Specification

f'(x):g(f(x))

Description

Similar to the mapper pattern, the functional overlay pattern consists of modifying a service by routing requests for the service to a transparent wrapper function. The wrapper function may provide, for example, access control, logging or other interception operations before issuing a sub-request for the original unwrapped service. The response from the sub-request is transparently returned as the result for the original request.

Usage

This pattern can be used to modify or augment an existing service without requiring any higher order consumers of that service to themselves be rewritten.

The URI gatekeeper accessor is frequently used as a transparent functional overlay to provide access control over a URI address space.