12. A non-transitory computer-readable medium having stored thereon program code executable by a computer system, the program code comprising: code that causes the computer system to receive a runtime policy set for a web service endpoint, the runtime policy set identifying a list of all web service policies whose attachment metadata indicates attachment to the web service endpoint; code that causes the computer system to receive constraint expression for each web service policy in the runtime policy set, each constraint expression specifying one or more runtime values; code that causes the computer system to aggregate the web service policies included in the runtime policy set into a set of groups based on each web service policy's corresponding constraint expression, each group in the set of groups grouping web service policies by constraint expression; code that causes the computer system to identify one or more groups in the set of groups whose constraint expressions indicate grouped web service policies are simultaneously attached to the web service endpoint at runtime; code that causes the computer system to apply one or more validation rules to the web service policies in the one or more groups; code that causes the computer system to evaluate the one or more runtime values at runtime, wherein the evaluating determines whether the attachment metadata of a corresponding web service policy in a group validated by the one or more validation rules is enforced attaching the corresponding web service policy to the web service endpoint; and code that causes the computer system to control execution behavior of the web service endpoint based on the one or more runtime values and the corresponding web service policy, wherein the execution behavior includes at least one of authentication, authorization, message encryption, or message logging.
12. A non-transitory computer-readable medium having stored thereon program code executable by a computer system, the program code comprising: code that causes the computer system to receive a runtime policy set for a web service endpoint, the runtime policy set identifying a list of all web service policies whose attachment metadata indicates attachment to the web service endpoint; code that causes the computer system to receive constraint expression for each web service policy in the runtime policy set, each constraint expression specifying one or more runtime values; code that causes the computer system to aggregate the web service policies included in the runtime policy set into a set of groups based on each web service policy's corresponding constraint expression, each group in the set of groups grouping web service policies by constraint expression; code that causes the computer system to identify one or more groups in the set of groups whose constraint expressions indicate grouped web service policies are simultaneously attached to the web service endpoint at runtime; code that causes the computer system to apply one or more validation rules to the web service policies in the one or more groups; code that causes the computer system to evaluate the one or more runtime values at runtime, wherein the evaluating determines whether the attachment metadata of a corresponding web service policy in a group validated by the one or more validation rules is enforced attaching the corresponding web service policy to the web service endpoint; and code that causes the computer system to control execution behavior of the web service endpoint based on the one or more runtime values and the corresponding web service policy, wherein the execution behavior includes at least one of authentication, authorization, message encryption, or message logging. 13. The non-transitory computer-readable medium of claim 12 wherein the program code further comprises code that causes the computer system to store a set of validation errors that are generated as a result of applying the one or more validation rules.