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Scalable Abstractions

It has been a goal for some time in our industry to create reusable components. Unfortunately, there is little agreement on the meaning of the term component, nor on a related term, module (which some people consider synonymous with component). Proposed definitions usually start with assumptions about the platform, granularity, deployment and configuration scenarios, versioning issues, etc. (see [Szyperski1998]).

We’ll avoid that discussion and use the term component informally to refer to a grouping of types and packages that exposes coherent abstractions (preferably just one) for the services it offers, that has minimal coupling to other components, and that is internally cohesive.

All languages offer mechanisms for defining components, at least to some degree. Objects are the primary encapsulation mechanism in object-oriented languages. However, objects alone aren’t enough, because we quickly find that objects naturally cluster together into more coarse-grained aggregates, especially as our applications grow. Generally speaking, an object isn’t necessarily a component, and a component may contain many objects. Scala and Java offer packages for aggregating types. Ruby modules serve a similar purpose, as do C# and C++ namespaces.

However, these packaging mechanisms still have limitations. A common problem is that they don’t clearly define what is publicly visible outside the component boundary and what is internal to the component. For example, in Java, any public ...

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