Wednesday, February 03, 2010

"Strong" optionality

Richard Hall tells me Felix interprets optional imports as "strong", to use my earlier term. I believe there is enough latitude in the OSGi spec to permit this. Equinox takes the opposite interpretation and can discard optional imports of available packages in order to overcome uses constraints which would otherwise prevent resolution.

Regardless of that, I think that strong optionality is more likely to be what the user wants.

Without strong optionality, if the user provides a bundle which can satisfy some optional imports, they are not likely to be pleased if the resolver discards corresponding optional imports as the functions provided by the bundle will then be unavailable. Better to fail fast with diagnostics that allow the user to sort out the uses constraints.

Even worse, without strong optionality resolution may fail after a protracted attempt to discard combinations of optional imports in order to get a valid wiring. All the optional imports could be discarded before embarking upon a protracted search to ensure that the search is not in vain. Perhaps Equinox already does this, but I'm doubtful it as it could be criticised as optimising for the failure case.

OSGi resolution is NP-Complete. So what?

At last we have a proof, thanks to Robert Dunne, that the OSGi R4 resolution problem is NP-Complete. He showed that it is possible to represent a solution to the NP-Complete boolean satisfaction problem 3-SAT as a wiring of a carefully constructed set of OSGi bundles. After the idea arose of using a SAT solver to construct an OSGi resolver, some of us have been discussing the possibility of a SAT-based proof. The hard step was how to represent a boolean OR in terms of resolution, which Robert has now provided.

NP-Completeness tallies with the experience of those who have written an OSGi R4 resolver: it takes about a fortnight of intensive hacking, combined with sleepless nights, to crack the problem followed by a much longer period of bug fixing and optimisation. Now we know why: NP-Complete problems are hard to solve efficiently.

I'm partly to blame as I led the spec work on OSGi RFC 79 which added considerable function, and with it complexity, to the resolution algorithm. I presented some of the background at the 2004 OSGi Congress. The RFC 79 spec was one of the more interesting that the OSGi Core Platform Expert Group has worked on. There was a tension between addressing the basic use cases and creating something that was too hard to implement.

Fortunately, we developed a prototype resolver in the Equinox incubator. This provided useful feedback to the spec work, particularly when features were dropped from the spec which made the implementation more tractable. The team which created the prototype included Simon Burns, Steve Poole, and Tom Watson. Simon wrote the initial R4 resolver. Steve created a rigorous model-based test based on a slow, but functionally corrrect, resolver of his own. Tom provided much advice on the R3 resolver and eventually integrated the new resolver into Equinox and has been improving it ever since.

So what features emerged from this process that made the resolver NP-Complete? Well, crucial to the proof are the uses constraint and, its evil twin, optional imports. Steve Powell observes that the proof does not depend on the transitivity of the uses constraint. (Other, arguably much more evil, features in R4, require-bundle and fragments, also did not contribute to the resolver's intrinsic complexity.)

This suggests some practical ways round the problem. Examples occasionally crop up when the time to resolve a set of bundles becomes unacceptable. How can we work around such issues if we are willing to put in the effort?

Firstly, uses constraints can be avoided by preventing types from one package leaking out through the interface of another package. This is an application of the Law of Demeter.

Secondly, in some cases it may be possible to remove, or at least cut down, optional imports by using mandatory imports instead and splitting the function which has those dependencies across bundles so that the user's dependencies determine which bundles are needed.

These approaches are feasible only for new code. Bundles generated from popular Java libraries tend to exhibit both uses constraints and optional imports.

Another possibility we've been considering is a "strong" form of optionality in which the optional import cannot be discarded if it is locally satisfiable, i.e. before uses constraints are applied. Certainly the current definition of optional import is critical to Robert's proof. The challenge is to convince ourselves that R4 resolution with strong optionality in place of the current definition is no longer NP-Complete or, conversely, to prove the opposite.

I'm undecided. Strong optionality drastically reduces the search space in some cases. On the other hand, I feel there may be a proof based on 3-SAT in which multiple bundles import, but do not export, packages.


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