What differentiates Web services from an SOA?
Web services are an enabling technology upon which you can build an SOA, but Web services can also be used in many non-SOA ways. For example, in a Java application you could wholesale swap use of EJB/RMI with Web services inside a single application and the application may be no closer to being part of an SOA.
An SOA is an architecture which allows services to be reused across projects and application teams. Successful reuse has fundamental requirements like the ability to version services without impacting consumers -- something traditional distributed systems have been poor at in real world scenarios. In contrast, Web services make reuse much easier because of the self describing nature of messages (consumers that don't understand part of the message can safely ignore it) and because of the ability to seamlessly introduce intermediaries between consumers and providers to, for example, add a layer of additional security outside the application or to execute a zero-downtime version rollover while consumers continue to use the service.
The easiest way to objectively measure how close you are to having an SOA is to measure the number of different reuses of services in your environment (not the number of services themselves). That is, if you have a hundred Web services, each used by only one consumer, you couldn't really consider your environment an SOA yet (there's no reuse). If, on the other hand, you have only one service, but it's used by a hundred different consumer applications, you can safely say you have an SOA.
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