CSC/ECE 517 Fall 2009/wiki2 17 va

Service Oriented Architecture 3 (or SOA) describes how to design a software system and connect the separate components using services. Reflection and metaprogramming are two powerful concepts that directly support the principles of SOA. This article covers the following goals:

1. Provides a simple understanding of reflection, metaprogramming, and SOA's.
2. Explains how these concepts are interrelated, specifically how reflection and metaprogramming support SOA's.

An overview of the concepts

Service Oriented Architecture

Service Oriented Architecture (or SOA) is a concept in computing that defines the interaction of different software in terms of protocols and functionality. A SOA can be viewed as containing multiple services which may be linked as desired by protocols. SOA is designed so that each functional unit of software in a system is isolated from each other and provides a service without directly making subroutine calls to each other. This is generally accomplished by passing messages. 3

SOA offers the following technical advantages:

• Offers services across platforms
• Allows services to be split across systems and networks
• Allows services to be location independent and close to their business units
• Provides a bridge between incompatible technologies
• Leverages existing technologies
• Reduces dependency on custom systems

The information being used to communicate between services must contain sufficient detail about the characteristic of the data and the data itself and must remain independent of the underlying platform and programming language. Although SOA does not specify the format to transfer data, programmers have generally used XML for this purpose. However, Metadata (data that describes the actual data) 6 in the SOA must: 3

• Be easy to configure
• Provide an easy way to discover services
• Provide an easy way to incorporate services
• Remain coherent
• Preserve data integrity
• Be easy to manage

A simple example of metadata is a README file that specifies the inputs and outputs to a program. The inputs and outputs of the program would be the data, and the README file is metadata that describes this data.

SOA does not limit the protocol used to transfer the data. A wide variety of technologies can be used including SOAP, REST and RPC. This is generally left for the programmer of the system.

We created this simplified image to illustrate the basic principles of service oriented architectures.

• Services may be used by multiple applications.
• Services do not call on each other.
• Services communicate via protocols.
• An example protocol is provided, in which the application passes x to the Sin function, and Sin(x) is returned.
• Because of the protocol link, services may be written in different programming languages, or even be hosted on different computers with different operating systems, which allows for great flexibility.

Metaprogramming

Metaprogramming is the writing of computer programs that write other computer programs. This allows the programmer to complete tasks faster than if (s)he had to code everything manually. A compiler is an excellent example of a metaprogram. It allows the programmer to code in a higher level language and then the compiler will convert that program into machine language. 7

• Example: Metaprogramming in Ruby [2]

 class Person
   attr_accessor :name
 end

In the example above, attr_accessor is used to create 'setters' and 'getters' for the name attribute via metaprogramming. From this statement, code is automatically generated that provides a function to write the name (setter) and a function to read the name (getter).

Reflection

Reflection is a specific type of meta-programming and emphasizes on dynamic program modification. It is the ability of a programming language to inspect its own code and can therefore be used to extend the language beyond its usual capabilities. For example, it is possible to use reflection to inspect an object for a desired function and execute it dynamically.

• Example: Reflection in Perl 5

 # without reflection
 my $foo = Foo->new();
 $foo->hello();

 # with reflection
 my $class  = "Foo";
 my $method = "hello";
 my $object = $class->new();
 $object->$method();

In the example with reflection, $object and $method can be determined and even changed at run time. This allows the program to change its own behavior.

How metaprogramming and reflection enhance SOA

Service-oriented Architecture is enhanced when the underlying programs are able to dynamically adapt to the data received. Metaprogramming and Reflection make this possible.

The services in a Service-oriented Architecture make use of the data passed around between them. When the data passed around represents code, for example 'print "Hello World"' in a language independent way, the receiving service would use metaprogramming to write code to just that. The service would then use reflection to determine was is available, in our case 'print', and would perform that task. If the service logic was to "repeat 3 times", then the code would ultimately print "Hello World" that number of times.

XML may be used to transfer the message as follows:

• Example XML Metaprogramming:

 <message>
   <method>print</method>
   Hello World
 </message>

A more practical example would be a service that examines the code passed to it for security holes. These examples both make use of metaprogramming and reflection.

References

1. http://www.service-architecture.com/web-services/articles/service-oriented_architecture_soa_definition.html - Brief explanation of SOA

2. Thomas, Dave (2006). Programming Ruby, The Pragmatic Programmers' Guide.

3. http://en.wikipedia.org/wiki/Service_oriented_architecture - A very detailed explanation of SOA

4. http://www.javaworld.com/javaworld/jw-06-2005/jw-0613-soa.html - A more easy to understand explanation of SOA

5. http://en.wikipedia.org/wiki/Reflection_(computer_science) - Explanation of reflection

6. http://en.wikipedia.org/wiki/Metadata - Explanation of metadata

7. http://en.wikipedia.org/wiki/Metaprogramming - Explanation of metaprogramming