ffirs.fm Page ii Wednesday, March 21, 2007 11:14 AM ffirs.fm Page i Wednesday, March 21, 2007 11:14 AM P A T T E R N - O R I E N T E D S O F T W A R E A R C H I T E C T U R E ffirs.fm Page ii Wednesday, March 21, 2007 11:14 AM ffirs.fm Page iii Wednesday, March 21, 2007 11:14 AM PATTERN-ORIENTED SOFTWARE ARCHITECTURE On Patterns and Pattern Languages Volume 5 Frank Buschmann, Siemens, Munich, Germany Kevlin Henney, Curbralan, Bristol, UK Douglas C. Schmidt, Vanderbilt University, Tennessee, USA ffirs.fm Page iv Wednesday, March 21, 2007 11:14 AM Copyright © 2007 John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England Telephone (+44) 1243 779777 Email (for orders and customer service enquiries): [email protected] Visit our Home Page on www.wiley.com All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London W1T 4LP, UK, without the permission in writing of the Publisher. Requests to the Publisher should be addressed to the Permissions Department, John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex PO19 8SQ, England, or emailed to [email protected], or faxed to (+44) 1243 770620. Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The Publisher is not associated with any product or vendor mentioned in this book. This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the Publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. Other Wiley Editorial Offices John Wiley & Sons Inc., 111 River Street, Hoboken, NJ 07030, USA Jossey-Bass, 989 Market Street, San Francisco, CA 94103-1741, USA Wiley-VCH Verlag GmbH, Boschstr. 12, D-69469 Weinheim, Germany John Wiley & Sons Australia Ltd, 42 McDougall Street, Milton, Queensland 4064, Australia John Wiley & Sons (Asia) Pte Ltd, 2 Clementi Loop #02-01, Jin Xing Distripark, Singapore 129809 John Wiley & Sons Canada Ltd, 22 Worcester Road, Etobicoke, Ontario, Canada M9W 1L1 Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN-13: 978-0-471-48648-0 Typeset in 10/13 Bookman-Light by Laserwords Private Limited, Chennai, India Printed and bound in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire This book is printed on acid-free paper responsibly manufactured from sustainable forestry in which at least two trees are planted for each one used for paper production. ffirs.fm Page v Wednesday, March 21, 2007 11:14 AM † For Anna, Bebé , and Martina Frank Buschmann For Carolyn, Stefan, and Yannick Kevlin Henney For Lori, Bronson, Mom, and Dad Douglas C. Schmidt For John Frank Buschmann, Kevlin Henney, Douglas C. Schmidt † Bebé, July 3, 1999 ffirs.fm Page vi Wednesday, March 21, 2007 11:14 AM ftoc.fm Page vii Wednesday, March 21, 2007 11:12 AM Table of Contents Foreword by Richard P. Gabriel . . . . . . . xiii Foreword by Wayne Cool . . . . . . . . . . . . . xxiii About the Authors . . . . . . . . . . . . . . . . . . . xxix About this Book . . . . . . . . . . . . . . . . . . . . . xxxi Guide to the Reader . . . . . . . . . . . . . . . . . . xxxvii 0 Beyond the Hype . . . . . . . . . . . . . . . . . . . . . . . 1 0.1 Beginnings… . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 0.2 A Story of Success… with Some Exceptions . . . . 3 0.3 Pattern Definitions and their Interpretations . . . 8 0.4 Toward a Deeper Understanding of Patterns . . . . 19 Part I Inside Patterns . . . . . . . . . . . . . . . . . . . . . . . . 25 1 A Solution to a Problem and More . . . . . . . . . . 29 1.1 A Solution to a Problem . . . . . . . . . . . . . . . . . . . 30 1.2 A Process and a Thing . . . . . . . . . . . . . . . . . . . . 32 1.3 Best of Breed . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 1.4 Forces: the Heart of Every Pattern . . . . . . . . . . . 36 1.5 The Context: Part of a Pattern or Not? . . . . . . . . 42 1.6 Genericity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 ftoc.fm Page viii Wednesday, March 21, 2007 11:12 AM viii Contents 1.7 A Diagram Says More than a Thousand Words… or Less . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 1.8 Evocative Names Help Pattern Recollection . . . . 54 1.9 Patterns are Works in Progress . . . . . . . . . . . . . 57 1.10 A Pattern Tells a Story and Initiates a Dialog . . . 61 1.11 A Pattern Celebrates Human Intelligence . . . . . . 62 1.12 From a Problem–Solution Statement to a Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 2 A Million Different Implementations . . . . . . . 65 2.1 Does One Size Fit All? . . . . . . . . . . . . . . . . . . . . 66 2.2 Patterns and Frameworks . . . . . . . . . . . . . . . . . 77 2.3 Patterns and Formalisms . . . . . . . . . . . . . . . . . 84 2.4 A Million and One… and then Some . . . . . . . . . 87 3 Notes on Pattern Form . . . . . . . . . . . . . . . . . . 91 3.1 Style and Substance . . . . . . . . . . . . . . . . . . . . . 92 3.2 The Function of Form . . . . . . . . . . . . . . . . . . . . 96 3.3 Elements of Form . . . . . . . . . . . . . . . . . . . . . . . 97 3.4 Details, Details . . . . . . . . . . . . . . . . . . . . . . . . . 102 3.5 Aerial View . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 3.6 Different Pattern Forms . . . . . . . . . . . . . . . . . . . 110 3.7 Style and Substance (Redux) . . . . . . . . . . . . . . . 116 Part II Between Patterns . . . . . . . . . . . . . . . . . . . . . . 117 4 Pattern Islands? . . . . . . . . . . . . . . . . . . . . . . . 121 4.1 Patterns Connect . . . . . . . . . . . . . . . . . . . . . . . 122 4.2 A Design Experiment: Patterns as Islands . . . . . 123 4.3 A Second Design Experiment: Interwoven Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 4.4 Pattern Density . . . . . . . . . . . . . . . . . . . . . . . . . 131