7. The system of claim 6 , the operations comprising: selecting one or more of the search parameters as encoded parameters; generating a session identifier as a representation of the encoded parameters; and associating the encoded parameters with the session identifier in one or more storage devices.
6. A system comprising one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving, by location-based search engine that includes (i) a web crawler, (ii) a query processor, (iii) a uniform resource identifier constructor, and (iv) a uniform resource identifier interpreter, a query comprising search parameters that (i) specify a desired location and one or more desired amenities of a rental unit being sought by a user of the location-based search engine, and (ii) are specified through one or more interactions between the user and one or more drop down controls included on a search parameter entry graphical user interface associated with the location-based search engine; selecting, by the uniform resource identifier constructor, two or more of the search parameters that (i) specify the desired location and the one or more desired amenities of a rental unit being sought by the user of the location-based search engine, and (ii) are specified through the one or more interactions between the user and the one or more drop down controls included on the search parameter entry graphical user interface associated with the location-based search engine; for each of the selected search parameters: generating, by the uniform resource identifier constructor, a natural language representation of the selected search parameter; and determining, by the uniform resource identifier constructor, using rules that indicate a relative importance of the search parameters that specify the one or more desired amenities to users of the search engine, or to the web crawler for search engine optimization purposes, a rank of the selected parameter among the one or more selected parameters, wherein search parameters that are more frequently searched by users of the search engine are assigned a higher relative importance than search parameters that are less frequently searched by the users of the search engines; generating, by the uniform resource identifier constructor, a uniform resource identifier comprising the natural language representations of the selected parameters arranged in an order based on the ranks of the selected search parameters, including the search parameters that specify the one or more desired amenities of the rental unit being sought by the user, that were determined using the rules that indicate the relative importance of the search parameters to the users of the search engine, or to the web crawler for search engine optimization purposes; obtaining, by the query processor, one or more search results identified as responsive to the query; providing, by the location-based search engine, a search results page graphical user interface that includes the uniform resource identifier and the one or more search results for output, the uniform resource identifier including a natural language representation of the desired location and a natural language representation of one or more of the desired amenities of the rental unit; and in response to subsequently receiving the generated uniform resource identifier including the natural language representation of the desired location and the natural language representation of one or more of the desired amenities of the rental unit, generating, by the uniform resource identifier interpreter, a subsequent query that includes the same search parameters for execution by the location-based search engine.