The OMG Portable Object Adapter specification provides before-after filters. The experimental W3C Jigsaw server does support server-side filters. A number of http servers can act as proxy servers to receive a URL from a client and pass it on to a server, first performing some intermediary operation, then returning a page through the proxy to the client, possibly side-effecting the page transfer. Common uses for proxies have been page caching mechanisms and security firewalls. The purpose of the URL interceptor is to trap and allow for one or many side-effects at the point before or after of client send, server receive, server send, client receive.
Services implement some behavior that a third party can specify.
A service can itself be extensible. Services can be dynamically installed and placed on the client, server, or in between for different purposes. The purpose of composition specifications is to sequence a collection of services so they are executed in a known order. Service Support Infrastructure. Services may or may not share a common service support infrastructure.