this type of high availability.

Note

You can achieve high availability at process level with this option. It does not help if the whole machine crashes.

You start the SAP Web Dispatcher with the option -auto_restart to achieve the following behavior.

The necessary resources (shared memories, sockets, administration structures) of the SAP Web Dispatcher are loaded when it is started. This process is then duplicated by the system call fork(), that is, there are now two processes that know exactly the same shared memories, sockets and administration structures. The original process (father process F) takes over the monitoring function (watchdog), whilst the new process (child process C) takes care of the actual tasks of the SAP Web Dispatcher (forwarding requests, load balancing,…).

The monitoring process V then checks every n seconds the status of the SAP Web Dispatcher process C.

If the K process ends (whether this is required or unexpected), the watchdog V recognizes this and the state of the process that was frozen after it was started is duplicated again (fork).

This has the following advantages:

Fast availability of the SAP Web Dispatcher, because the resources do not have to be created for the first time
"Zero downtime", because requests to the SAP Web Dispatcher do not get lost but are stored in the system queue and can be processed after the process is duplicated by the SAP Web Dispatcher work process (C). Requests that were being processed when the crash happened will, of course, be lost.

Ending the highly available SAP Web Dispatcher

If you want to end the SAP Web Dispatcher, ensure that you end the watchdog process (V) (this is started if you end the other process!). To do this, send a SIGINT to the watchdog’s PID. You can take this from the trace file dev_webdisp_watchdog.

Ending the watchdog also ends the SAP Web Dispatcher work process C.

Note

If only the SAP Web Dispatcher work process is ended with SIGINT, the watchdog starts this process again.