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Dealing with Loose Coupling in a Service-Oriented Architecture


In this conclusion to a two-part article series on loose coupling in SOA, we wrap up our discussion of the forms of loose coupling, and explain how to handle its complexity. This article is excerpted from chapter four of the book SOA in Practice: The Art of Distributed System Design, written by Nicolai Josuttis (O'Reilly, 2008; ISBN: 0596529554). Copyright © 2008 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

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By: O'Reilly Media
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January 19, 2010
TABLE OF CONTENTS:
  1. · Dealing with Loose Coupling in a Service-Oriented Architecture
  2. · 4.2.5 Binding
  3. · 4.2.8 Compensation
  4. · 4.2.9 Control of Process Logic
  5. · 4.3 Dealing with Loose Coupling

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Dealing with Loose Coupling in a Service-Oriented Architecture - 4.3 Dealing with Loose Coupling
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The forms of loose coupling discussed in this chapter are only some (more or less typical) examples. Again, note that there are no hard and fast rules: you will have to decide on the appropriate amount of loose coupling for your specific context and architecture.

I have seen very different decisions made with regard to different types of coupling. As I mentioned earlier, the policy of one of my customers was to avoid asynchronous communication whenever possible, based on the experience that it led to race conditions at runtime that were very hard, or even impossible, to reproduce in a development environment, and therefore almost impossible to fix. Another customer in the same domain had a policy that synchronous calls were allowed only for reading service calls because the performance was not good enough for writing service calls.

Note that you might also have to decide about combinations of forms of loose coupling. For example, one important decision youíll have to make is whether an ESB should be separated from a backend via a protocol, or via an API (see Section 5.3.3). Separating via an API usually means that the ESB provides libraries each backend or backend adapter has to use. So, deployment and binding become issues. On the other hand, using a common API, you can hide some aspects of synchronous or asynchronous communications inside the ESB.

You might ask which forms of loose coupling are typical. To my best knowledge, there is no answer. All I can say is that the larger systems are, the more loosely they should be coupled.

4.4  Summary

  • Loose coupling is a fundamental concept of SOA (and large distributed systems in general) aimed at reducing dependencies between different systems.
  • There are different forms of loose coupling, and you will have to find the mixture of tight and loose coupling thatís appropriate for your specific context and project.
  • Any form of loose coupling has drawbacks. For this reason, loose coupling should never be an end in itself.
  • The need to map data is usually a good property of large systems.
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