A web service is a piece of computer code that is running somewhere in the world, and that you can talk to. Because you are communicating with a computer and not a human being, you have to document exactly on how to communicate. This is done by a protocol. There exists multiple types of protocols. You can think of a website as a web service. You ask for information using the URL, and you receive an answer following the html standard.
The following architecture is the foundation of web services:
A service provider is a company or person that builds a web service. They intend to create a service that is widely being adopted. Therefore, they publish their service at a service registry. A service registry is nothing else than a big library for looking up web services. You want to call an existing web service, and are therefore the service requestor. You look for a web service that matches your needs in the service registry. Once you find a matching service, you want to call its functions, using your own piece of software. To do that, you bind yourself to this service provider, by following the specific communication protocol.
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