1. A method comprising: determining whether an information source requested by a user has been modified within a specified length of time based on a Last-Modified field of a Hypertext Transfer Protocol (HTTP) header of a message associated with the information source; in response to determining that the information source has been modified within the specified length of time, creating an entry in a modified information source log that identifies the information source; retrieving information from the information source based on the modified information source log; and updating a search engine index based on the information; wherein the method is performed by one or more computing devices.