1. A retrieval system, comprising: a storage system; and a search engine server coupled to the storage system, for retrieving a data object satisfying search criteria from among a plurality of data objects in accordance with a search request, wherein: the storage system comprises: a first logical unit for storing an index including a plurality of index records; a second logical unit for storing a rule that defines how to output search results; at least one third logical unit for storing a snapshot of the index; and at least one fourth logical unit for storing a snapshot of the rule; each of the plurality of index records is created for any one of the plurality of data objects, and includes storage location information indicating where the data object is stored and a keyword list of at least one keyword included in the data object; and the search engine server is configured to: select the third logical unit and the fourth logical unit based on a search request upon reception of the search request; search the index stored in the selected third logical unit with a keyword included in the search request for an index record whose keyword list includes the keyword; and output the storage location information of the data object included in at least one extracted index record in an order according to the rule stored in the selected fourth logical unit.