by Roy Thomas Fielding Doctor of Philosophy in Information and Computer Science University of California, Irvine, 2000 Professor Richard N. Taylor, Chair

ABSTRACT OF THE DISSERTATION

The World Wide Web has succeeded in large part because its software architecture has been designed to meet the needs of an Internet-scale distributed hypermedia system. The Web has been iteratively developed over the past ten years through a series of modifications to the standards that define its architecture. In order to identify those aspects of the Web that needed improvement and avoid undesirable modifications, a model for the modern Web architecture was needed to guide its design, definition, and deployment.

Software architecture research investigates methods for determining how best to partition a system, how components identify and communicate with each other, how information is communicated, how elements of a system can evolve independently, and how all of the above can be described using formal and informal notations. My work is motivated by the desire to understand and evaluate the architectural design of network-based application software through principled use of architectural constraints, thereby obtaining the functional, performance, and social properties desired of an architecture. An architectural style is a named, coordinated set of architectural constraints.

This dissertation defines a framework for understanding software architecture via architectural styles and demonstrates how styles can be used to guide the architectural design of network-based application software. A survey of architectural styles for network-based applications is used to classify styles according to the architectural properties they induce on an architecture for distributed hypermedia. I then introduce the Representational State Transfer (REST) architectural style and describe how REST has been used to guide the design and development of the architecture for the modern Web.

REST emphasizes scalability of component interactions, generality of interfaces, independent deployment of components, and intermediary components to reduce interaction latency, enforce security, and encapsulate legacy systems. I describe the software engineering principles guiding REST and the interaction constraints chosen to retain those principles, contrasting them to the constraints of other architectural styles. Finally, I describe the lessons learned from applying REST to the design of the Hypertext Transfer Protocol and Uniform Resource Identifier standards, and from their subsequent deployment in Web client and server software.

INTRODUCTION

Excuse me ... did you say ‘knives’? — City Gent #1 (Michael Palin), The Architects Sketch [111] As predicted by Perry and Wolf [105], software architecture has been a focal point for software engineering research in the 1990s. The complexity of modern software systems have necessitated a greater emphasis on componentized systems, where the implementation is partitioned into independent components that communicate to perform a desired task. Software architecture research investigates methods for determining how best to partition a system, how components identify and communicate with each other, how information is communicated, how elements of a system can evolve independently, and how all of the above can be described using formal and informal notations. A good architecture is not created in a vacuum. All design decisions at the architectural level should be made within the context of the functional, behavioral, and social requirements of the system being designed, which is a principle that applies equally to both software architecture and the traditional field of building architecture. The guideline that “form follows function” comes from hundreds of years of experience with failed building projects, but is often ignored by software practitioners. The funny bit within the Monty Python sketch, cited above, is the absurd notion that an architect, when faced with the goal of designing an urban block of flats (apartments), would present a building design with all the components of a modern slaughterhouse. It might very well be the best slaughterhouse design ever conceived, but that would be of little comfort to the prospective tenants as they are whisked along hallways containing rotating knives.