What is SOA?
A service - a repeatable business task (eg: get exchange rate)
Service orientation - how a business is integrated as services
Service oriented architecture (SOA) - an IT architectural style that provides service orientation
A web service is an implementation scenario of SOA, which is encompassed by SaaS or Software as a Service.
What is a web service?
Simply put, a web service is a piece of software focusing on a specific task, designed to be used through a set of methods and parameters.
For example, imagine you are creating an app that has the requirement to get the conversion rate between Sri Lankan Rupee and USD. You can go for the tiring method of saving your own information in a database and write code to get the data. Or you can use a web service (say developed by company-A) that does that specific task for you, where you have to use a method (say getExchangeRate) to get required information. Sometimes, you will have to pass certain parameters too (like the type of currencies, LKR and USD).
(Update: this post presents a sample RESTful exchangerate service.)
The beauty of a web service is encapsulation. In the above scenario, you did not have to worry about how the data was stored or calculated.
With that in mind, let’s take a look at the definition of web services again.
Web services is a technology for transmitting data over the internet and allow programmatic access to data through standard internet protocols.
Who provides and uses web services?
In the above scenario, you were the service requester and company-A is the service provider. There has to be a service broker who will handle the directory and support the registration of services.
The language used to exchange data is XML and the communication is provided by SOAP (Simple Object Access Protocol). The services are described using WSDL (Web Services Description Language) and they are listed and searched for with the help of UDDI (Universal Discovery Description and Integration).
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