1. A system comprising: memory storing an index of documents, the index comprising at least one posting list that maps a term to documents; at least one processor; and memory storing instructions that, when executed by the at least one processor cause the system to: receive an update for the posting list, responsive to receiving the update: generate a snapshot record, the snapshot record including a portion of the posting list that is to be changed by the update, the snapshot record being smaller than the entire posting list, and modify the portion of the posting list using the update, use a main query processing engine to generate a search result for a subsequent query based on the updated posting list without applying the snapshot record, and responsive to determining that the main query processing engine is unavailable, use a secondary query processing engine to generate a search result for a subsequent query by applying the snapshot record to the updated posting list.