This book emphasizes practical solutions over theoretical formalisms. The basic ideas behind many presented patterns are well-known to experienced system developers—for example, dispatching, demultiplexing, callbacks, and configuration—and are sometimes variants of more general OO patterns—for example, proxies, adapters, and facades. This book's main contribution centers on in-depth engineering solutions based upon these ideas. Middleware developers must resolve a wide range of forces including throughput, responsiveness, dependability, interoperability, portability, extensibility, and accommodating legacy software. The diversity and severity of these forces accounts for the complexity of middleware patterns, as opposed to those seen in smaller-scale OO applications and concurrent programming.