1. A method by one or more computing systems of an online social network, comprising: receiving, from a third-party system, an introspection query requesting a data structure of a specific data type to be retrieved from one or more hierarchical graphs associated with a social-networking system, each of the hierarchical graphs comprising one or more nodes, each of the nodes representing one or more data items; and each of the data items having a particular one of a plurality of data types; retrieving, using the introspection query, the data structure of the specific data type from the one or more hierarchical graphs, the data structure including (1) a name of the specific data type, and (2) for each of one or more fields of the data structure, a name and a description of the field; receiving, from the third-party system, a data query corresponding to the data structure specified in the definition, the data query requesting one or more of the data items; retrieving, using the data query, the one or more requested data items from the hierarchical graphs; and validating the retrieved data items by determining whether each of the retrieved data items corresponds to the data structure.