Portable Distributed Objects

From Academic Kids

Portable Distributed Objects, or PDO, is a programming API for creating object oriented code that runs anywhere on a network of computers. It was created using the OpenStep system, whose use of Objective C made the package very easy to write. It was characterized by its very light weight and high speed, things that cannot be claimed for similar systems like CORBA.

Versions of PDO were available for Solaris, HP-UX and all versions of the OPENSTEP system. A version for working with Microsoft Component object model (then known as OLE) was also available, called D'OLE. D'OLE allowed you to write distributed code using PDO on any platform, which would be presented on Microsoft systems as if they were local OLE objects.

At the time PDO was one of a number of distributed object systems, which was, for a time in the early 1990s, the "next big thing" in programming. The idea was to produce "front end" applications on GUI-based microcomputers that would call code running on mainframe and minicomputers for the processing and data storage. At the time Microsoft was evolving OLE into the Component Object Model or COM, and the similar distributed version, DCOM. IBM was late in the evolution of their System Object Model (SOM/DSOM) and even Sun Microsystems was promoting their Distributed Objects Everywhere (there were a host of smaller players as well). With the exception of the limited functionality in COM, most of these systems were extremely heavyweight, tended to be very large and slow, and often were very difficult to use.

PDO on the other hand relied on a small number of features in the Objective C runtime to handle both portability as well as distribution. The key feature was the language's support for a "second chance" method in all classes; if a method call on an object failed because the object didn't support it (normally not even allowed in most languages due to strong typing), the runtime would then bundle the message into a compact format and pass it back into the object's forwardInvocation method.

The normal behavior for forwardInvocation was to return an error, including details taken from the message (the "invocation"). PDO instead supplied a number of new objects with forwardInvocation methods that passed the invocation object to another machine on the network, with various versions to support different networks and platforms. Calling methods on remote objects was almost invisible; after some network setup (a few lines typically) PDO objects were instantated locally and called as you would any other object on the system. The PDO object then forwarded the invocation to the remote computer for processing and unbundled the results when they were returned.

In comparison with the "standard", CORBA, PDO programs were typically 1/10th or less the size. For a while, when PDO was NeXT's "big thing" it was common for NeXT staffers to write into magazines showing how to re-implement a multi-page CORBA article in perhaps 15 lines of code. From a programming standpoint, there was simply nothing nearly as easy to use as PDO.

However, PDO was also reliant entirely on Objective C to function. This was a price most were unwilling to pay, as at the time C++ was the "next big language" and no one was terribly interested in anything else, no matter how good. PDO never saw much use, and quietly disappeared as a marketting push with the release of WebObjects in 1995.


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