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Interesting applications rarely live in isolation. Whether your sales application must interface with your inventory application, your procurement application must connect to an auction site, or your PDA’s PIM must synchronize with the corporate calendar server, it seems like any application can be made better by integrating it with other applications.
All integration solutions have to deal with a few fundamental challenges:
Over time, developers have overcome these challenges with four main approaches:
While all four approaches solve essentially the same problem, each style has its distinct advantages and disadvantages. In fact, applications may integrate using multiple styles such that each point of integration takes advantage of the style that suits it best.
What is Messaging?
This book is about how to use messaging to integrate applications. A simple way to understand what messaging does is to consider the telephone system. A telephone call is a synchronous form of communication. I can only communicate with the other party if the other party is available at the time I place the call. Voice mail on the other hand, allows asynchronous communication. With voice mail, when the receiver does not answer, the caller can leave him a message; later the receiver (at his convenience) can listen to the messages queued in his mailbox. Voice mail enables the caller to leave a message now so that the receiver can listen to it later, which is lot easier than trying to get the caller and the receiver on the phone at the same time. Voice mail bundles (at least part of) a phone call into a message and queues it for later consumption; this is essentially how messaging works.
Messaging is a technology that enables high-speed, asynchronous, program-to-program communication with reliable delivery. Programs communicate by sending packets of data called messages to each other. Channels, also known as queues, are logical pathways that connect the programs and convey messages. A channel behaves like a collection or array of messages, but one that is magically shared across multiple computers and can be used concurrently by multiple applications. A sender or producer is a program that sends a message by writing the message to a channel. A receiver or consumer is a program that receives a message by reading (and deleting) it from a channel.
The message itself is simply some sort of data structure—such as a string, a byte array, a record, or an object. It can be interpreted simply as data, as the description of a command to be invoked on the receiver, or as the description of an event that occurred in the sender. A message actually contains two parts, a header and a body. The header contains meta-information about the message—who sent it, where it’s going, etc.; this information is used by the messaging system and is mostly (but not always) ignored by the applications using the messages. The body contains the data being transmitted and is ignored by the messaging system. In conversation, when an application developer who is using messaging talks about a message, he’s usually referring to the data in the body of the message.
Asynchronous messaging architectures are powerful, but require us to rethink our development approach. As compared to the other three integration approaches, relatively few developers have had exposure to messaging and message systems. As a result, application developers in general are not as familiar with the idioms and peculiarities of this communications platform.
What is a Messaging System?
Messaging capabilities are typically provided by a separate software system called a messaging system or message-oriented middleware (MOM). A messaging system manages messaging the way a database system manages data persistence. Just as an administrator must populate the database with the schema for an application’s data, an administrator must configure the messaging system with the channels that define the paths of communication between the applications. The messaging system then coordinates and manages the sending and receiving of messages. The primary purpose of a database is to make sure each data record is safely persisted, and likewise the main task of a messaging system is to move messages from the sender’s computer to the receiver’s computer in a reliable fashion.
The reason a messaging system is needed to move messages from one computer to another is that computers and the networks that connect them are inherently unreliable. Just because one application is ready to send a communication does not mean that the other application is ready to receive it. Even if both applications are ready, the network may not be working, or may fail to transmit the data properly. A messaging system overcomes these limitations by repeatedly trying to transmit the message until it succeeds. Under ideal circumstances, the message is transmitted successfully on the first try, but circumstances are often not ideal.
In essence, a message is transmitted in five steps:
This diagram illustrates these five transmission steps, which computer performs each, and which steps involve the messaging system:
This diagram also illustrates two important messaging concepts:
The create, send, receive, and process steps may seem like unnecessary overhead. Why not simply deliver the data to the receiver? By wrapping the data as a message and storing it in the messaging system, the applications delegate to the messaging system the responsibility of delivering the data. Because the data is wrapped as an atomic message, delivery can be retried until it succeeds and the receiver can be assured of reliably receiving exactly one copy of the data.
Why Use Messaging?
Now that we know what messaging is, we should ask: Why use messaging? As with any sophisticated solution, there is no one simple answer. The quick answer is that messaging is more immediate than File Transfer, better encapsulated than Shared Database, and more reliable than Remote Procedure Invocation. However, that’s just the beginning of the advantages that can be gained using messaging.
Specific benefits of messaging include:
So there are a number of different reasons an application or enterprise may benefit from messaging. Some of these are technical details that application developers relate most readily to, whereas others are strategic decisions that resonate best with enterprise architects. Which of these reasons is most important depends on the current requirements of your particular applications. They’re all good reasons to use messaging, so take advantage of whichever reasons provide the most benefit to you.
Challenges of Asynchronous Messaging
Asynchronous messaging is not the panacea of integration. It resolves many of the challenges of integrating disparate systems in an elegant way but it also introduces new challenges. Some of these challenges are inherent in the asynchronous model while other challenges vary with the specific implementation of a messaging system.
So asynchronous messaging does not solve all problems, and can even create some new ones. Keep these consequences in mind when deciding which problems to solve using messaging.
Messaging is an asynchronous technology, which enables delivery to be retried until it succeeds. In contrast, most applications use synchronous function calls; for example: a procedure calling a sub-procedure, one method calling another method, or one procedure invoking another remotely through a remote procedure call (RPC) (such as CORBA and DCOM). Synchronous calls imply that the calling process is halted while the sub-process is executing a function. Even in an RPC scenario, where the called sub-procedure executes in a different process, the caller blocks until the sub-procedure returns control (and the results) to the caller. When using asynchronous messaging, the caller uses a send and forget approach that allows it to continue to execute after it sends the message. As a result, the calling procedure continues to run while the sub-procedure is being invoked.
Asynchronous communication has a number of implications. First, we no longer have a single thread of execution. Multiple threads enable sub-procedures to run concurrently, which can greatly improve performance and help ensure that some sub-processes are making progress even while other sub-processes may be waiting for external results. However, concurrent threads can also make debugging much more difficult. Second, results (if any) arrive via a callback. This enables the caller to perform other tasks and be notified when the result is available, which can improve performance. However, the caller has to be able to process the result even while it is in the middle of other tasks, and it has to be able to use the result to remember the context in which the call was made. Third, asynchronous sub-processes can execute in any order. Again, this enables one sub-procedure to make progress even while another cannot. But it also means that the subprocesses must be able to run independently in any order, and the caller must be able to determine which result came from which sub-process and combine the results together. So asynchronous communication has several advantages but requires rethinking how a procedure uses its sub-procedures.
Distributed Applications vs. Integration
This book is about enterprise integration—how to integrate independent applications so that they can work together. An enterprise application often incorporates an n-tier architecture (a more sophisticated version of a client/server architecture) enabling it to be distributed across several computers. Even though this results in processes on different machines communicating with each other, this is application distribution, not application integration.
Why is an n-tier architecture considered application distribution and not application integration? First, the communicating parts are tightly coupled—they dependent directly on each other, so that one tier cannot function without the others. Second, communication between tiers tends to be synchronous. Third, an application (n-tier or atomic) tends to have human users that will only accept rapid system response.
In contrast, integrated applications are independent applications that can each run by itself, but coordinate with each other in a loosely coupled way. This enables each application to focus on one comprehensive set of functionality and yet delegate to other applications for related functionality. Integrated applications communicating asynchronously don’t have to wait for a response; they can proceed without a response or perform other tasks concurrently until the response is available. Integrated applications tend to have a broad time constraint, such that they can work on other tasks until a result becomes available, and therefore are more patient than most human users waiting real-time for a result.
Commercial Messaging Systems
The apparent benefits of integrating systems using an asynchronous messaging solution have opened up a significant market for software vendors creating messaging middleware and associated tools. We can roughly group the messaging vendors’ products into the following four categories:
The patterns in this book are vendor-independent and apply to most messaging solutions. Unfortunately, each vendor tends to define their own terminology when describing messaging solutions. In this book we have striven to choose pattern names that are technology- and product-neutral, yet descriptive and easy to use conversationally.
Many messaging vendors have incorporated some of this book’s patterns as features of their products, which simplifies applying the patterns and accelerates solution development. Readers who are familiar with a particular vendor’s terminology will most likely recognize many of the concepts in this book. To help these readers map the pattern language to the vendor-specific terminology, the following tables map the most common pattern names to their corresponding product feature names in some of the most widely-used messaging products.
This book is structured as a set of patterns organized into a pattern language. Books such as Design Patterns, Pattern Oriented Software Architecture, Core J2EE Patterns, and Patterns of Enterprise Application Architecture have popularized the concept of using patterns to document computer-programming techniques. Christopher Alexander pioneered the concept of patterns and pattern languages in his books A Pattern Language and A Timeless Way of Building. Each pattern represents a decision that the reader must make and the considerations that go into that decision. A pattern language is a web of related patterns where each pattern leads to others, guiding the reader through the decision making process. This approach is a powerful technique for documenting an expert’s knowledge so that it can be readily understood and applied by non-experts.
A pattern language teaches the reader how to solve a limitless variety of problems within a bounded problem space. Because the overall problem that is being solved is different every time, the path through the patterns and how they’re applied is also unique. In this way, this book was written for anyone using any messaging tools for any application, but can be applied specifically for you and the specific application of messaging that you are facing.
Just using the pattern form does not guarantee that a book contains a wealth of knowledge. It is not just enough to simply say, “When you face this problem, apply this solution.” For a reader to truly learn from a pattern, it has to document why the problem is difficult to solve, consider possible solutions that in fact don’t work well, and explain why the solution offered is the best available. Likewise, the patterns need to connect to each other so as to walk the reader from one problem to the next. In this way, the pattern form can be used to teach the reader not just what solutions to apply, but how to solve problems the author could not have predicted. These are goals we strive to accomplish in this book.
Patterns should be prescriptive, meaning that they should tell you what to do. They don’t just describe a problem, and they don’t just describe how to solve it, they tell you what to do to solve it. Each pattern represents a decision the reader must make: “Should I use Messaging?” “Would a Reply Message help me here?” The point of the patterns and the pattern language is to help the reader make decisions that lead to a good solution for his specific problem, even if the authors didn’t have that specific problem in mind, and even if the reader doesn’t have the knowledge and experience to develop that solution on his own.
There is no one universal pattern form; different books use various structures. We used a style that is fairly close to the Alexandrian form, which was first popularized for computer programming in Smalltalk Best Practice Patterns by Kent Beck. We like the Alexandrian form because it results in patterns that are more prose-like. As a result, even though each pattern follows an identical, well-defined structure, the format avoids headings for each individual sub-section, which disrupt the flow of the discussion. To improve navigability, the format uses style elements such as bolding, indentation, and pictures to help the reader identify important sections even at a quick glance.
This pattern language uses the following pattern structure:
The beauty in describing solutions as patterns is that it not only teaches the reader how to solve the specific problems discussed, but also how to create designs that solve problems the authors were not even aware of. As a result, these patterns for messaging describe not only messaging systems that exist today, but may also apply to new ones created well after this book is published.
Integration solutions consist of many different pieces—applications, databases, endpoints, channels, messages, routers, etc. If we want to describe an integration solution, we need to define a notation that accommodates all these different components. To our knowledge, there is no widely used, comprehensive notation that is geared towards the description of all aspects of an integration solution. The Unified Modeling Language (UML) does a fine job of describing object-oriented systems with class and interaction diagrams, but it does not contain semantics to describe messaging solutions. The UML Profile for EAI [UMLEAI] enriches the semantics of collaboration diagrams to describe message flows between components. This notation is very useful as a precise visual description of a system that can serve as the basis for code generation as part of a model-driven architecture (MDA). We decided not to adopt this notation for two reasons. First, the UML Profile does not capture all the patterns described in our pattern language. Second, we were not looking to create a precise visual specification, but images that have a certain ‘sketch’ quality to them. We wanted pictures that are able to convey the essence of a pattern to the reader at a quick glance—very much like Alexander’s sketch. That’s why we decided to create our own ‘notation’. Luckily, unlike the more formal notation, ours does not require you to read a large manual. A simple picture should suffice:
This simple picture shows a message being sent to a component over a channel. We use the word component very loosely here—it can indicate an application that is being integrated, an intermediary that transforms or routes the message between applications, or a specific part of an application. Sometimes, we also depict a channel as a three-dimensional pipe if we want to highlight the channel itself. Often times we are more interested in the components and draw the channels as simple lines with arrow heads. The two notations are equivalent. We depict the message as a small tree with a round root and nested, square elements. The tree elements can be shaded or colored to highlight their usage in a particular pattern. Many messaging systems allow messages to contain tree-like data structures, for example XML documents. Also, depicting messages in this way allows us to provide a quick visual description of transformation patterns—it will be easy to show a pattern that adds, re-arranges or removes fields from the message.
When we describe application designs—for example, messaging endpoints or examples written in C# or Java—we do use standard UML class and sequence diagrams to depict the class hierarchy and the interaction between objects. The UML notation is widely accepted as the standard way of describing these types of solutions (if you need a refresher on UML, have a look at [UML]).
Examples and Interludes
We have tried to underline the broad applicability of the patterns by including implementation examples using a variety of integration technologies. The potential downside of this approach is that you may not be familiar with each technology that is being used in an example. That’s why we made sure that reading the examples is strictly optional — all relevant points are discussed in the pattern description. Therefore, you can safely skip the examples without risk of losing out on important detail. Also, where possible, we provided more than one implementation example using different technologies.
When presenting example code we focused on readability over runnability. A code segment can help remove any potential ambiguity left by the solution description and many application developers and architects prefer looking at 30 lines of code as opposed to reading many paragraphs of text. To support this intent we often only show the most relevant methods or classes of a potentially larger solution. We also omitted most forms of error checking to highlight the core function implemented by the code. Most code snippets do not contain in-line comments as the code is explained in the paragraphs before and after the code segment.
Providing a meaningful example for a single integration pattern is challenging. Enterprise integration solutions typically consist of a number of heterogeneous components, spread across multiple systems. Likewise, most integration patterns do not operate in isolation but rely on other patterns to form a meaningful solution. To highlight the collaboration between multiple patterns we included more comprehensive examples as interludes at the end of the major sections of the book. These solutions illustrate many of the trade-offs involved in designing a more comprehensive messaging solution.
All code samples should be treated as illustrative tools only and not as a starting point for development of an integration solution. For example, almost all examples lack any form of error checking or concern for robustness, security, or scalability.
We tried as much as possible to base the examples on software platforms that are available free of charge or as a trial version. In some cases, we used commercial platforms (such as TIBCO ActiveEnterprise or Microsoft BizTalk) to illustrate the difference between developing a solution from scratch and using a commercial tool. We presented those example in such a way that they are educational even if you do not have access to the required run-time platform. For many examples, we use relatively bare-bones messaging frameworks such as JMS or MSMQ. This allows us to be more explicit in the example and focus on the problem at hand instead of distracting from it with all the features a more complex middleware toolset may provide.
The Java examples in this book are based on the JMS 1.1 specification, which is part of the J2EE 1.4 specification. By the time this book is published, most messaging and application server vendors will support JMS 1.1. You can download Sun’s reference implementation of the JMS specification from Sun’s Web site: http://java.sun.com/j2ee.
The Microsoft .NET examples are based on Version 1.1 of the .NET Framework and are written in C#. You can download the .NET Framework SDK from Microsoft’s Web site: http://msdn.microsoft.com/net.
Organization of this Book
The pattern language in this book, as with any pattern language, is a web of patterns referring to each other. At the same time, some patterns are more fundamental than others, forming a hierarchy of big-concept patterns that lead to finer-detailed patterns. The big-concept patterns form the load-baring members of the pattern language. They are the main ones, what we term root patterns, that provide the foundation of the language and support the other patterns.
This book groups patterns into chapters by level-of-abstraction and by topic area. The following diagram shows the root patterns and their relationship to the chapters of the book.
The most fundamental pattern is Messaging; that’s what this whole book is about. It leads to the six root patterns—which are in the Messaging Systems chapter—namely Message Channel, Message, Pipes and Filters, Message Router, Message Translator, and Message Endpoint. In turn, each of these root patterns leads to its own chapter in the book (except Pipes and Filters, which is not specific to messaging but is the basis of the routing and transformation patterns).
The pattern language is divided into eight chapters, which follow the hierarchy described above:
These eight chapters go together to teach you what you need to know about connecting applications using messaging.
With any book that has a lot to teach, it’s hard to know where to start, both for the authors and the readers. Reading all of the pages straight through assures covering the entire subject area, but isn’t the quickest way to get to the issues that are of the most help. Starting with a pattern in the middle of the language can be like starting to watch a movie that’s half over; you see what’s happening but don’t understand what it means.
Luckily, the pattern language is formed around root patterns (as described earlier). These root patterns collectively provide an overview of the pattern language, and individually provide starting points for delving deep into the details of messaging. To get an overall survey of the language without reviewing all of the patterns, start with reviewing the root patterns. To jump into the middle of the language, jump in at a root pattern, a place where the language has finished discussing one major topic and is now starting another.
Chapter 1: Integration Styles provides an overview of the four main application integration techniques and settles on Messaging as being the best overall for many integration opportunities. Read this chapter if you are unfamiliar with issues involved in application integration and the pros and cons of the various approaches that are available. If you just want to know what’s so great about messaging, go straight to that pattern. If you’re already convinced that messaging is the way to go and want to get started with how to use messaging, you can skip the first chapter completely.
Chapter 2: Messaging Systems contains all of this pattern language’s root patterns (except Messaging, which is in the first chapter). For an overview of the pattern language, read (or at least skim) all of the patterns in this chapter. To dive deep on a particular topic, read its root pattern, then go to the patterns mentioned in its next section at the end of the pattern; those next patterns will all be in a chapter named after the root pattern.
The root patterns in this language are:
After the first two chapters, different types of messaging developers may be most interested in different chapters, based on the specifics of how each group uses messaging to perform integration:
Keep in mind that when reading a pattern, if you’re in a hurry, start by just reading the problem and solution (the two sentences in bold). This will give you enough information to determine if the pattern is of interest to you right now, and if you already know the pattern. If you do not know the pattern and it sounds interesting, go ahead and read the other parts.
Also remember that this is a pattern language, so the patterns are not necessarily meant to be read in the order they’re presented in the book. The book’s order teaches you about messaging by considering all of the relevant topics in turn and discussing related issues together. To use the patterns to solve a particular problem, start with an appropriate root pattern. Its context explains what patterns need to be applied before this one, even if they’re not the ones immediately preceding this one in the book. Likewise, the next section (the last paragraph of the pattern) describes what patterns to consider applying after this one, even if they’re not the ones immediately following this one in the book. Use the web of interconnected patterns, not the linear list of book pages, to guide you through the material.
Supporting Web Site
Please look for companion information to this book plus related information on enterprise integration at our Web site: www.enterpriseintegrationpatterns.com. You can also e-mail your comments, suggestions and feedback to us at email@example.com.
You should now have a good understanding of the following concepts which are fundamental to the material in this book:
You should also have a feel for how this book is going to teach you how to use messaging:
Now that you understand the basic concepts and how the material will be presented, you are now ready to start learning how to integrate applications using messaging.
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