Progress Rollbase provides scripting capabilities on the client-side and the server-side. The scripting capabilities can be used to access the Progress OpenEdge AppServer.
This whitepaper explains how to access the OpenEdge AppServer from Rollbase using Object Script.
Note that this whitepaper shows how to call the AppServer using JSON and the rbv_api.sendJSONRequest() method in Rollbase.
If you are 'on-premise' and want to call the AppServer from Rollbase using OpenClient APIs, then there is a second whitepaper, "Calling ABL Business Logic from Rollbase via OpenClient" available here
Note: due to the Community re-org, we had to move this doc from its previous location.