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<a name="Using-a-Distributed-Language"></a>
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Previous: <a href="Using-a-Middleware-Environment.html#Using-a-Middleware-Environment" accesskey="p" rel="prev">Using a Middleware Environment</a>, Up: <a href="Introduction-to-distributed-systems.html#Introduction-to-distributed-systems" accesskey="u" rel="up">Introduction to distributed systems</a> [<a href="About-This-Guide.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
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<a name="Using-a-Distributed-Language-1"></a>
<h4 class="subsection">1.1.3 Using a Distributed Language</h4>
<p>Rather than defining a new language like the CORBA IDL, an alternative is
to extend an existing programming language with distributed
features. The distributed object paradigm provides a more
object-oriented approach to programming distributed systems. The notion
of a distributed object is an extension to the abstract data type that
allows the services provided in the type interface to be called
independently of where the actual service is executed. When combined
with object-oriented features such as inheritance and polymorphism,
distributed objects offer a more dynamic and structured computational
environment for distributed applications.
</p>
<p>The Distributed Systems Annex (DSA) of Ada defines several
extensions that allow the user to write a distributed system entirely in
Ada. The types of distributed objects, the services they provide, and
the bodies of the remote methods to be executed are all defined in conventional
Ada packages. The Ada model is analogous to the Java/RMI model.
In both languages, the IDL is replaced by well-defined language constructs.
Therefore, the language supports both remote procedure
calls and remote object method invocations transparently, and the semantics
of distribution are consistent with the rest of the language.
</p>
<p>A program written in such a language is intended to communicate with a
program written in the same language, but this apparent restriction has
several useful consequences. The language can provide more powerful
features because it is not constrained by the common features available
in all host languages. In Ada, the user will define a specification of
remote services and implement them exactly as he would for ordinary,
non-distributed services. His Ada environment will compile them to
produce a stub file (on the caller side) and a skeleton file that
automatically includes the body of the services (on the receiver
side). Creating objects, obtaining or registering object references or
adapting the object skeleton to the user object implementation are made
transparent because the language environment has a full control over the
development process.
</p>
<p>Comparing with multi-threaded programming once again, the language
extension solution is equivalent to the solution adopted for tasking
facilities in Ada. Writing a distributed application is as simple as
writing a concurrent application: there is no binding consideration and
no code to wrap. The language and its run-time system take care of
most issues that would divert the programmer’s attention from the
application domain.
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