5. A method of scheduling a search engine crawler using a scheduler system, comprising: at a server with one or more processors and memory, and one or more programs stored in the memory that execute on the one or more processors: selecting a first subset of document identifiers from a set of document identifiers corresponding to documents on a network; computing priority scores for the first subset of document identifiers; forming a second subset of document identifiers based on the priority scores and status data collected during one or more previous crawls by the search engine crawler and by removing from the first subset one or more document identifiers identified as unreachable in a plurality of prior crawls or associated with download errors in a plurality of prior crawls; and scheduling for crawling the second subset of document identifiers; wherein each priority score is computed as a product of a respective page importance and a respective boost factor, the respective boost factor being a scalar that is used to demote or promote the priority score of the respective document identifier.
5. A method of scheduling a search engine crawler using a scheduler system, comprising: at a server with one or more processors and memory, and one or more programs stored in the memory that execute on the one or more processors: selecting a first subset of document identifiers from a set of document identifiers corresponding to documents on a network; computing priority scores for the first subset of document identifiers; forming a second subset of document identifiers based on the priority scores and status data collected during one or more previous crawls by the search engine crawler and by removing from the first subset one or more document identifiers identified as unreachable in a plurality of prior crawls or associated with download errors in a plurality of prior crawls; and scheduling for crawling the second subset of document identifiers; wherein each priority score is computed as a product of a respective page importance and a respective boost factor, the respective boost factor being a scalar that is used to demote or promote the priority score of the respective document identifier. 6. The method of claim 5 , further comprising: applying at least one exception filter to at least a plurality of the document identifiers to identify document identifiers to be excluded from a next crawl.