1. A method for managing the lifetime of semantically-identified data, comprising: maintaining a graph data structure in storage of a computing device, said graph data structure being defined by a graph data structure definition and having data items and relationships therebetween that are semantically identified; storing lifetime information in at least one of said graph data structure and said graph data structure definition, said lifetime information describing the period during which at least one of said data items in said graph data structure is valid; and communicating said at least one data item to a software system only if said period is unexpired.