1. A method comprising: detecting, by a computer system, occurrence at a web service endpoint of a runtime event; updating, by the computer system, a runtime context component maintained for the web service endpoint based on the runtime event prior to evaluating one or more policies with respect to the web service endpoint and the runtime event, the runtime context component specifying a set of data regarding a current context within a runtime environment of the web service endpoint that includes information relevant to the runtime event; identifying, by the computer system, a web service policy whose metadata indicates attachment to the web service endpoint; determining, by the computer system, whether the web service policy is to be attached to the web service endpoint using the runtime context component and a constraint expression associated with web service policy being dependent on one or more runtime values specified by the runtime context component, the determining comprising evaluating the constraint expression in view of the one or more runtime values specified by the runtime context component; enforcing, by the computer system, the web service policy at the web service endpoint with respect to the detected runtime event when the web service policy is determined to be attached to the web service endpoint based on satisfying the constraint expression; and ignoring, by the computer system, the metadata indicating attachment of the web service policy at the web service endpoint when the web service policy is determined not to be attached to the web service endpoint based on failing to satisfying the constraint expression.