Sunday, February 7, 2010

Service Oriented Architecture

Service Oriented Architecture or SOA was a hot new buzz word 3 - 4 years ago. While still around, it has been pushed to the background by other hot paradigms like Software as a Service (Saas) and cloud computing. You don't see vendors highlighting the SOA qualities of the products that much. Is SOA dead ?

In this blog I will briefly describe the principles of SOA and where I think it stands today.

In SOA, you build complex software services by coupling together other coarse grained software services, each of which provides a distinct service. The coupling between services is loose coupling. The idea is that you should be able to replace a service or add a new one without lot of coding or rework.

The participating services do not have to be written in the same programming language. Some could have been developed in java, others in .net, some others in c++ and so on.

The interaction between services may be based on asynchronous messaging. Instead of using APIs, services use messages to exchange data between each other. This is in contrast to strongly typed APIs.

Each service provides the abstraction for 1 particular task such as computing the credit score for an application. Each service can be optimized to do the one task it does really well.

Bigger applications are composed by assembling other services that provide a specific function.

An example of a reusable service is a service for creating, updating, viewing customer data. Every other application in the enterprise can use this service for customer information. The service may be available via an end point such as http URL or a messaging destination. The customer data that the service accepts and returns may be described using XML.

Some of the technologies the enable SOA are:

1. Web Services
2. Service Component architecture (SCA)
3. Service Data Objects (SDO)
4. Enterprise messaging
5. XML

Web Services and SCA are used to build services. The data or meta data can be exchanged between services as straight XML or using SDOs. The messaging transport can be SOAP over HTTP in the case of web services or HTTP in the case of REST. One can use asynchronous messaging protocols like JMS when persistent messages are required.

We know things like abstraction , encapsulation, res use from the early days of object oriented design and design patterns. So SOA is not telling us any thing radically new. However it sort of reminds us once more to apply these design principles in large enterprise application environments.

SOA is not without problems. Weakly typed message based protocols as encouraged by SOA are typically hard to debug. Describing metadata in XML and using it in runtime work well for simple interfaces, but is more problematic for complex interfaces. Parsing XML is expensive and adds a performance overhead. Propagating contexts from one service to another can be an issue when the services are controlled by different teams. Due to the complexity, significant testing needs to be done before rolling out such services to users.

In conclusion, SOA reminds us of some useful design principles. These same principles are applicable even if you are building a cloud and offering your software as service. However you want to pay close attention to the know pitfalls when they apply to your environment.

In the future blogs, I will provide an introduction to some of the technologies that enable you to build SOA services.