Enterprise Integration Styles
Enterprise integration is the task of making disparate applications work together to produce a unified set of functionality. These applications can be custom developed in house or purchased from third-party vendors. They likely run on multiple computers, which may represent multiple platforms, and may be geographically dispersed. Some of the applications may be run outside of the enterprise by business partners or customers. Other applications might not have been designed with integration in mind and are difficult to change. These issues and others like them make application integration complicated. This chapter explores multiple integration approaches that can help overcome these challenges.
Application Integration Criteria
What makes good application integration? If integration needs were always the same, there would be only one integration style. Yet, like any complex technological effort, application integration involves a range of considerations and consequences that should be taken into account for any integration opportunity.
The fundamental criterion is whether to use application integration at all. If you can develop a single, standalone application that doesn't need to collaborate with any other applications, you can avoid the whole integration issue entirely. Realistically, though, even a simple enterprise has multiple applications that need to work together to provide a unified experience for the enterprise's employees, partners, and customers.
The following are some other main decision criteria.
Application coupling Integrated applications should minimize their dependencies on each other so that each can evolve without causing problems to the others. As explained in Chapter 1, "Solving Integration Problems Using Patterns," tightly coupled applications make numerous assumptions about how the other applications work; when the applications change and break those assumptions, the integration between them breaks. Therefore, the interfaces for integrating applications should be specific enough to implement useful functionality but general enough to allow the implementation to change as needed.
Intrusiveness When integrating an application into an enterprise, developers should strive to minimize both changes to the application and the amount of integration code needed. Yet, changes and new code are often necessary to provide good integration functionality, and the approaches with the least impact on the application may not provide the best integration into the enterprise.
Technology selection Different integration techniques require varying amounts of specialized software and hardware. Such tools can be expensive, can lead to vendor lock-in, and can increase the learning curve for developers. On the other hand, creating an integration solution from scratch usually results in more effort than originally intended and can mean reinventing the wheel.
Data format Integrated applications must agree on the format of the data they exchange. Changing existing applications to use a unified data format may be difficult or impossible. Alternatively, an intermediate translator can unify applications that insist on different data formats. A related issue is data format evolution and extensibilityhow the format can change over time and how that change will affect the applications.
Data timeliness Integration should minimize the length of time between when one application decides to share some data and other applications have that data. This can be accomplished by exchanging data frequently and in small chunks. However, chunking a large set of data into small pieces may introduce inefficiencies. Latency in data sharing must be factored into the integration design. Ideally, receiver applications should be informed as soon as shared data is ready for consumption. The longer sharing takes, the greater the opportunity for applications to get out of sync and the more complex integration can become.
Data or functionality Many integration solutions allow applications to share not only data but functionality as well, because sharing of functionality can provider better abstraction between the applications. Even though invoking functionality in a remote application may seem the same as invoking local functionality, it works quite differently, with significant consequences for how well the integration works.
Remote Communication Computer processing is typically synchronousthat is, a procedure waits while its subprocedure executes. However, calling a remote subprocedure is much slower than a local one so that a procedure may not want to wait for the subprocedure to complete; instead, it may want to invoke the subprocedure asynchronously, that is, starting the subprocedure but continuing with its own processing simultaneously. Asynchronicity can make for a much more efficient solution, but such a solution is also more complex to design, develop, and debug.
Reliability Remote connections are not only slow, but they are much less reliable than a local function call. When a procedure calls a subprocedure inside a single application, it's a given that the subprocedure is available. This is not necessarily true when communicating remotely; the remote application may not even be running or the network may be temporarily unavailable. Reliable, asynchronous communication enables the source application to go on to other work, confident that the remote application will act sometime later.
So, as you can see, there are several different criteria that must be considered when choosing and designing an integration approach. The question then becomes, Which integration approach best addresses which of these criteria?
Application Integration Options
There is no one integration approach that addresses all criteria equally well. Therefore, multiple approaches for integrating applications have evolved over time. The various approaches can be summed up in four main integration styles.
File Transfer Have each application produce files of shared data for others to consume and consume files that others have produced.
Shared Database Have the applications store the data they wish to share in a common database.
Remote Procedure Invocation Have each application expose some of its procedures so that they can be invoked remotely, and have applications invoke those to initiate behavior and exchange data.
Messaging Have each application connect to a common messaging system, and exchange data and invoke behavior using messages.
This chapter presents each style as a pattern. The four patterns share the same problem statementthe need to integrate applicationsand very similar contexts. What differentiates them are the forces searching for a more elegant solution. Each pattern builds on the last, looking for a more sophisticated approach to address the shortcomings of its predecessors. Thus, the pattern order reflects an increasing order of sophistication, but also increasing complexity.
The trick is not to choose one style to use every time but to choose the best style for a particular integration opportunity. Each style has its advantages and disadvantages. Applications may integrate using multiple styles so that each point of integration takes advantage of the style that suits it best. Likewise, an application may use different styles to integrate with different applications, choosing the style that works best for the other application. As a result, many integration approaches can best be viewed as a hybrid of multiple integration styles. To support this type of integration, many integration and EAI middleware products employ a combination of styles, all of which are effectively hidden in the product's implementation.
The patterns in the remainder of this book expand on the Messaging integration style. We focus on messaging because we believe that it provides a good balance between the integration criteria but is also the most difficult style to work with. As a result, messaging is still the least well understood of the integration styles and a technology ripe with patterns that quickly explain how to use it best. Finally, messaging is the basis for many commercial EAI products, so explaining how to use messaging well also goes a long way in teaching you how to use those products. The focus of this section is to highlight the issues involved with application integration and how messaging fits into the mix.