Design Notes

The adaptor interface is undoubtedly simple; in this section, we will ask ourselves whether it is too simplistic. The Adaptor implementation is still at the level of a prototype but, as we shall see in the next few pages, is significant enough to challenge us on all the issues that the people working with object persistence are trying to grapple with.

Design Goals

I wanted the Adaptor API to be transparent ; that is, to be able to change the type of persistent store at will. The idea was to write small prototypes without messing around with databases and then migrate to a database for the real thing by simply changing the adaptor. Further, I wanted to retain the flexibility of an object living in multiple persistent stores concurrently, because that is the only way to copy objects from one store to another.

I wanted to retain the best features of memory-based data structures (navigability, speed, ease of use) and those of databases (transactions, concurrency, queries), where available. Finally, I did not want the adaptor to break object encapsulation, which means that the implementation could not assume anything about how a module stores instance-specific information and, more subtly, how it constructs its objects.

Object Encapsulation

One important stricture that we easily forget is that an object is not just data. The three serialization modules we saw in the last chapter—FreezeThaw, Data::Dumper, and Storable — all make this assumption. They look past an object ...

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