Hi Simon,

IIS doesn't know which server you want to use - this is handled by the CSP Gateway module.

To get this done you need individual application url paths for each server. In the CSP Web Gateway Management you can define a cache server for each application path:

Select Add Server and create a new server configuration for cache server (Y) 

Option 1:
http(s)://<host>/csp/... --> server X   (all url paths except /csp/api)
http(s)://<host>/csp/api --> server Y

1. Select Application Access
2. Create a new application configuration. (e.g /csp/api) and select the server (Y) as Server 0
3. Edit the /csp application and select the server (X) as Server 0

Option 2:
http(s)://<host>/csp/... --> server X
http(s)://<host>/api /... --> server Y

In IIS add a new application (e.g api) and add the same handler mappings like for csp.

In the CSP Webgateway management you do nearly the same like in option 1, except that you create the new application for /api and map it to the server (Y)

See also: https://docs.intersystems.com/latest/csp/docbook/Doc.View.cls?KEY=GCGI_r...