Incidentally, I started a project that will test different databases' performance. The (yet, incomplete) version is here, and the C# class that specifically deals with Caché interaction is here (a subclass of the main database class). The I/O ops are very simple, only dealing with 2 fields.
Ensemble is a RAD platform; it allows users to create workflows, RIA's, and rule-based logic (hence, it can work as an interface engine). It contains an application server, workflow server, document server all in one - not surprising, given the Caché platform's own relatively wide array of offerings, on which Ensemble is based. As far as I can tell without having seen the product, it is really a set of extensions built in the Caché environment to provide messaging, workflow, and portal services, with some industry-specific features such as HL7/EDI, and endpoints for BPEL applications, database access, and other EAI connectors. Ensemble also offers data (SSIS-style? not too difficult to understand, and resulting in federated databases, as already implemented in the Caché application server external database gateway) and object transformation (Java --> .NET ORBing? I am not sure how is this done, I assume through instantiating VM's for each of the supported platforms and performing marshaling between the objects).
I assume that messaging is implemented in the Caché application server - not entirely different from the original MSMQ.
As far as RAD capabilities (as so far I have mostly talked about infrastructure), Ensemble offers some graphical code generators for BPM; I am assuming it also supports the Caché development environment (ObjectScript, MVBasic and Basic).
In Microsoft terms, Ensemble is basically VS + SQLServer + SSIS + WCF + WWF + BPEL parser + BizTalk + customizations. In bold, the middleware stack.
On closer inspection, it appears that the inter-VM object conversion is in fact introspection- and proxy-based instantiation of .NET objects which are made available by Ensemble to Caché's native VM's. Ensemble runs a .NET VM which can execute .NET objects natively through worker threads. I am curious if this requires a Windows Server to be available at runtime - not sure how distributed can the Ensemble installation be.