Pattern-Oriented Software Architecture Volume 3: Patterns for Resource Management

1 Introduction

“A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.”

Douglas Adams

A resource is an entity that is available in limited supply such that there exists a requestor, the resource user, that needs the entity to perform a function, and there exists a mechanism, the resource provider, that provides the entity on request. In the context of software systems, a resource can include, among other things, memory, synchronization primitives, file handles, network connections, security tokens, database sessions, and local as well as distributed services. A resource can be anything from a heavyweight object such as an application server component [VSW02] to a finegrained lightweight object such as a file handle.

Determining what is a resource can sometimes be challenging. For example, in programming environments an image, such as a JPEG or GIF file, is often referred to as a resource. In reality, however, there are no acquisition and release semantics defined for an image—instead, it is the data that makes up the image that constitutes a resource. Therefore a more accurate representation would be to treat the memory an image is using as a resource, which needs to be acquired when the image is loaded and released when the image is no longer needed.

There are numerous ways of categorizing resources. In the simplest, resources can be viewed as either reusable or non-reusable. Reusable resources ...