1. A computer-implemented method performed by data processing apparatus comprising one or more computers in data communication, the method comprising: receiving, for each resource a plurality of resources that are each accessible by a user device, action data describing actions supported by each resource, each action being an action that can be performed by the user device processing the resource; for each resource; determining, from the action data, an action type for each action supported by the resource; determining, from the action data, action parameters for each action supported by the resource; and indexing the action data, action types and action parameters for the resource in an index that is searchable by a search engine.