Patent ID: 8161138

Claim:
A replication system for replicating data between sites, comprising: a first site for providing system functions that accompany writing or reading of data; a second site: that receives update information of said first site and that retains a replica of data of said first site by applying this update information to itself; that can receive a write request command or a read request command for providing at least a portion of said system functions in place of said first site or as assistance to said first site even when all update information of said first site has not been applied; and when executing said command, that acquires the necessary update information if the most recent data that are the target of this command have not yet been retained, and then executes said command; and when executing said command, that executes said command without acquiring the update information if said most recent data that are the target of this command have already been retained; and a transfer device having the functions of receiving said update information from said first site, causing completion of processing for replication in said first site, and continuously executing processes for successively transferring said update information to said second site; wherein if said second site, when executing said command, still has not retained said most recent data that are the target of the command, and moreover, if the command is the write request command, said second site executes said command without acquiring from said transfer device the most recent data that are the target of said command, and after executing said command, does not apply to itself update information of the data that are the target of said command even if the update information are transferred in.