First-class driver stack

[prdoyle]

BoskDriver is designed to be a composable interface that combines functionality by stacking drivers in layers. However, this stacking occurs only during construction; after that, only the driver objects themselves implicitly "know" the stack structure, with each driver having a reference to its downstream driver.

Examples of things we currently can't do:

• List the layers for observability
• Look up a driver with a given type (eg. "give me the MongoDriver")
  • (What if there are more than one?)
• Dynamically modify the driver stack (do we actually want this?)

Perhaps the current DriverFactory and DriverStack abstractions should evolve into more than just a discipline for instantiating BoskDriver objects.

[prdoyle]

I'm thinking we replace DriverFactory with DriverLayer. The layer object would act as a factory, but would also offer a bunch of metadata about the driver.

DriverStack would then be pretty much a list of DriverLayer objects. It should be possible to interpose on a driver stack at bosk creation time and modify the list, especially to insert more layers. Such interposing modules could be installed using Java's Service Provider Interface, so the existence of a particular library could automatically cause additional layers to be injected, configured by metadata from the other layers and/or the stack as a whole.