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Parallel WebDispatchers - WebDynpro termination with "A request for server & was sent to server &."

Jan 10 at 06:12 AM

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Hi,

We are moving our SAP systems from on-premise into Azure, and as part of the move we are implementing 2 (parallel) Web Dispatchers in front of our ECC system to handle HTTP (WebDynpro) traffic. The 2 parallel Web Dispatchers are implemented for HA, and are load balanced by an Azure Internal Load Balancer (ILB) using simple round robin. WebDispatcher version is 7.45.

Generally this seems to be working fine, however we do get intermittent errors "A request for server & was sent to server &.", terminating the WebDynpro function.

I have seen 2-3 previous posts regarding this error, however couldn't find any exact "solution" or cause. I can't see any related information in the WebDispatcher log, and I'm not really sure where else to start investigating.

Has anyone else encountered and resolved this issue? Would be especially interested if anyone has implemented a similar setup in Azure.

Thanks!

Todd

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2 Answers

Yogesh Patel
Jan 10 at 02:29 PM
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Hello Todd Scholes,

Can you please look at SAP help link below?

High Availability of the SAP Web Dispatcher

https://help.sap.com/saphelp_nw74/helpdata/en/48/9a9a6b48c673e8e10000000a42189b/content.htm?loaded_from_frameset=true

First line under section : High Availability with Parallel Web Dispatchers

You can provide high availability with parallel SAP Web Dispatchers too. Both Web Dispatchers have the same configuration and forward their requests to the target system. Since stickiness is achieved by using a cookie, a stateful request will get to the correct application server in any case.

In your case this is what not happening what I have highlighted.

Thank you

Yogesh

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Thanks for the response Yagesh. Would be interested if you have any thoughts on my response to Isaias below as well!

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Isaias Freitas
Jan 10 at 11:57 PM
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Hello Todd,

The feature highlighted by Yogesh (in bold) is somewhat new.

What is the patch level of the Web Dispatcher 7.45 being used?

You might consider upgrading the Web Dispatcher to the latest patch level of the 7.49 release, as this is the recommended version (read the SAP Note 908097 - S-user required).

Regards,

Isaías

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Hi Isaias - thanks very much for the response and the information about this being somewhat new. We are on patch 618.

We are in the middle of a migration project, and had originally been using 7.49 however had to revert to 7.45 due to an issue with 7.49.

What is interesting is that yesterday we disabled one of our WebDispatchers...so we basically had an Azure load balancer "balancing" across a single WebDispatcher, and we still got the error. To me, this seems to indicate that it's either caused by WebDispatcher configuration, or it's due to the Load Balancer being in front......as opposed to being an issue with parallel WebDispatchers. Would you agree with this? Any other suggestions on how I can further troubleshoot?

Thanks again - really appreciate the help.

Todd

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Hello,

You might need to remove Azure load balancer and use one virtual host name (DNS) to point both web dispacherts and try again.

-Yogesh

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Hello Todd,

You are welcome!

If the issue persisted even with one Web Dispatcher only, then I would agree that the issue would not be occurring because of the parallel Web Dispatchers.

You can verify whether the Azure load balancer is tampering with HTTP headers / cookies.

It might be removing headers/cookies that SAP needs (like "MYSAPSSO2").

Regards,

Isaías

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