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<?xml version="1.0" encoding="UTF-8"?>
<chapter id="psa-intro">
<title>Introduction to Spring Services</title>
<sect1>
<title>Introduction</title>
<para>The goal of Spring's integration with distributed technologies is to
adapt plain .NET objects so they can be used with a specific distributed
technology. This integration is designed to be as non-intrusive as
possible. If you need to expose an object to a remote process then you can
define an exporter for that object. Similarly, on the client side you
define an corresponding endpoint accessor. Of course, the object's methods
stil need to be suitable for remoting, i.e. course grained, to avoid
making uncessary and expensive remote calls. </para>
<para>Since these exporters and client side endpoint accessors are defined
using meta data for Spring IoC container, you can easily use dependency
injection on then to set initial state and to 'wire up' the presentation
tier, such as web forms, to the service layer. In addition, you may apply
AOP aspects to the exported classes and/or service endpoints to apply
behavior such as logging, security, or other custom behavior that may not
be provided by the target distributed technology. The Spring specific
terminology for this approach to object distribution is known as Portable
Service Abstractions (PSA). As a result of this approach, you can decide
much later in the development process the technical details of how you
will distribute your objects as compared to traditional code centric
approaches. Changing of the implementation is done though configuration of
the IoC container and not by recompilation. Of course, you may choose to
not use the IoC container to manage these objects and use the exporter and
service endpoints programmatically.</para>
<para>The diagram shown below is a useful way to demonstrate the key
abstractions in the Spring tool chest and their interrelationships. The
four key concepts are; plain .NET objects, Dependency Injection, AOP, and
Portable Service Abstractions. At the heart sits the plain .NET object
that can be instantiated and configured using dependency injection. Then,
optionally, the plain object can be adapted to a specific distributed
technology. Lastly, additional behavior can be applied to objects. This
behavior is typically that which can not be easily address by traditional
OO approaches such as inheritance. In the case of service layer, common
requirements such as 'the service layer must be transactional' are
implemented in a manner that naturally expresses that intention in a
single place, as compared to scattered code across the service
layer.</para>
<para><mediaobject>
<imageobject>
<imagedata fileref="images/spring-triangle.png" format="PNG"
scale="50" />
</imageobject>
</mediaobject>Spring implements this exporter functionality by creating
a proxy at runtime that meets the implementation requirements of a
specific distributed technology. In the case of .NET Remoting the proxy
will inherit from MarshalByRef, for EnterpriseServices it will inherit
from ServicedComponent and for aspx web services, WebMethod attributes
will be added to methods. Client side functionality is often implemented
by a thin layer over the client access mechanism of the underlying
distributed technology, though in some cases such as client side access to
web services, you can the option to create a proxy on the fly from the
.wsdl definition, much like you would have done using the command line
tools.</para>
<para>The common implementation theme for you as a provider of these
service objects is to implement an interface. This is generally considered
a best practice in its own right, you will see most pure WCF examples
following this practice, and also lends itself to a straightforward
approach to unit testing business functionality as stub or mock
implementations maybe defined for testing purposes.</para>
<para>The assembly <literal>Spring.Services.dll</literal> contains support
for .NET Remoting, Enterprise Services and ASMX Web Services. Support for
WCF services is planned for Spring 1.2</para>
</sect1>
</chapter>
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