Methods and apparatus, including computer program products, that include providing to a client a definition of an object class representing attributes of a first collection of data elements and a list of possible operations on the first collection, each data element having attributes and possible operations in common with other data elements from the first collection. The method also includes customizing a usage policy of the first collection for a configuration of the client, receiving from the client a first request to execute an operation from the list on one or more data elements from the first collection, checking the first request against the definition and the usage policy, and executing the operation on the one or more data elements of the first collection.