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This is basically the same as Coding to interfaces, but played out in the real world of when there are various engineering complexities such as immutability of published interfaces and implementations, etc.


Consider the following situation:

A OS-level object-oriented library with a set of published interfaces and an implementation provided by the OS.

Some extension points are provided so that third parties can extend specific behaviors by implementing a subset of the published interfaces and registering them with the OS.

A programmer looks at the whole set of published interfaces, and says to self, "it seems like I can re-implement most of the functionalities (with better algorithms), including those areas which aren't designated as extensible." And so the programmer worketh.

Only when the programmer's own implementation mash up (COM-style) with the OS implementation and fails miserably, did the programmer realize that the suite of objects as implemented by the OS rely heavily on communications using non-published interfaces, because the published interface was minimalistic and blocked many opportunities to optimizations(*).

(*) In encoding/decoding/trans-coding (of data/image/sound/video compression) tasks, it is well known that certain pipelines stages can cancel each other if they perform the exact opposite operation / inverse, e.g. G(F(x)) == x.

A typical interaction is like:

  1. Consumer asks Producer if it implements a special interface X that only vendor V knows.
  2. If yes, Consumer talks to Producer using interface X and see if they are exact opposite (#1).
  3. If yes, Consumer asks Producer for its "upper stage" (i.e. bypassing Producer) (#2) so that in effect both are being cancelled out.

(#1) and (#2) are capabilities that are missing in the minimalistic published interface. On first glance it seems that the vendor might have a chance to provide them as well, but choose not to.

It might also be the case that providing those performance-oriented interfaces would severely pollute the namespace.


The end result is that whenever a programmer provides its own implementation of a certain class, it would either (i) fail miserably, or (ii) perform very slowly, because it could not interact with the rest of the suite using the performance-enhanced internal interfaces only known to that suite.

Is this a more frequent problem with some flavors of Object-Oriented technology? Or is it more common with some flavors of Component-based Engineering?

Advocates of OOP will argue that Refactoring and Publishing those interfaces would solve the problem. This assumes that it is possible to distribute a new version of the library along with new set of interface. For some technology this is not possible.


The way it matters to is that its QueryInterface method allows run-time query (and run-time response) of additional interfaces implemented by a class. Its Java equivalent would be like a library that makes heavy use of instanceof for interfaces internal to the package.

To Reviewers: I'm open to suggestions on trimming down the question to its essence.

The correct term seems to be Mixin, thanks to this question.


This answer is partly relevant, but my example focuses on the lost opportunities for optimizations within a single library. Basically, if a third-party implementer didn't do both:

  1. Implement both encoder and decoder
  2. Implement its own proprietary interface on both the encoder and decoder, in order to detect and bypassing operations that cancel out each other

Then the implementation will only get "baseline performance" when inter-operating with different encoders/decoders from other vendors.

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up vote 2 down vote accepted

As a general rule, Public API should not change over time (or should at least be stable) so that existing applications that depends on them doesn't break while more functionality for additional applications are supported.

On the contrary, Private API should be allowed (nearly freely) without which it is not quite possible to effectively bring that evolution we really need.

If you are finding that given Public API is cause of bottleneck, then it is the issue with the prior API at the first place. In your specific case, for transcoding example (which is what i work on), the Encoding API and Decoding API should NOT change, but both modules should have additional API that support transcoding.

Of courses, within such API, the change should be controlled such that internal/private classes don't arbitrary change their roles/ else there are chances to break the original behavior.

Is this a more frequent problem with some flavors of Object-Oriented technology? Or is it more common with some flavors of Component-based Engineering?

NO. It's only dependent on the design not inherently a limitation of OO or Components.

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In my actual work, the encoders and decoders expose a vendor ID, which can be queried and compared. It is assumed that if the encoders and decoders are from the same "vendor", they could then start talking to each other using their proprietary interfaces. However, a third-party implementer who only implement one side (only one of decoder/encoder) would not be able to talk efficiently with any vendor's optimized interface. Likewise, a third-party implementer who didn't implement any proprietary optimized interface also won't get any automatic benefit. Which leads to a "fall-back" behavior. –  rwong Jan 17 '12 at 8:12
    
@rwong if you are referring to actual MPEG class codecs transcoding interface (which provides partial decode/encode) should be a super set of encoding/decoding sides not replacement. It should be Public API rather than Private API. The real trouble with transcoding API is heterogeneity of the symbols across variety of formats and acceptable intermediate symbols. It's possible, but it is not quite there in public domain, because there is no money in doing this! –  Dipan Mehta Jan 17 '12 at 10:23
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