The Web Services Description Language (WSDL) currently represents the service description layer within the Web service protocol stack.
In a nutshell, WSDL is an XML grammar for specifying a public interface for a Web service. This public interface can include the following:
Information on all publicly available functions.
Data type information for all XML messages.
Binding information about the specific transport protocol to be used.
Address information for locating the specified service.
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Service Buses: Connectors, Caching, Policies, Scripting, Messaging
Infrastructure Components: SOAP bindings, XML parsers, Message service bindings, clustering, replication and high availability
--ESB:
An enterprise service bus (ESB) is a pattern of middleware that unifies and connects services, applications and resources within a business. it's a new way of looking at how to integrate applications, coordinate resources and manipulate information,
the ESB pattern enables the connection of software running in parallel on different platforms, written in different programming languages and using different programming models.