1. A computer system, comprising a plurality of computers coupled to one another via a network, for executing a service by using databases constructed of storage areas included in the plurality of computers, each of the plurality of computers including: a processor; a storage apparatus coupled to the processor; and a network interface configured to communicate to and from the plurality of computers other than the each of the plurality of computers via the network, the each of the plurality of computers being configured to hold performance management information for managing respective performances of the plurality of computers, the database storing a plurality of pieces of data formed of a key and a data value, the each of the plurality of computers being allocated thereto: a plurality of pieces of master data managed by the each of the plurality of computers as a master based on a distributed algorithm for determining a management range indicating a range of the keys allocated to the each of the plurality of computers; and a plurality of pieces of replicated data of the plurality of pieces of master data managed by one of the plurality of computers other than the each of the plurality of computers, the computer system further comprising: a performance information management part configured to obtain information on the performance from the each of the plurality of computers and update the performance management information based on the obtained information; and a cluster configuration management part configured to determine, based on the performance management information, the management range of the plurality of pieces of master data managed by the each of the plurality of computers and a plurality of sub-computers configured to hold the plurality of pieces of replicated data of the plurality of pieces of master data managed by the each of the plurality of computers.