The increasing complexity of software systems, together with the agile way in which we develop and evolve them today, demand an approach that makes the architecture of such systems deal with change. The solution to this challenge is called modularity. Modularity is a core design principle that makes code easier to understand and maintain by splitting it into small, isolated modules. This is not a new concept, but becomes more relevant each day. A modular code base lets us change, refactor, or simply replace code of modules without breaking other parts of the system. This makes modularity the ultimate agile tool.
Bringing modularity from the design to the runtime is not always easy and straightforward and cannot be done with plain Java alone. The only mature modularity solution for Java is OSGi. Over the years, OSGi has had the reputation of being too complex and difficult to use. Recently a lot has changed in the area of tooling and frameworks, however, bringing it on par with the development experience of traditional development stacks. With this we can focus on writing code while keeping an architectural focus on modularity.
This book describes how to apply those tools and techniques available today, and gives pragmatic insights into getting up and running with the required technology in no time. Using easy to understand, concise, but real-world code samples, we explore the application of modularity toward a new breed of web applications; software as a service over the Internet, more affectionately known as cloud applications. Along the way, we address typical cloud-age topics such as RESTful Web Services, NoSQL, Provisioning, Elasticity, Auto-Scaling, Hot Updates, and Automated Failover.
Primarily, this book is targeted to experienced enterprise Java developers who have a keen interest in modularity and who are looking for solutions to overcome some of today’s most advanced software development problems: how to deal with change and how to manage complexity in a code base. The authors of this book use the tools and techniques described throughout the book in everyday development and have put some sophisticated cloud applications in production using them. A significant part of what you will learn here is based upon OSGi. We will focus on practical advice, rather than the more theoretical approach commonly found in other books and resources.
The book will also be useful to developers who have tried using OSGi in the past, but for some reason got stuck. Because OSGi is not a product owned by a single vendor, it is sometimes hard to find hands-on, to-the-point information on how to get things working. To those readers, this book can be an approachable and practical guide to become unstuck and get things going again. By no means is this book a complete OSGi reference, but it does contain everything that you need to know in order to build sophisticated applications in the cloud age.
This book is organized into three parts. Part I introduces the concepts of modularity from a high-level perspective and translates these to actual implementation details. Part II focuses on developing cloud applications and the tools and technologies required. Part III deals with deploying these applications to production and setting up advanced cloud infrastructure capabilities such as auto-scaling and failover.
Additionally, Appendix A describes an example application that puts everything from this book together in a showcase. The code of this application is hosted online, and the appendix contains instructions on how to get it.
The following typographical conventions are used in this book:
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Constant width - Used for program listings, as well as within paragraphs to refer to program elements such as variable or function names, databases, data types, environment variables, statements, and keywords.
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Constant width bold - Shows commands or other text that should be typed literally by the user.
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Constant width italic - Shows text that should be replaced with user-supplied values or by values determined by context.
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