A Structural Pattern For Designing Transparent Layered Services

Facilities for instrumentation or runtime “printf” debugging are commonly implemented by instrumentation or debugging statements interspersed within program code. Such hardwiring makes the programs dependent on assumptions about the implementations of these services (meta-facilities) that are used by almost every program module. As a result, program modules become difficult to reuse in contexts where the meta-services may be absent or have different implementations. This pattern shows how to organize the meta-level code so that it can be changed with minimal effects on the original program.