1. A method comprising: determining an indication of an amount of resources available to a search engine to process a subset of a plurality of search queries; selecting the search engine to operate in one of a synchronous mode or an asynchronous mode based on the determined indication and on an arrival rate at which the subset of search queries is received; and operating the search engine in accordance with the selected mode to determine at least one document responsive to a given search query of the plurality of search queries.