Replication system having the capability to accept commands at a standby-system site before completion of updating thereof

Abstract

A replication system rapidly switches from a normal-system site to a standby-system site in the event of a problem and reliably maintains functions, and moreover, reduces drops in performance. A first site provides system functions that accompany writing and reading of data. A relay transfer device receives update information from the first site, causes completion of processes for replication in the first site, and continuously executes processes for successively transferring the update information to a second site. The second site receives the update information of the first site and applies this update information to itself to retain a replica of the data of the first site. If, when executing a write request command or a read request command to provide at least a portion of the system functions, the second site has not yet retained the most recent data that are the target of the command, the second site acquires the necessary update information and then executes the command.

1 . 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    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.    
     
     
         2 . A replication system according to  claim 1 , wherein if said second site, when executing said command, still has not retained said most recent data that are the target of the command, said second site requests said transfer device to immediately transfer said update information.  
     
     
         3 . A replication system according to  claim 1 , wherein if said second site, when executing said command, still has not retained said most recent data that are the target of the command, said second site waits for the transfer of said update information from said transfer device.  
     
     
         4 . A replication system according to  claim 1 , 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 a 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.  
     
     
         5 . A replication system according to  claim 1 , 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 a 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 instructs said transfer device not to transfer the update information of the data that are the target of said command.  
     
     
         6 . A replication system according to  claim 1 , wherein said second site retains address information of said update information that is retained in said transfer device and consults said address information to determine whether or not said most recent data that are the target of said command are retained.  
     
     
         7 . A replication system according to  claim 1 , wherein said transfer device, in a process for successively transferring said update information to said second site, transfers said update information to said second site in accordance with transfer priority that is based on a policy that has been set beforehand.  
     
     
         8 . A replication system according to  claim 7 , further comprising a policy control device for controlling settings of said policy.  
     
     
         9 . A replication system according to  claim 8 , wherein said policy control device is connected to at least said transfer device and sends instructions for controlling said policy to said transfer device.  
     
     
         10 . A replication system according to  claim 8 , wherein said policy control device detects the state of at least one of said first site and said second site and controls said policy in accordance with said state that has been detected.  
     
     
         11 . A replication system according to  claim 8 , wherein said policy control device controls said policy in accordance with requests from at least one of said first site and said second site.  
     
     
         12 . A replication system according to  claim 8 , wherein said policy control device detects the state of the communication network and reflects the state of said communication network in the control of said policy.  
     
     
         13 . A replication device in a replication system for retaining update information of a first site that provides system functions that accompany writing or reading of data and for continuously executing processes for successively transferring the update information to a second site, wherein said replication device constitutes said second site; said replication device comprising: 
 an execution processor for executing 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 of said first site;    a data storage unit for retaining a replica of said first site;    a data application unit for applying said update information to said data storage unit; and    a data acquisition unit having the functions of receiving update information that has been transferred in and sending the update information to said data application unit, and when said execution processor executes said command, if the most recent data that are the target of the command are still not retained in said data storage unit, acquiring necessary update information before the execution of said command.    
     
     
         14 . A transfer device in a replication system for implementing data replication between sites; said transfer device being between a first site that provides system functions that accompany writing or reading of data and a second site that receives update information of said first site and that applies said update information to itself to retain a replica of data of said first site and that executes a write request command or a read request command to provide at least a portion said system functions in place of said first site or as assistance to said first side; said transfer device transferring said update information of said first site to said second site; and said transfer device comprising: 
 a temporary storage unit for receiving said update information from said    first site and causing completion of processes for replication in said first site;    a policy storage unit for storing a policy relating to the transfer priority of said update information that has been received by said temporary storage unit to said second site; and    a data transfer unit for transferring said update information that has been received by said temporary storage unit to said second site in accordance with said transfer priority of said policy that has been stored in said policy storage unit.    
     
     
         15 . A policy control device in a replication system for retaining update information of a first site that provides system functions that accompany writing or reading of data and for continuously executing processes for successively transferring said update information to a second site, said policy control device setting a policy relating to the transfer of said update information to said second site; and said policy control device comprising: 
 a state detection unit for detecting the state of said replication system;    a policy determination unit for determining transfer priority of data in accordance with states that have been detected by said state detection unit; and    a policy instruction unit for setting said transfer priority that has been determined by said policy determination unit as said policy.    
     
     
         16 . A replication method for implementing replication of data between a first site that provides system functions that accompany writing and reading of data and a second site that receives update information of said first site and applies said update information to itself to retain a replica of said first site and that provides at least a portion of said system functions in place of said first site or as assistance to said first site, said replication method comprising steps of: 
 retaining said update information from said first site, causing completion of processes for implementing replication in said first site, and successively transferring said update information that has been retained to said second site;    determining whether the most recent data that are the target of the command are retained in said second site when said second site executes a write request command or a read request command; and    transferring necessary update information to said second site and then executing said command if the most recent data that are the target of said command are not retained in said second site.    
     
     
         17 . A replication method for implementing replication of data between a first site that provides system functions that accompany writing and reading of data and a second site that receives update information of said first site and applies said update information to itself to retain a replica of said first site and that provides at least a portion of said system functions in place of said first site or as assistance to said first site, said replication method comprising steps of: 
 storing a policy relating to transfer priority of said update information to said second site;    retaining said update information of said first site and causing completion of processes for implementing replication in said first site, when data are updated in said first site; and    transferring said update information that has been retained to said second site in accordance with said transfer priority of said policy.    
     
     
         18 . A replication method according to  claim 17 , further comprising steps of: 
 detecting states of said replication system;    determining the transfer priority of data in accordance with states that have been detected; and    controlling said policy such that said policy is set to said transfer priority that has been determined.    
     
     
         19 . A program in a replication system for retaining update information from a first site that provides system functions that accompany writing and reading of data and for successively transferring said update information to a second site to retain a replica of said first site in said second site, said program causing a computer to function as said second site; and said program causing a computer to execute processes of: 
 successively applying said update information that has arrived by transfer to said replica;    determining whether the most recent data that are the target of the command are retained as said replica when executing 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; and    acquiring the necessary update information before executing said command if said most recent data have still not been retained.    
     
     
         20 . A program in a replication system that includes a first site that provides system functions that accompany writing or reading of data, a second site that receives update information of said first site and applies said update information to itself to retain a replica of the data of said first site and that executes a write request command or a read request command to provide at least a portion of said system functions in place of said first site or as assistance to said first site, and a transfer device that transfers said update information of said first site to said second site; said program causing a computer to function as said transfer device; and said program causing said computer to execute processes of: 
 storing a policy relating to transfer priority of said update information to said second site;    receiving said update information from said first site and causing completion of processes for implementing replication in said first site; and    transferring said update information that has been received from said first site to said second site in accordance with said transfer priority of said policy.    
     
     
         21 . A program in a replication system for retaining update information from a first site that provides system functions that accompany writing or reading of data and for continuously executing processes of successively transferring said update information to a second site, said program causing a computer to function as a policy control device that sets a policy that relates to the transfer of said update information to said second site; said program causing said computer to execute processes of: 
 detecting states of said replication system;    determining transfer priority of data in accordance with states that have been detected; and    setting said transfer priority that has been determined as said policy.