CSC/ECE 517 Fall 2010/ch6 6j SO

Service-oriented architecture

Introduction

SOA (service-oriented architecture) has become a buzzword of late. Although the concepts behind SOA have been around for over a decade now, SOA has gained extreme popularity of late due to web services. Before we dive in and talk about what SOA is and what the essentials behind SOA are, it is a useful first step to look back at the evolution of SOA. To do that, we have to simply look at the challenges developers have faced over the past few decades and observe the solutions that have been proposed to solve their problems.

The Problem

Early programmers realized that writing software was becoming more and more complex. They needed a better way to reuse some of the code that they were rewriting. When researchers took notice of this, they introduced the concept of modular design. With modular design principles, programmers could write subroutines and functions and reuse their code. This was great for a while. Later, developers started to see that they were cutting and pasting their modules into other applications and that this started to create a maintenance nightmare; when a bug was discovered in a function somewhere, they had to track down all of the applications that used the function and modify the code to reflect the fix. After the fix, the deployment nightmare began. Developers didn't like that; they needed a higher level of abstraction.

Researchers proposed classes and object-oriented software to solve this, and many more, problems. Again, as software complexity grew, developers started to see that developing and maintaining software was complex and they wanted a way to reuse and maintain functionality, not just code. Researchers offered yet another abstraction layer to handle this complexity -- component-based software. Component-based software is/was a good solution for reuse and maintenance, but it doesn't address all of the complexities developers are faced with today. Today, we face complex issues like distributed software, application integration, varying platforms, varying protocols, various devices, the Internet, etc. Today's software has to be equipped to answer the call for all of the above. In short, SOA (along with web services) provides a solution to all of the above. By adopting a SOA, you eliminate the headaches of protocol and platforms and your applications integrate seamlessly.

Key Components of SOA

The first step in learning something new is to understand its vocabulary. In the context of SOA, we have the terms service, message, dynamic discovery, and web services. Each of these plays an essential role in SOA.

Service

A service in SOA is an exposed piece of functionality with three properties:

1. The interface contract to the service is platform-independent.

2. The service can be dynamically located and invoked.

3. The service is self-contained. That is, the service maintains its own state.

A platform-independent interface contract implies that a client from anywhere, on any OS, and in any language, can consume the service. Dynamic discovery hints that a discovery service (e.g., a directory service) is available. The directory service enables a look-up mechanism where consumers can go to find a service based on some criteria. For example, if I was looking for a credit-card authorization service, I might query the directory service to find a list of service providers that could authorize a credit card for a fee. Based on the fee, I would select a service (see Figure 1.1). The last property of a service is that the service be self-contained.