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Loose Coupling in a Service-Oriented Architecture


If you're looking for an introduction to one of the key concepts behind building a service-oriented architecture (SOA), you've come to the right place. This two-part article explains why loose coupling is important to SOA, the forms of loose coupling, and how to deal with it. It 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 15, 2010
TABLE OF CONTENTS:
  1. · Loose Coupling in a Service-Oriented Architecture
  2. · 4.2.1 Asynchronous Communication
  3. · 4.2.2 Heterogeneous Data Types
  4. · 4.2.3 Mediators

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Loose Coupling in a Service-Oriented Architecture - 4.2.3 Mediators
(Page 4 of 4 )

A third form of loose coupling has to do with how a service call performed by a consumer finds the provider that has to process this request. With a “point-to-point” approach, the sender sends the request to one specific physical system using its physical address. This is like sending a letter to a specific postal address, such as 42 Broadway in New York, NY.

This is a tightly coupled approach. What happens if the receiver moves houses? What happens if the receiver is out of order, or is getting flooded with too many messages? Mechanisms for failover and load balancing are required. That is, you need some kind of intermediary to switch between different physical receivers.  

In principle, there are two kinds of mediators:

  1. The first type tells you the correct endpoint for your service call before you send it. That is, you still have point-to-point connections, but with the help of these mediators, you send your service calls to different addresses. Such a mediator is often called a broker or name server. You ask for a service using a symbolic name, and the broker or name server tells you which physical system to send your request to. This form of mediation requires some additional intelligence at the consumer site. 
     
  2. The second type chooses the right endpoint for a request after the consumer sends it. In this case, the consumer sends the request to a symbolic name, and the infrastructure (network, middleware, ESB) routes the call to the appropriate system according to intelligent routing rules.

In practice, very different flavors of both forms of mediation occur. For example, there are service buses that send messages using a broadcasting approach, so that the sender sends a request to a logical receiver, and any of the different providers providing the requested service can process the call. On the other hand, Web Services are technically point-to-point connections, where you typically use a broker to find the physical endpoint of a request, and/or you insert some so-called interceptors that route your request at runtime to an available provider.

Chapter 5 will discuss details of how to deal with mediation in an ESB.

Please check back next week for the conclusion to this article.


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