1. A computer-implemented method for indexing documents, comprising: storing, at one or more computing devices, an index that includes references to each of a plurality of previously indexed documents; receiving, at the one or more computing devices, a first set of one or more topics relating to a text of a document and a weight for each topic in the first set, where the weight represents how strongly the topic is associated with the document; generating, at the one or more computing devices, a first update to the index by inserting a reference to the document for each of the one or more topics in the first set; receiving a second set of one or more topics relating to the text of the document and a weight for each topic in the second set; and generating, at the one or more computing devices, a second update to the index by inserting the reference to the document for topics in the second set that are not in the first set and removing the reference to the document for topics in the first set that are not in the second set, wherein the second set is generated in response to a triggering event.
1. A computer-implemented method for indexing documents, comprising: storing, at one or more computing devices, an index that includes references to each of a plurality of previously indexed documents; receiving, at the one or more computing devices, a first set of one or more topics relating to a text of a document and a weight for each topic in the first set, where the weight represents how strongly the topic is associated with the document; generating, at the one or more computing devices, a first update to the index by inserting a reference to the document for each of the one or more topics in the first set; receiving a second set of one or more topics relating to the text of the document and a weight for each topic in the second set; and generating, at the one or more computing devices, a second update to the index by inserting the reference to the document for topics in the second set that are not in the first set and removing the reference to the document for topics in the first set that are not in the second set, wherein the second set is generated in response to a triggering event. 8. The method of claim 1 , wherein each topic in the index is capable of being associated with a set of documents.