Wink Open Source Software: Hydro

Hydro (like ICE) allows you to bridge language barriers by letting components written in different programming languages communicate with each other. ZeroC's ICE product currently includes implementations for C++, Java, C#, Python, Ruby, VB, and PHP. Hydro adds support for O'Caml. The ICE protocol is rich enough to make the representation of the data to communicate easy and convenient to use, and adopts an object-oriented type system. ZeroC sees ICE as a superior successor to CORBA.

Hydro includes a runtime, and a generator called hydrogen. The runtime is fully asynchronous, based on Ocamlnet's event queues. The generator is used to create a thin layer on top of the runtime that maps abstract Hydro types to specific O'Caml types. The Hydro engine therefore appears to the programmer as if it was typed using the input IDL file. Unlike the generators shipped by ZeroC, hydrogen does not create marshalling code -- we see this as the task of the runtime and not of the language mapping layer. Because of this design decision we have high confidence in the correctness of the generator.

Although we want Hydro to be an implementation whose protocol is fully compatible with ICE and that can process all its IDL files, we take some freedom to deviate from what is described in ZeroC's "ICE manual". ICE was designed with mainstream object-oriented languages in mind, and therefore lacks many of the advanced constructs available in O'Caml (variants, for example). Conversely, ICE includes constructs such as hierarchical exceptions and downcasts which do not exist in O'Caml (and for good reason!). To deal with the "impedance mismatch," we implemented emulations for these concepts. On the other hand, O'Caml offers advanced features which ought to be exploited by a good implementation. For example, in O'Caml, asynchronous networking code is quite natural, so we have chosen to make this the default style. So while Hydro is quite straightforward, the O'Caml programmer should not expect a design which is a perfect one-to-one translation of what ZeroC describes.

For more details, see the README file coming with the distribution.

Hydro is implemented in the Objective Caml language. Version 0.7.1 can be downloaded under a BSD style license, and without any warranties. Further links: code, manual.