Simple text searching systems rely on inverted index to look up the set of documents that contain a given word or a term. In this recipe, we implement a simple inverted index building application that computes a list of terms in the documents, the set of documents that contains each term, and the term frequency in each of the documents. Retrieval of results from an inverted index can be as simple as returning the set of documents that contains the given terms or can involve much more complex operations such as returning the set of documents ordered based on a particular ranking.