Abstract:
Provided is a controller in a computer system, the computer system including a plurality of data storage systems, and at least one controller for controlling access to data stored in the plurality of data storage systems, the each controller including: an interface coupled to the network; a processor coupled to the interface; and a storage unit coupled to the processor, in which: the storage unit holds attribute information indicating whether to permit access to the data; and the processor is configured to: receive a writing request of the data from a client computer coupled to the network; judge whether each of the each controller permits the requested writing based on the held attribute information and information of a location where the each controller is installed; and write the data in a data storage system controlled by a controller judged to permit the writing.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese patent application JP 2007-249835 filed on Sep. 26, 2007, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND 
     This invention relates to access control, and more particularly to data management and access control based on a storage permitted location and an access permitted location of data in a group of computer systems intercoupled via a network. 
     With higher performance and lower prices of computer systems, the use of the computer systems have recently been diffused in various industries and applications. Accordingly, data conventionally handled on paper media or the like have been computerized, and electronically stored by the computer system. 
     Additionally, a form where a plurality of computer systems are intercoupled via a network has rapidly progressed. Distributed management and distributed processing of data can be realized, and so availability, reliability, and performance which had been difficult to be realized only by one computer system has become possible. 
     In the form of intercoupling the plurality of computer systems via the network, it has become more important to provide a technology of efficiently managing data and a technology of controlling access for the purpose of improving convenience for computer system users. 
     An overlay network technology of building and providing a logical network structure by hiding a physical network structure for intercoupling a plurality of computer systems has recently been used. The overlay network technology enables transparent access to the computer system irrespective of an installation location of the computer system to be used. 
     By using the overlay network technology, for example, file share services of a peer-to-peer form for distributing and storing shared files can be realized on the computer systems which build the overlay network. 
     In the file share services, the users can know on which of the computer systems shared files are present by requesting file acquisition based on identification information of the shared files to be obtained. Once the presence location of the shared file is known, each user can obtain the shared file by accessing a relevant arbitrary computer system. 
     In the case of accessing the arbitrary computer system, the user doesn&#39;t have to know where the computer system to be accessed is actually present but has to know only identification information of the computer system to be accessed in the overlay network. Based on the identification information, the user can access the computer system where the shared file is present via the overlay network. 
     Conventionally, to build the overlay network, identification information of a participating computer system, and identification information in the network which becomes necessary for accessing via an actual network are necessary. For example, the former is a host name of the computer system, and the latter is an IP address allocated to the computer system. 
     The computer system participating in the overlay network has to manage such identification information, and exchange identification information with the other computer system participating in the overlay network to update contents. 
     By executing the updating properly, the computer system can dynamically participate in or withdraw from the overlay network as occasion demands, and the computer systems which participate in the overlay network can be easily managed. 
     For management of the computer systems which participate in the overlay network, two methods are available, i.e., a method of managing information of all participating computer systems in each computer system, and a method of managing only information of some computer systems in each computer system. As each computer system can understand all the computer systems participating in the overlay network, the former method has a feature that a node storing a shared file is searched for in the case of actually accessing the shared file, and efficiency is high when access is made to the shared file. On the other hand, as update information has to be reflected in all the participating computer systems each time the computer system participates in or withdraws from the overlay network, there is a problem of reduced efficiency for managing the participating computer systems. 
     In the case of the latter method, each computer system manages information of some computer systems among the computer systems participating in the overlay network. Accordingly, updating of information managed by each computer system each time a computer system participates in or withdraws from the overlay network can be minimized, and its influence can be reduced. In the case of the latter method, when access to the shared file is actually made, by making an inquiry to the other computer systems participating in the overlay network, information of the computer system which has participated in or withdrawn from the network can be obtained. In this case, overheads occur because of the inquiry made about the information of the computer system. However, the overheads are much smaller as compared with those when the management information of the computer systems participating in the overlay network is updated. Especially, overheads are conspicuously large when a large-scale overlay network is run. 
     SUMMARY 
     Regarding the latter method, JP 2007-28400 A discloses a technology of reducing actual network loads by building an overlay network based on localities of participating computer systems. According to the technology described in JP 2007-28400 A, when a computer system that is to participate in the overlay network transmits request information of participation in the overlay network, information indicating a reachable range of the transmitted request information is set. Thus, propagation of the request information of participation can be locally limited, and updating of management information of each computer system can be locally suppressed when the computer system participates in the overlay network. 
     In the case of the technology described in JP 2007-28400 A, however, when file share services using the overlay network are provided, the location of a computer system which stores a shared file cannot be designated or controlled. For example, if a shared file whose distributable or sharable countries or regions are limited because of a copyright or other problems is shared among the file share services described in JP 2007-28400 A, the shared file may be stored in a computer system installed in a country or a region where distribution and sharing are not permitted. Management of the file which cannot be used in the computer system reduces storage use efficiency of the computer system. Additionally, because the location (country or region) where an access requester of access to the shared file is present is not identified at the time of access control, there arises a problem in that control carried out to deny access for a user present in a location where access not permitted is difficult. For example, because where the access requester is present is not identified, access to contents denied for access from the US cannot be denied to users permitted for access in Japan during their stay in the US. 
     In the case of the technology described in JP 2007-28400 A, communication charges between the computer systems in the overlay network or between Internet service providers (ISP) for providing network connection services are not taken into consideration. Thus, even when network loads can be reduced, extra communication fees may be charged. The increase of communication charges may increase ISP loads, consequently reducing quality of ISP network services. 
     A problem of the former case is that control of the shared file storage location and access control to the shared file are carried out without taking localities in the overlay network into consideration. It is expected that information shared and distributed via the network will increase, and information involving rights regarding copyrights will be contained. Accordingly, for providing file share services using the overlay network, a technology solving this problem will be necessary. 
     A problem of the latter case is that the network is used without taking actual communication charges of the network into consideration. It is expected that a network represented by the Internet will come into wider use, thereby increasing an information distribution amount. In such a case, to enable the ISP providing network connection services to continuously provide proper network service quality, a technology capable of minimizing use of a network which increases ISP loads will be necessary. 
     A representative aspect of this invention is as follows. That is, there is provided a controller installed in a computer system, the computer system having: a plurality of data storage systems for storing copies of data in a distributed manner; at least one controller for controlling access to the data stored in the plurality of data storage systems; and a network for coupling the at least one controller. The each controller comprising: an interface coupled to the network; a processor coupled to the interface; and a storage unit coupled to the processor. The storage unit holds attribute information indicating whether to permit access to the data. The processor is configured to: receive a writing request of the data from a client computer coupled to the network; judge whether the each controller permits the requested writing based on the held attribute information and information of a location where the each controller is installed; and write the data in a data storage system controlled by a controller judged to permit the writing. 
     According to the aspect of this invention, by designating the installation location of the storage destination controller of shared data and the access permitted location for each piece of the data, access from the location where the access is not permitted can be denied. Moreover, by suppressing storage of the data in the controller installed in the location where the storage of the shared data is not permitted, unnecessary communication can be removed to suppress wasteful use of the storage system. 
     At the time of accessing the data, by carrying out control to preferentially access the data of the controller with a minimum communication charge, communication charges necessary for file share services can be reduced. The reduction of communication charges enables reduction of loads of the ISP for providing network connection services, and the ISP can provide sustainable network services of proper quality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein: 
         FIG. 1  is a configuration diagram showing a configuration of a computer system in accordance with a first embodiment of this invention. 
         FIG. 2  is a block diagram showing a hardware configuration of the storage node in accordance with the first embodiment of this invention; 
         FIG. 3  is a block diagram showing a hardware configuration of the client node in accordance with the first embodiment of this invention; 
         FIG. 4  is a diagram showing a configuration of the node management table in accordance with the first embodiment of this invention; 
         FIG. 5  is a diagram showing a configuration of the node installation location information management table in accordance with the first embodiment of this invention; 
         FIG. 6  is a diagram showing a configuration of the shared file metadata management table in accordance with the first embodiment of this invention; 
         FIG. 7  is a diagram showing schematically a shared file which is stored in the storage nodes in accordance with the first embodiment of this invention; 
         FIG. 8  is a flowchart showing a node registration process in accordance with the first embodiment of this invention; 
         FIG. 9  is a flowchart showing a node search process in accordance with the first embodiment of this invention; 
         FIG. 10  is a flowchart showing a file registration process in accordance with the first embodiment of this invention; 
         FIG. 11  is a flowchart showing a file migration/replication process in accordance with the first embodiment of this invention; 
         FIG. 12  is a flowchart showing a file access process in accordance with the first embodiment of this invention; 
         FIG. 13  is a configuration diagram showing a configuration of a computer system in accordance with a second embodiment of this invention. 
         FIG. 14  is a block diagram showing a hardware configuration of the management server in accordance with the second embodiment of this invention; 
         FIG. 15  is a diagram showing a configuration of the ISP information management table in accordance with the second embodiment of this invention; 
         FIG. 16  is a diagram showing a configuration of the node management table in accordance with the second embodiment of this invention; 
         FIG. 17  is a diagram showing a configuration of the node installation location information management table in accordance with the second embodiment of this invention; 
         FIG. 18  is a flowchart showing a node search process in accordance with the second embodiment of this invention; and 
         FIG. 19  is a flowchart showing a file access process in accordance with the second embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, the preferred embodiments of this invention will be described bellow. 
     First Embodiment 
     First, a system according to a first embodiment of this invention will be described. 
       FIG. 1  illustrates a configuration of the computer system according to the first embodiment of this invention. 
     The computer system includes ISPs  1  to  5 . The ISPs  1  to  5  are intercoupled via Internet  10 . An authentication server  3000  is coupled to the Internet  10 . 
     A storage node A 1100  and client nodes  2000  and  2100  are coupled to the ISP  1 . A storage node B 1200  and client nodes  2200  and  2300  are coupled to the ISP  2 . A storage node C 1300  and client nodes  2400  and  2500  are coupled to the ISP  3 . A storage node D 1400  and client nodes  2600  and  2700  are coupled to the ISP  4 . A storage node E 1500  and client nodes  2800  and  2900  are coupled to the ISP  5 . 
     The storage node A 1100  cooperates with the other storage node (e.g., storage node B  1200 ) via the Internet  10  to build an overlay network, thereby providing file share services. The storage nodes B 1200 , C 1300 , D 1400 , and E 1500  have similar configuration to the storage node A. 
     The client node  2000  is a device of a user who utilizes file share services. The client nodes  2100 ,  2200 ,  2300 ,  2400 ,  2500 ,  2600 ,  2700 ,  2800 , and  2900  have similar configuration to the client node  2000 . 
     The ISPs  2  to  5  have similar configuration to the ISP  1 . The ISPs  1  to  3  are included in Area  11 . The ISPs  4  and  5  are included in Area  12 . The Areas  11  and  12  indicate the same location (e.g., country or region). 
     The authentication server  3000  authenticates a presence location of a client (e.g., client node  2000 ) which requests access. 
       FIG. 1  shows five ISPs. However, any number of ISPs may be set. A network structure to which each ISP is coupled is not limited to the form shown in  FIG. 1 . An arbitrary network structure (e.g., ring type or star type) may be employed. 
     In an example shown in  FIG. 1 , one storage node is coupled to each ISP. However, any number of storage nodes may be coupled. Two client nodes are coupled to each ISP. However, any number of client nodes may be coupled. 
       FIG. 2  illustrates a hardware configuration of the storage node A 1100  according to the first embodiment of this invention. 
     The storage node A 1100  includes a processor  1110 , a memory  1120 , an external storage system I/F  1140 , and a network IF  1150 . These components are intercoupled via a bus  1160 . The storage node A 1100  is coupled to an external storage system  1170  via the external storage system I/F  1140 . 
     The processor  1110  executes a program stored in the memory  1120  to control the entire storage node A 1100 . 
     The memory  1120  temporarily stores the program and/or data executed by the processor  1110 . The memory  1120  may include a semiconductor memory such as a RAM. 
     The memory  1120  stores an external storage system I/F control program  1121 , a network I/F control program  1122 , a local file system control program  1123 , a distributed file system control program  125 , a node management table  4000 , a node installation location information management table  4100 , a shared file metadata management table  4200 , and a cache memory  1130 . 
     The external storage system I/F control program  1121  controls the external storage system I/F  1140 . The network I/F control program  1122  controls the network I/F  1150 . 
     The local file system control program  1123  contains a cache memory control subprogram  1124 . The local file system control program  1123  controls a file system provided by the storage node A 1100 . The cache memory control subprogram  1124  controls the cache memory  1130 . 
     The distributed file system control program  1125  contains a query request control subprogram  1126 , a query response control subprogram  1127 , a basic control subprogram  1131 , a file registration subprogram  1132 , a file replication/migration subprogram  1133 , and a file access subprogram  1134 . 
     The distributed file system control program  1125  controls file share services which have used the overlay network. 
     The query request control subprogram  1126  controls a query request transmitted to the other storage node constituting the overlay network. The query request is a request transmitted when information (e.g., identification information of the other storage node constituting the overlay network) is desired to be obtained from the other storage node. 
     The query response control subprogram  1127  receives a query request from the other storage node constituting the overlay network, executes a process of obtaining requested information, and controls a response of a processed result. 
     The basic control subprogram  1131  manages information of the storage node A 1100 . For example, the basic control subprogram  1131  registers information (identification information) regarding the storage node A 1100  in the node management table  4000 . 
     The file registration subprogram  1132  registers a shared file in the overlay network. 
     The file replication/migration subprogram  1133  replicates or migrates the shared file registered in the overlay network. 
     The file access subprogram  134  controls access requested to the shared file registered in the overlay network from a client (e.g., client node  2100 ). 
     The node management table  4000  holds identification information of a storage node present in the overlay network. The node management table  4000  will be described below referring to  FIG. 4 . 
     The node installation location information management table  4100  holds information regarding a location where a storage node is present. The node installation location information management table  4100  will be described below referring to  FIG. 5 . 
     The shared file metadata management table  4200  holds metadata registered in the shared file stored in the storage node. The shared file metadata management table  4200  will be described below referring to  FIG. 6 . 
     The cache memory  1130  is used for shortening access response time when the local file system control program  2023  accesses a file managed by the file system. 
     The external storage system I/F  1140  is an interface for accessing the external storage system  1170 . The network I/F  1150  is an interface for accessing the other system coupled via the network. 
     The external storage system  1170  stores a shared file. The external storage system  1170  may include, for example, a hard disk drive (HDD). Alternatively, a semiconductor memory device such as a flash memory may be used. 
       FIG. 3  illustrates a hardware configuration of the client node  2000  according to the first embodiment of this invention. 
     The client node  2000  includes a processor  2010 , a memory  2020 , an external storage system I/F  2040 , and a network IF  2050 . These components are intercoupled via a bus  2060 . The client node  2000  is coupled to an external storage system  2070  via the external storage system I/F  2040 . 
     The processor  2010  executes a program stored in the memory  2020  to control the entire client node  2000 . 
     The memory  2020  temporarily stores the program and/or data executed by the processor  2010 . The memory  2020  may include a semiconductor memory such as a RAM. 
     The memory  2020  stores an external storage system I/F control program  2021 , a network I/F control program  2022 , a local file system control program  2023 , a distributed file system client control program  2025 , the node management table  4000 , and a cache memory  2030 . 
     The external storage system I/F control program  2021  controls the external storage system I/F  2040 . The network I/F control program  2022  controls the network I/F  2050 . 
     The local file system control program  2023  contains a cache memory control subprogram  2024 . The local file system control program  2023  controls a file system provided by the client node  2000 . The cache memory control subprogram  2024  controls the cache memory  2030 . 
     The distributed file system client control program  2025  contains a query request control subprogram  2026 , and a basic control subprogram  2031 . 
     The distributed file system client control program  2025  controls file share services which have used the overlay network. 
     The query request control subprogram  2026  transmits a query request, a file registration request, a shared file access request, and the like to the storage node for providing file share services using the overlay network. 
     The basic control subprogram  2031  manages information of the client node  2000 . For example, the basic control subprogram  2031  registers information (identification information) regarding a storage node accessed by the client node  2000  in the node management table  4000 . 
     The node management table  4000  holds only information regarding the storage node directly accessed first from the client node  2000 . The node management table  4000  of the client node  2000  and the node management table  4000  of the storage node A 1100  may be synchronized to hold the same information. In the case of holding the same information, the same information as that of the node management table  4000  held by the storage node A 1100  may be held in the node management table  4000  of the client node  2000 . 
     The cache memory  2030  is used for shortening access response time when the local file system control program  2023  accesses a file managed by the file system. 
     The external storage system I/F  2040  is an interface for accessing the external storage system  2070 . The network I/F  2050  is an interface for accessing the other system coupled via the network. 
     The external storage system  2070  stores a program or user data. The external storage system  2070  may include, for example, a hard disk drive (HDD). Alternatively, a semiconductor memory device such as a flash memory may be employed. 
       FIG. 4  illustrates a configuration of the node management table  4000  according to the first embodiment of this invention. 
     The node management table  4000  holds identification information of a storage node recognized to be present in the overlay network. 
     The node management table  4000  includes a node name  4010  and identification information  4020 . 
     The node name  4010  is information for identifying a storage node in the overlay network. In the node name  4010  shown in  FIG. 4 , information of a character string is stored. However, information of a numerical value such as node ID may be stored. 
     The identification information  4020  is identification information of a storage node recognized in a normal network. The identification information  4020  is used for designating an access destination when the storage node is accessed via the network. In the identification information  4020  shown in  FIG. 4 , an IP address is stored. However, for example, ID information for specifying a storage node may be stored. Alternatively, information of a character string may be stored. 
     In an example shown in  FIG. 4 , five pieces of storage node information are stored. “STORAGE NODE A” and “ 10 .  20 .  30 .  40 ” are respectively stored in a node name  4010  and identification information  4020  of a first line of the node management table  4000 . These indicate that a storage node named “STORAGE A” participating in the overlay network is present, and “STORAGE NODE A” can be accessed based on information of the destination “ 10 .  20 .  30 .  40 ”. 
       FIG. 5  illustrates a configuration of the node installation location information management table  4100  according to the first embodiment of this invention. 
     The node installation location information management table  4100  manages information regarding a location where a storage node is present. 
     The node installation location information management table  4100  includes node installation location information  4110 . The node installation location information  4110  is information for identifying a location where a storage node is installed. 
     In the node installation location information  4100  shown in  FIG. 5 , certain information or all pieces of information regarding an actual address are stored. However, for example, identification information (IP address) may be stored in a tiered manner. Alternatively, ID information for specifying a location may be stored. 
     In an example shown in  FIG. 5 , two pieces of information regarding an installation location of a storage node are stored. In the node installation location information  4110  of the node installation location information management table  4100 , “JAPAN” and “TOKYO” are stored. These indicate that the storage node A 1100  is installed in a location called “TOKYO” of “JAPAN”. 
       FIG. 6  illustrates a configuration of the shared file metadata management table  4200  according to the first embodiment of this invention. 
     The shared file metadata management table  4200  manages metadata registered in a shared file stored in the external storage system  1170  coupled to the storage node A 1100  for each shared file. 
     The shared file data management table  4200  includes, in addition to existing metadata (e.g., storage date and keyword), information of storing permitted location  4210 , information of storing denied location  4220 , information of access permitted location  4230 , and information of access denied location. 
     The information of storing permitted location  4210  is for specifying a location where a shared file can be stored. Specifically, a client (e.g., client node  2000 ) can store a shared file only in an external storage system coupled to a storage node installed in a location designated by the information of storing permitted location  4210 . 
     In contrast to the information of storing permitted location  4210 , the information of storing denied location  4220  is information for designating a location where a shared file cannot be stored. Specifically, a client (e.g., client node  2000 ) cannot store any shared file in an external storage system coupled to a storage node installed in a location designated by the information of storing denied location  4220 . 
     The information of access permitted location  4230  is information for designating a location where access is permitted when a request of access to a shared file is received. Specifically, only an access requester (e.g., client node  2000 ) judged to be present in a location designated by the information of access permitted location  4230  can access a shared file. The access to the shared file means reading of the shared file, writing of the shared file, copying of the shared file, or migration of the shared file. 
     In contrast to the information of access permitted location  4230 , the information of access denied location  4240  is information for designating a location where access is denied when a request of access to a shared file is received. Specifically, an access requester judged to be present in a location designated by the information of access denied location  4240  cannot access a shared file. 
     In an example shown in  FIG. 6 , one piece of information is stored. However, a plurality of pieces of information may be stored as occasion demands. 
     When information of the same location is stored in the information of storing permitted location  4210  and the information of storing denied location  4220 , contents stored in the information of storing denied location  4220  are preferentially used. When information of the same location is stored in the information of access permitted location  4230  and the information of access denied location  4240 , contents stored in the information of access denied location  4240  are preferentially stored. 
     As shown in  FIG. 6 , when no value is stored, “Null” is stored. When “Null” is stored, there is no location to be designated. 
     In an example shown in  FIG. 6 , the information of storing permitted location  4210 , the information of storing denied location  4220 , the information of access permitted location  4230 , and the information of access denied location  4240  are stored in the shared file metadata management table  4200 . 
     “JAPAN”, “Null”, “Null”, and “UNITED STATES” are stored in the information of storing permitted location  4210 , the information of storing denied location  4220 , the information of access permitted location  4230 , and the information of access denied location  4240  of a first line of the shared file metadata management table  4200 . These indicate that a shared file can be stored in a storage node installed in a location “JAPAN”, and access from an access requester present in a location “UNITED STATES” is denied. 
       FIG. 7  schematically illustrates which of the storage nodes a shared file is stored in the system configuration of the first embodiment of this invention. 
     In an example shown in  FIG. 7 , “A”, “B”, “C”, and “D” are stored as shared files in the external storage systems. In this case, the shared file “A” is distributed to be stored in two locations, i.e., the external storage system  1170  coupled to the storage node A 1100  and the external storage system  1370  coupled to the storage node C 1300 . The shared file “B” is distributed to be stored in the external storage system  1170  coupled to the storage node A 1100 , and the external storage system  1570  coupled to the storage node E 1500 . The shared file “C” is distributed to be stored in the external storage system  1270  coupled to the storage node B 1200 , the external storage system  1370  coupled to the storage node C 1300 , and the external storage system  1470  coupled to the storage node D 1400 . The shared file “D” is distributed to be stored in the external storage system  1470  coupled to the storage node D 1400 , and the external storage system  1570  coupled to the storage node E 1500 . 
     It is presumed that for the shared file “A”, information “Area  11 ” is stored in the information of storing permitted location  4210  of the shared file metadata management table  4200 . In this case, when the shared file “A” is registered in file share services, a storage node which becomes a storage destination of a shared file is selected from the storage nodes A 1100 , B 1200 , and C 1300  included in the “Area  11 ”. 
       FIG. 8  is a flowchart showing a node registration process according to the first embodiment of this invention, which is executed by the basic control subprogram  1131  and the query request control subprogram  1126 . 
     The node registration process is executed by a participating storage node (here, referred to as storage node A 1100 ) when a new storage node participates in the overlay network. 
     First, the basic control subprogram  1131  of the storage node A 1100  gets an area for storing information of the storage node A 1100  in the node management table  4000 , and initializes the got area (S 101 ). 
     For the overlay network which is a target of participation, the basic control subprogram  1131  of the storage node A 1100  judges whether another storage node for registering node information (e.g., node name and identification information) of the storage node A 1100  is present (S 102 ). In other words, in the step S 102 , whether the storage node A 1100  is a first storage node to participate in the overlay network is judged. 
     If another storage node for registering the node information is not present, the storage node A 1100  is a first storage node to participate in the overlay network. Accordingly, the node information of the storage node A 1100  does not have to be registered in the other storage node. In this case, the process proceeds to step S 108 . 
     On the other hand, if another storage node for registering the node information is present, the other storage node has participated in the overlay network. Accordingly, the storage node A 1100  has to register own node information in the other storage node participating in the overlay network. In this case, the process proceeds to step S 103 . To judge whether the other storage node participates in the overlay network, it can be judged based on whether identification information (e.g., IP address) regarding the other storage node is held when participating in the overlay network. 
     The query request control subprogram  1126  of the storage node A 1100  requests node registration in the overlay network together with information regarding the storage node A 1100  to the other storage node which participates in the overlay network (S 103 ). 
     When participating in the overlay network, node information provided beforehand is designated to be transmitted to a storage node (hereinafter, referred to as target node) which requests node registration. The node information transmitted together with the node registration request contains a node name of the storage node A 1100 , and identification information (e.g., IP address) in the network. 
     Upon reception of the node registration request, a node registration process is executed by the basic control subprogram  1131  and the query response control subprogram  1127 . 
     First, the query response control subprogram  1127  of the target node receives the node registration request (S 104 ). 
     The basic control subprogram  1131  of the target node stores information of the storage node A 1100  which has requested the node registration in the node management table  4000  of the target node (S 105 ). 
     The query response control subprogram  1127  of the target node transmits the node information stored in the node management table  4000  to the storage node A 1100  (S 106 ). Specifically, the basic control subprogram  1131  of the target node reads the node information (e.g., node name  4010  and identification information  4020 ) stored in the node management table  4000 , and the query response control subprogram  1127  of the target node transmits the read node information together with a response of node registration success to the storage node A 1100 . 
     The basic control subprogram  1131  of the storage node A 1100  stores the node information transmitted from the target node in the node management table  4000  of the storage node A 1100  (S 107 ). Specifically, the query request subprogram  1126  of the storage node A 1100  receives a response of the node registration from the target node. When the response is a node registration success, the basic control subprogram  1131  of the storage node A 1100  stores the node information transmitted together with the node registration response in the node management table  4000  of the storage node A 1100 . Then, the process is finished. 
     In step S 108 , the basic control subprogram  1131  of the storage node A 1100  stores the node information of the storage node A 1100  in the node management table  4000  (S 108 ). Then, the process is finished. 
       FIG. 9  is a flowchart showing a node search process according to the first embodiment of this invention, which is executed by the basic control subprogram  1131  and the query request control subprogram  1126 . 
     The node search process is carried out when the storage node (here, referred to as storage node A 1100 ) participating in the overlay network searches for other participating storage nodes. 
     First, the basic control subprogram  1131  of the storage node A 1100  selects a necessary number of arbitrary storage nodes from a list of storage nodes registered in the node management table  4000  (S 111 ). 
     The necessary number of storage nodes is selected, because, when node search is requested to all the storage nodes registered in the node management table  4000 , node information of the entire overlay network may be obtained, but process loads of storage node searching and updating of the node management table  4000  may increase. Thus, in this case, the necessary number of nodes is selected, and node search is requested to the selected storage node. For a method of selecting storage nodes, for example, a method of randomly selecting storage nodes from the node management table  4000  is available. The necessary number can be changed flexibly by an administrator. 
     The query request control subprogram  1126  of the storage node A 1100  transmits contents of the node management table  4000  held by the storage node A 1100  to the storage node (hereinafter, referred to as target node) selected in the step S 111  to request acquisition of node information (e.g., node name and identification information (S 112 ). 
     The contents of the node management table  4000  to be transmitted contain all data (e.g., node name and identification information). Only certain data (e.g., only identification information) may be contained. 
     Upon reception of the node information acquisition request, a node search process is executed by the basic control subprogram  1131  and the query response control subprogram  1127 . 
     First, the query response control subprogram  1127  of the target node receives the node information acquisition request transmitted in the step S 112  (S 113 ). 
     The basic control subprogram  1131  of the target node stores node information transmitted from the storage node A 1100  which has requested the acquisition of node information in the node management table  4000  of the target node (S 114 ). 
     The query response control subprogram  1127  of the target node transmits the node information stored in the node management table  4000  to the storage node A 1100  (S 115 ). Specifically, the basic control subprogram  1131  of the target node read the node information stored in the node management table  4000 . The query response control subprogram  1127  of the target node transmits the read node information together with a response to the node information acquisition request to a request source (storage node A 1100 ) of the node information acquisition. 
     As in the case of the step S 112 , the contents of the node information to be transmitted contain all data (e.g., node name  4010  and identification information  4020 ). Only certain data (e.g., only identification information  4020 ) may be contained. 
     The basic control subprogram  1131  of the storage node A 1100  stores the node information transmitted from the target node in the step S 115  in the node management table  4000  of the storage node A 1100  (S 116 ). Specifically, the query request subprogram  1126  of the storage node A 1100  receives a response to the node information acquisition request from the target node. The basic control subprogram  1131  of the storage node A 1100  stores the node information transmitted together with the response in the node management table  4000  of the storage node A 1100 . Then, the process is finished. 
     The node search process is carried out to, for example, periodically update information. When node registration is requested, the node search process may be executed to quickly propagate information of the storage node which has requested the node registration to the other storage node. 
     The node search process may be executed each time a node is registered in the overlay network or a storage node withdraws from the network. 
     Only an outline of a node withdrawal process will be described without using any drawings. For example, upon execution of the node search process, when the target node tries to access a withdrawn storage node, by using a storage node which has not responded within a predetermined time as a withdrawn node, information of a storage node not responded from the own node management table  4000  is deleted. 
       FIG. 10  is a flowchart showing the file registration process carried out by the file registration subprogram  1132  according to the first embodiment of this invention. 
     The file registration process is executed by a storage node (here, referred to as storage node A 1100 ) which requests registration of a shared file when a new shared file is registered in the overlay network. 
     First, the file registration subprogram  1132  obtains the number of stored redundancies of a new shared file to be registered (hereinafter, referred to as target file) to store it in a temporary variable “S” (S 121 ). The number of stored redundancies is the number of copies of the target file to distribute and store the target file. For the number of stored redundancies, for example, a value designated as metadata of the target file is used. Alternatively, a value defined by the entire system may be used. 
     The file registration subprogram  1132  obtains node list information from the node management table  4000  to store it in a temporary area (S 122 ). The node list information contains, for example, a node name and identification information. 
     The file registration subprogram  1132  judges whether a process of step S 124  and the following steps has been finished for all the storage nodes registered in the node list information stored in the step S 122  (S 123 ). 
     If the process of the step S 124  and the following steps has been finished for all the storage nodes, the necessary number of storage nodes (number of stored redundancies) which become storage destinations of the target file is not secured. Thus, the process proceeds to step S 133 . 
     On the other hand, if the process of the step S 124  and the following steps has not been finished for all the storage nodes, the file registration subprogram  1132  has to get the necessary number of storage nodes which become storage destinations of the target file. In this case, the process proceeds to the step S 124 . 
     The file registration subprogram  1132  selects storage destination node candidates from the node list information obtained in the step S 122  to create a storage destination node candidate list (S 124 ). For a method of selecting the storage destination node candidates, for example, a method of selecting storage node candidates in order registered in the node list information is available. Alternatively, a method of randomly selecting storage node candidates may be used. Otherwise, a method of selecting storage node candidates by using hash values of node names so that the storage nodes of the storage destination can be uniformly distributed may be used. 
     The file registration subprogram  1132  judges whether a process of step S 127  and the following steps has been finished for all the storage nodes registered in the storage destination node candidate list created in the step S 124  (S 125 ). 
     If the process of the step S 127  and the following steps has been finished for all the storage nodes, the process proceeds to step S 126 . 
     On the other hand, if the process of the step S 127  and the following steps has been finished for all the storage nodes, the file registration subprogram  1132  has to get the necessary number of storage nodes which become storage destinations of the target file. In this case, the process proceeds to the step S 127 . 
     The file registration subprogram  1132  selects one storage node not subjected to a process of step S 128  and the following steps from the storage destination node candidate list (S 127 ). For a method of selecting one storage node, a method of randomly selecting a storage node from the storage destination node candidate list is available. 
     The file registration subprogram  1132  judges whether the information of storing denied location  4220  of the target file contains location information of the selected storage node (S 128 ). Specifically, the file registration subprogram  1132  refers to the information of storing denied location  4220  stored as metadata of the target file in the shared file metadata management table  4200  to judge whether a location indicated by the information of storing denied location  4220  contains node installation location information  4110  stored in the node installation location information management table  4100  of the storage node selected in the step S 127 . 
     If the location information of the selected storage node is contained, the file registration subprogram  1132  judges that the target file cannot be stored in the selected storage node. In this case, the process returns to the step S 125 . 
     On the other hand, if the location information of the selected storage node is not contained, the process proceeds to step S 129 . 
     The file registration subprogram  1132  judges whether the information of storing permitted location  4210  of the target file contains the location information of the selected storage node (S 129 ). Specifically, the file registration subprogram  1132  refers to information of storing permitted location  4210  stored as metadata of the target file in the shared file metadata management table  4200  to judge whether a location indicated by the information of storing permitted location  4210  contains node installation location information  4110  stored in the node installation location information management table  4100  of the storage node selected in the step S 127 . 
     If the location information of the selected storage node is not contained, the file registration subprogram  1132  judges that the target file cannot be stored in the selected storage node. In this case, the process returns to the step S 125 . 
     On the other hand, if the location information of the selected storage node is contained, the process proceeds to step S 130 . 
     The file registration subprogram  1132  decides the selected storage node to be a storage destination node to decrement the temporary variable “S” (S 130 ). 
     The file registration subprogram  1132  judges whether the temporary variable “S” is “0” (S 131 ). 
     If the temporary variable “S” is not “0”, it is judged that the necessary number of storage nodes for storing the target file has not been secured. In this case, the process returns to the step S 125 . 
     On the other hand, if the temporary variable “S” is “0”, it is judged that the necessary number of storage nodes for storing the target file has been secured. In this case, the process proceeds to step S 132 . 
     The file registration subprogram  1132  copies the target file by a number of times equal to the number of stored redundancies to store the target file in the storage destination node decided in the step S 130  (S 132 ). If the storage destination node is another storage node (target node), the query request control program  1126  requests the target node to store the target file. The query response control program  1127  of the storage node requested to store the target file receives the storage request of the target file to store the target file. Then, a result of the storage is transmitted to a request source of the target file storage request. 
     In step S 126 , the file registration subprogram  1132  deletes node information of the storage node registered in the storage destination node candidate list from the node list information stored in the temporary area (S 126 ). Then, the process returns to the step S 123 . 
     In step S 133 , the file registration subprogram  1132  becomes an error because the necessary number of storage destination nodes cannot be found. In this case, the process is finished (S 133 ). 
       FIG. 11  is a flowchart showing a file replication/migration process executed by the file replication/migration subprogram  1133  according to the first embodiment of this invention. 
     When the shared file (hereinafter, referred to as target file) registered in the overlay network is copied or migrated over the storage nodes, the file replication/migration process is executed by the storage node (here, referred to as storage node A 1100 ) which requests copying or migration. 
     The file replication/migration process enables adjustment of response performance of file access by increasing/decreasing the number of copies of the shared file, and maintenance of access availability to the shared file by migrating the shared file stored in a storage node to be stopped to another storage node beforehand. 
     First, the file replication/migration subprogram  1133  obtains node list information from the node management table  4000  to store it in a temporary area (S 141 ). The node list information contains, for example, a node name and identification information. 
     The file replication/migration subprogram  1133  judges whether a process of step S 143  and the following steps has been finished for all the storage nodes registered in the node list information stored in the step S 141  (S 142 ). 
     If the process of the step S 143  and the following steps has been finished for all the storage nodes, no storage node which becomes a storage destination is secured. Thus, the process proceeds to step S 150 . 
     On the other hand, if the process of the step S 143  and the following steps has not been finished for all the storage nodes, the file replication/migration subprogram  1133  has to get a storage node which become a storage destination of the target file. In this case, the process proceeds to the step S 143 . 
     The file replication/migration subprogram  1133  selects storage destination node candidates from the node list information obtained in the step S 141  to create a storage destination node candidate list (S 143 ). For a method of selecting the storage destination node candidates, for example, a method of selecting storage node candidates in order registered in the node list information is available. Alternatively, a method of randomly selecting storage node candidates may be used. Otherwise, a method of selecting storage node candidates by using hash values of node names so that the storage nodes of the storage destination can be uniformly distributed may be used. 
     The file replication/migration subprogram  1133  judges whether a process of step S 146  and the following steps has been finished for all the storage nodes registered in the storage destination node candidate list created in the step S 143  (S 144 ). 
     If the process of the step S 146  and the following steps has been finished for all the storage nodes, the process proceeds to step S 145 . 
     On the other hand, if the process of the step S 146  and the following steps has been finished for all the storage nodes, the file replication/migration subprogram  1133  has to get storage nodes which become storage destinations of the target file. In this case, the process proceeds to the step S 146 . 
     The file replication/migration subprogram  1133  selects one storage node not subjected to a process of step S 147  and the following steps from the storage destination node candidate list (S 146 ). For a method of selecting one storage node, a method of randomly selecting a storage node from the storage destination node candidate list is available. 
     The file replication/migration subprogram  1133  judges whether the information of storing denied location  4220  of the target file contains location information of the selected storage node (S 147 ). Specifically, the file replication/migration subprogram  1133  refers to the information of storing denied location  4220  stored as metadata of the target file in the shared file metadata management table  4200  to judge whether a location indicated by the information of storing denied location  4220  contains node installation location information  4110  stored in the node installation location information management table  4100  of the storage node selected in the step S 146 . 
     If the location information of the selected storage node is contained, the file replication/migration subprogram  1133  judges that the target file cannot be copied or migrated to the selected storage node. In this case, the process returns to the step S 144 . 
     On the other hand, if the location information of the selected storage node is not contained, the process proceeds to step S 148 . 
     The file replication/migration subprogram  1133  judges whether the information of storing permitted location  4210  of the target file contains the location information of the selected storage node (S 148 ). Specifically, the file replication/migration subprogram  1133  refers to information of storing permitted location  4210  stored as metadata of the target file in the shared file metadata management table  4200  to judge whether a location indicated by the information of storing permitted location  4210  contains node installation location information  4110  stored in the node installation location information management table  4100  of the storage node selected in the step S 146 . 
     If the location information of the selected storage node is not contained, the file replication/migration subprogram  1133  judges that the target file cannot be copied or migrated to the selected storage node. In this case, the process returns to the step S 144 . 
     On the other hand, if the location information of the selected storage node is contained, the process proceeds to step S 149 . 
     The file replication/migration subprogram  1133  copies or migrates the target file to the storage node (storage destination node) judged to contain the location information in the step S 148  (S 149 ). If the storage destination node is another storage node (target node), the query request control program  1126  requests the target node to copy or migrate the target file. The query response control program  1127  of the storage node requested to copy or migrate the target file receives the replication or migration request of the target file to store the replicated or migrated target file. Then, the target node transmits a result of the replication or the migration to the storage node A 1100  which has requested the replication or the migration of the target file. 
     In step S 145 , the file replication/migration subprogram  1133  deletes node information of the storage node registered in the storage destination node candidate list from the node list information stored in the temporary area (S 145 ). Then, the process returns to the step S 142 . 
     In step S 150 , the file replication/migration subprogram  1133  becomes an error as storage destination nodes cannot be found. In this case, the process is finished (S 150 ). 
       FIG. 12  is a flowchart of the file access process carried out by the file access subprogram  1134  according to the first embodiment of this invention. 
     The file access process is executed by a storage node (here, referred to as storage node A 1100 ) which requests access when an access request of a shared file (hereinafter, referred to as target file) registered in the overlay network is received. 
     First, the file access subprogram  1134  searches for a target file of an access request to create a list of nodes in which the target file is present (S 161 ). For the searching of the target file, the query request control subprogram  1126  selects a plurality of arbitrary storage nodes from the node management table  4000  to ask whether the target file is present in the plurality of selected storage nodes. For a method of selecting storage nodes, for example, a method of randomly selecting a plurality of storage nodes is available. 
     The query response control subprogram  1127  of the storage node (target node) selected in the step S 161  receives a query request to check whether the target file is present, and transmits its result as a response to the storage node A 1100 . If the target file is present, the query response control subprogram  1127  of the target node transmits the response together with information of metadata of the target file to the storage node A 1100 . 
     The file access subprogram  1134  judges whether a target file has been found based on the response received from the target node in the step S 161  (S 162 ). 
     If a target file has not been found, the file access subprogram  1134  judges that a target file of an access request has not been found. In this case, the process proceeds to step S 169 . 
     On the other hand, if a target file has been found, the file access subprogram  1134  has to judge whether access can be requested to the target file. In this case, the process proceeds to step S 163 . 
     The file access subprogram  1134  obtains information of access permitted/denied location of the target file (S 163 ). Specifically, the file access subprogram  1134  refers to the shared file metadata management table  4200  of the target file to obtain information of access permitted location  4230  and information of access denied location  4240 . 
     The file access subprogram  1134  obtains information (presence location information) of a location (e.g., location where the client node  2000  is installed) where an access requester of the target file is present (S 164 ). 
     A method of obtaining information of the location where the access requester is present is, for example, a method of using location information of a controller (e.g., storage node A 1100 ) which has first received an access request from the access requester. Alternatively, a method in which the controller uses positional information obtained by a GPS as information of a presence location of the access requester may be used. Otherwise, a method in which a third party (e.g., authentication server  3000 ) authenticates a presence location at the time of an access request of the access requester, and an authentication result is used may be employed. 
     The file access subprogram  1134  judges whether the information of access denied location  4240  of the target file contains presence location information of the access requester (S 165 ). Specifically, the file access subprogram  1134  refers to the information of access denied location  4240  stored as metadata of the target file in the shared file metadata management table  4200  to judge whether a location indicated by the information of access denied location  4240  contains presence location information of the access requester obtained in the step S 164 . 
     If the presence location information of the access requester is contained, the file access subprogram  1134  judges that access of the access requester to the target file is not permitted. In this case, the process proceeds to step S 168 . 
     On the other hand, if the presence location information of the access requester is not contained, the process proceeds to step S 166 . 
     The file access subprogram  1134  judges whether the information of access permitted location  4230  of the target file contains the presence location information of the access requester (S 166 ). Specifically, the file access subprogram  1134  refers to information of access permitted location  4230  stored as metadata of the target file in the shared file metadata management table  4200  to judge whether a location indicated by the information of access permitted location  4230  contains the presence location information of the access requester obtained in the step S 164 . 
     If the presence location information of the access requester is not contained, the file access subprogram  1134  judges that access of the access requester to the target file is not permitted. In this case, the process proceeds to step S 168 . 
     On the other hand, if the presence location information of the access requester is contained, the file access subprogram  1134  judges that access of the access requester to the target file is permitted. In this case, the process proceeds to step S 167 . 
     The file access subprogram  1134  selects an arbitrary storage node from the list of nodes created in the step S 161 , and provides information of the selected storage node to the access requester. Then, the access requester accesses the storage node included in the provided information (S 167 ). 
     For a method of selecting an arbitrary storage node, for example, a method of randomly selecting a storage node from the list of nodes created in the step S 161  is available. Alternatively, a method of selecting a storage node closest to the access requester may be used. Otherwise, a method of selecting a storage node of shortest access time from the list of nodes may be used. 
     In step S 168 , as the file access subprogram  1134  judges that access to the target file is not permitted, the process is set as an error without permitting the access requester to access the target file (S 168 ). In this case, the process is finished. 
     In step S 169 , as the file access program  1134  cannot find a target file, the process is finished as an error (S 169 ). 
     The first embodiment of this invention has been described by way of the system for storing the shared file. However, the system may share and store, for example, data, information of a block unit, fixed length data, and a record in addition to the file. 
     According to the first embodiment of this invention, the storage node for storing the shared file can be designated for each shared file. Thus, by suppressing storage of the shared file in a storage node installed in a location not permitted to store the shared file, unnecessary communication can be removed, and wasteful storage use can be suppressed. Moreover, since a location for accessing the shared file can be designated for each shared file, access from a location where access is not permitted can be denied. 
     The first embodiment of this invention has been described by way of a configuration made by the storage node (controller). In addition, however, the embodiment can be configured as a control system or a control method. The first embodiment can be realized by various modes such as a computer program for realizing the storage node (controller), a recording medium for recording a program, and a data signal containing a program and realized in a carrier wave. 
     In the case of configuring the embodiment of this invention as a computer program or a recording medium for recording a program, it can be configured as a controller or an entire program for controlling the controller. Only modules which perform functions of the first embodiment may be provided. As a recording medium, for example, a flexible disk, a CD-ROM, a DVD-ROM, a punch card, a printed matter where codes such as barcodes have been printed, or various volatile or nonvolatile storage media readable by an internal or external storage system of a computer can be used. 
     Second Embodiment 
     According to the first embodiment, based on the system configuration shown in  FIG. 1 , the access to the shared file is controlled based on the storage destination node of the shared file and the presence location of the access requester. A second embodiment is directed to control for optimizing communication charges of a network. Description of modules of the second embodiment similar to those of the first embodiment will be omitted. 
       FIG. 13  illustrates a configuration of a system according to the second embodiment of this invention. 
     A difference from the first embodiment is that management servers  3100 ,  3200 ,  3300 ,  3400 , and  3500  are coupled to ISPs. 
     The management server  3100  provides information regarding network services provided by the ISP, and information regarding communication charges. The management servers  3200 ,  3300 ,  3400 , and  3500  have similar configuration to the management server  3100 . In the system shown in  FIG. 13 , one management server is coupled to one ISP. However, a plurality of management servers may be coupled. 
       FIG. 14  illustrates a hardware configuration of the management server  3100  according to the second embodiment of this invention. 
     The management server  3100  includes a processor  3110 , a memory  3120 , an external storage system I/F  3140 , and a network I/F  3150 . These components are intercoupled via a bus  3160 . The management server  3100  is coupled to an external storage system  3170  via the external storage system I/F  3140 . 
     The processor  3110  executes a program stored in the memory  3120  to control the entire management server  3100 . 
     The memory  3120  temporarily stores the program executed by the processor  3110  and/or data. The memory  3120  may include, for example, a semiconductor memory such as a RAM. 
     The memory  3120  stores an external storage system I/F control program  3121 , a network I/F control program  3122 , a local file system control program  3123 , a distributed file system management program  3125 , an ISP information management table  4300 , and a cache memory  3130 . 
     The external storage system I/F control program  3121  controls the external storage system I/F  3140 . The network I/F control program  3122  controls the network I/F  3150 . 
     The local file system control program  3123  contains a cache memory control subprogram  3124 . The local file system control program  3123  controls a file system provided by the management server  3100 . The cache memory control subprogram  3124  controls the cache memory  3130 . 
     The distributed file system management program  3125  contains a query request control subprogram  3126 , a query response control subprogram  3127 , a basic control subprogram  3131 , and a file registration subprogram  3132 . 
     The distributed file system management program  3125  manages information regarding network services provided by the ISP, and information regarding communication charges. 
     The query request control subprogram  3126  controls a query request transmitted to the other management server constituting the overlay network. 
     The query response control subprogram  3127  receives a query request from the other management server constituting the overlay network, executes a requested process, and controls a response of a processed result. 
     The basic control subprogram  3131  manages information of the management server  3100 . For example, the basic control subprogram  3131  stores information (identification information) regarding the management server  3100  in the ISP information management table  4300 . 
     The ISP information management table  4300  holds an ISP recognized by the management server  3100 , identification information of a management server present in the recognized ISP, and information regarding communication charges of a network for reaching the recognized ISP. The ISP information management table  4300  will be described later in detail referring to  FIG. 15 . 
     The cache memory  3130  is used for shortening access response time when access to a file managed by the file system is made. 
     The external storage system I/F  3140  is an interface for accessing the external storage system  3170 . The network I/F  3150  is an interface for accessing the other system coupled via the network. 
     The external storage system  3170  stores a program or user data. The external storage system  3170  may include, for example, a hard disk drive (HDD). Alternatively, a semiconductor memory device such as a flash memory may be used. 
       FIG. 15  illustrates a configuration of the ISP information management table  4300  according to the second embodiment of this invention. 
     The ISP information management table  4300  holds an ISP recognized by the management server  3100 , identification information of a management server present in the recognized ISP, and information regarding communication charges of the network for reaching the recognized ISP. 
     The ISP information management table  4300  contains an ISP name  4310 , management server identification information  4320  of the ISP, and communication charges  4330 . 
     The ISP name  4310  is information for identifying the ISP in the network. In the management server identification information  4320  of the ISP shown in  FIG. 15 , information of a character string is stored. However, for example, information of a numerical value such as ID for identifying the ISP may be stored. 
     The management server identification information  4320  of the ISP is identification information of the management server of the ISP coupled to a normal network. The management server identification information  4320  of the ISP is used for designating an access destination when access is made to the management server via the network. In the management server identification information  4320  of the ISP shown in  FIG. 15 , an IP address is stored. However, for example, ID information for specifying the management server may be stored. Information of a character string may be stored. 
     The communication charges  4330  are communication charges from an ISP network to which the management server  3100  itself is coupled to a network of an ISP to which a management server of a communication target is coupled. For the communication charges  4330 , the amount of money paid from one ISP to another is used. The amount of money to be paid by someone for facilities or operations may be used. 
     In an example shown in  FIG. 15 , five pieces of ISP information are stored. As an ISP name  4310 , ISP management server identification information  4320 , and communication charges  4330  of a first line of the ISP information management table  4300 , “ISP  1 ”, “ 10 .  20 .  30 .  200 ”, and “ 30 ” are respectively stored. These indicate that a management server named “ISP  1 ” is present in the network, the “ISP  1 ” can be accessed based on information of the destination “ 10 .  20 .  30 .  200 ”, and the amount of money indicated by the communication charges “ 30 ” has to be paid to access the “ISP  1 ”. 
     Contents of the ISP information management table  4300  are updated by periodically exchanging information between the management servers. A method of updating the contents is similar to that of the flowchart of the node search process shown in  FIG. 9 . In other words, the contents are updated by making inquiries about the contents of the ISP information management table  4300  to the other management server. 
       FIG. 16  illustrates a configuration of the node management table  4000  according to the second embodiment of this invention. 
     A difference from the first embodiment is that the node management table  4000  contains node communication charges  4030 . 
     The node communication charges  4030  are communication charges generated when communication with a target storage node is carried out. The node communication charges  4030  are updated by taking updating of the contents of the node management table  4000  as an opportunity. Information regarding the communication charges is obtained by making an inquiry to a management server corresponding to the management server identification information  4140  of the node installation location information management table  4100  described later referring to  FIG. 17 . A method of obtaining the information regarding the communication charges is similar to that of the flowchart of the node search process shown in  FIG. 9 . In other words, the information is obtained by making an inquiry about communication charges to the other management server. 
     In an example shown in  FIG. 16 , as a node name  4010 , identification information  4020 , and node communication charges  4030  of a first line of the node management table  4000 , “STORAGE NODE A”, “ 10 .  20 .  30 .  40 ”, and “ 30 ” are respectively stored. These indicate that a storage node named “STORAGE NODE A” participating in the overlay network is present, the “STORAGE NODE A” can be accessed based on information of the destination “ 10 .  20 .  30 .  40 ”, and the amount of money indicated by the communication charges “ 30 ” has to be paid to access the “STORAGE NODE A”. 
       FIG. 17  illustrates a configuration of the node installation location information management table  4100  according to the second embodiment of this invention. 
     A difference from the first embodiment is that the node installation location information management table  4100  includes used ISP information  4130  and management server identification information  4140  of a used ISP. 
     The used ISP information  4130  is information for identifying an ISP which provides network services to which the storage node is coupled. When the storage node uses a plurality of ISPs, a plurality of pieces of information for identifying the ISPs are stored. 
     The management server identification information  4140  of the used ISP is identification information of a management server coupled to the ISP indicated by the used ISP information  4130  in the network. 
     The used ISP information  4130  and the management server identification information  4140  of the used ISP are similarly stored when the node installation location information  4110  is stored. 
     In an example shown in  FIG. 17 , as the used ISP information and the management server identification information  4140  of the used ISP of the node installation location information management table  4100 , “ISP  2 ” and “ 10 .  100 .  30 .  200 ” are respectively stored. These indicate that an ISP for providing network services to be used by the storage node has a name “ISP  2 ”, and identification information of a management server belonging to the “ISP  2 ” is “ 10 .  100 .  30 .  200 ”. 
       FIG. 18  is a flowchart showing a node search process according to the second embodiment of this invention. 
     A difference from the first embodiment is that step S 117  is added between the steps  114  and  115  of  FIG. 9 , and step S 118  is added after the step S 116  of  FIG. 9 . 
     In the step S 117 , the query response control subprogram  1127  of the target node makes an inquiry about communication charges to a management server of each ISP to reflect the charges in the node management table  4000 , based on the management server identification information  4140  of the node installation location information management table  4100  (S 117 ). 
     If necessary information cannot be obtained only by the management server of an inquiry destination, the management server of the inquiry destination obtains information by sequentially asking the other management servers registered in the ISP information management table  4300  stored in its own server. 
     In the step S 118 , the query request control subprogram  1126  of the storage node A 1100  makes an inquiry about communication charges to a management server of each ISP. The basic control subprogram  1131  of the storage node A 1100  reflects the communication charges obtained from the management server of each ISP in the node management table  4000  (S 118 ). Then, the process is finished. 
       FIG. 19  is a flowchart showing a file access process according to the second embodiment of this invention. 
     A difference from the first embodiment is that step S 170  is executed in location of the step S 167  of  FIG. 12 . 
     In the step S 170 , the file access subprogram  1134  selects a node of lowest node communication charge from the list of nodes created in the step S 161 , and provides information of the selected storage node to an access requester. Then, the access requester accesses the storage node whose information has been provided (S 170 ). 
     According to the first embodiment, the access destination of the target file is arbitrarily decided. According to the second embodiment, however, the storage node of low communication charges can be selected. Thus, by executing control to preferentially access a shared file of a storage node whose communication charge is lowest, communication charges for file share services can be reduced. 
     While the present invention has been described in detail and pictorially in the accompanying drawings, the present invention is not limited to such detail but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.