Duet FAQ: Why the two separate components, Duet server and Add-On are required?

I recently received this question from a reader:

Hi Eran,

for Duet the following main components are needed: Duet server and Add-On server. In the following I listed a couple of questions related to the Duet system landscape:

1.) Can you please explain, why the two separate components Duet server AND Add-On are needed?
a) Is it because the user mapping between ADS world and SAP world?
b) Is it also important in a multi-domain landscape? (eg. Clients, Exchange server, Duet server in one domain and ERP system in another domain)
c) When does the user need different physical machines?
d) When does the user need different J2EE instances?
e) When is it possible to use one single J2EE engine to install both components?
d) I assume it is also because of scalability, right?

Would be great to have more info about this topic!

THANKS & Cheers
Stefan

Well Stefan, the answer to your question is as follows:

The various components in Duet’s architecture are gathered to logical groups (Duet Server, SAP Add On) that are in charge of different product flows. While the Duet Server is mainly in charge of the deployment flow and the user initial authentication and URL resolution, the SAP Add On is in charge of the processing the ongoing web service calls.

There are various ways to install those logical components on physical machines. In the minimal installation the Duet server and the SAP Add On can be installed on the same machine and leverage one J2EE engine.

You’re also right about the scalability reason. Basically, a typical Duet landscape only requires a single Duet Server but can contain several Add Ons to optimize performance (for example, an Add On per backend system or geographical location.)