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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 - 4.2.5 Binding
(Page 2 of 5 )

Binding (the task of correlating symbols of different modules or translation units) allows a form of loose coupling that is similar to weak type checking. Again, you have to decide between early binding, which allows verification at compile or start time, and late binding, which happens at runtime.

4.2.6  Platform Dependencies

In Table 4-1, there are two forms of coupling that deal with the question of whether or not certain platform constraints apply. Making a decision about the general one, platform dependencies, is easy. Of course, you have more freedom of choice if platform-independent solutions are preferred over strong platform dependencies. The second form is discussed next.

4.2.7  Interaction Patterns

A special form of platform dependencies has to do with which interaction patterns are used in service signatures (i.e., which programming paradigms are provided to design service interfaces). Which are the fundamental data types, and how can you combine them? A wide range of questions must be considered:

  1. Are only strings supported, or can other fundamental data types (integers, floats, Booleans, date/time types, etc.) be used?
  2. Are enumerations (limited sets of named integer values) supported?
  3. Can you constrain possible values (e.g., define certain string formats)?
  4. Can you build higher types (structures, records)?
  5. Can you build sequence types (arrays, lists)?
  6. Can you design relations between types (inheritance, extensions, polymorphism)?

The more complicated the programming constructs are, the more abstractly you can program. However, you will also have more problems mapping data to platforms that have no native support for a particular construct.

Based on my own experience (and that of others), I recommend that you have a basic set of fundamental types that you can compose to other types (structures, records) and sequences (arrays). Be careful with enumerations (an anachronism from the time when each byte did countóuse strings instead), inheritance, and polymorphism (even when XML supports it).

In general, be conservative with types, because once you have to support some language construct, you canít stop doing so, even if the effort it requires (including the ability to log and debug) is very high. For more on data types, see Section 15.5.


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