Friday, June 22, 2007

Representations of module dependencies

One of the key differences between JSR 291 and JSR 277 is the way in which module dependencies are represented, satisfied, and managed. Both module systems express dependencies using import declarations with version ranges and so, in general, there can be multiple ways of satisfying any particular dependency.

Using an electronics analogy, there can be multiple different ways of wiring a module's dependencies to other modules. Without further constraints, the choice of wiring is essentially non-deterministic, which would make the module system hard to understand and unpredictable in behaviour. The non-determinism needs to be removed.

JSR 291 removes the non-determinism by specifying rules for how dependencies should be satisfied in the presence of a given collection of modules, some of which are already resolved (or 'wired') and others which are unresolved (not yet 'wired').

JSR 277 also specifies some rules to remove the non-determinism, but it allows a module to implement an import policy if the module wants to override these rules. The hope is that, in many simple cases, the import policy will not be needed. But, in general, JSR 277 removes the non-determinism using a combination of rules in the specification and user-written import policies.

Why is this difference important? Well, an import policy is a piece of Java code running inside the module whose dependencies need to be satisfied. So the import policy is in the unusual position of not being able to use the modules upon which its module depends. This is a potential source of errors.

But a more important difference relates to the need to be able predict the behaviour of a collection of modules. This is crucial when managing the dependencies of modules. With JSR 291, an external management system is able to read the dependency declarations in each module and apply the rules in the specification to determine how these modules will be wired up, whether there are any missing dependencies, and, if there are, how such dependencies could be satisfied.

With JSR 277 the position is quite different when import policies are used. The only way to determine the behaviour of an import policy is to execute it. But even then, there is no guarantee that the import policy will give the same result each time it runs. Also, if there are missing dependencies, it is not feasible to examine the import policy to determine how the missing dependencies could be satisfied.

(For further reading, I recommend Peter Kriens' JSR 277 review from last October.)

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