TECHNICAL FIELD The present invention relates to a dual web service system and method for providing a web service in a web server in a P2P environment using a central server, and more particularly to a dual web service system and method wherein a central server backs up and buffers a web site provided by a web server which provides content through the web site, and the central server substitutes for the web server and provides the web service through the backup web site when the web server cannot provide the web service due to its failure.

BACKGROUND ART As a network has been developed and a client/server model has been introduced, the client can now obtain required information without the need to store the information in its PC by accessing the server and retrieving and downloading the information from the server each time the client requires the information. Recently, a peer-to-peer (P2P) model has been suggested as a modification of the client/server model. The P2P model is a technology in which each computer on a network functions as a client or server as needed and is connected to each other computer to allow the users to share files. The P2P model is characterized in that it compensates for limited capacity and overload of the server in conventional server-based computing environments and provides new concept services to users or corporations. That is, as a method to overcome problems in the server-based system in which the server alone contains and uses all data, the P2P model distributes data to be shared over the network, thereby ensuring efficient management of data. In the conventional P2P environment, a central server is provided at the center of a network which is established for client PCs connected to the central server, and specific directories of the client PCs connected to the network are set as 'shared' so that each client PC communicates data directly with each other. However, in the conventional P2P environment, data of a client PC, which is not connected to the central server, cannot be shared. That is, data sharing of the client PC in the network depends on whether or not the client PC is connected to the central server. The dependency on the connection to the central server causes some problems. For example, a web server may be implemented in a specific client PC in the P2P environment so that it functions as a web server client PC and provides a web service to other client PCs through a web site. However, if the web server of the web server client PC is inoperable for whatever reason, client PCs requesting access to the web server client PC must wait indefinitely until the web service is restored. In addition, the conventional central server is provided as a simple medium for implementing the network supporting the P2P service for data sharing, and has a limited function similar to that of a domain name server which converts the domain name to an IP address and provides the IP address. In an environment in which the web server client PC, together with other PCs, is connected to a router, the web server client PC cannot be accessed from the outside with its IP address alone until a proper URL address of the web server client PC is specified.

DISCLOSURE OF INVENTION Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a system and method for providing a dual web service in a web server in a P2P environment using a central server, wherein a central server is provided with a function to convert a domain name to an IP address or a URL address so that each individual client PC can serve as a web server, and a buffering function to back up data of a web server client PC, and the central server substitutes for the web server client PC and serves as a web server when the web server client PC cannot provide the web service for whatever reason. In order to accomplish the above object, the present invention provides a dual web service system supporting a web service function of individual client PCs (personal computers) in a P2P (peer-to-peer) environment, which includes a requesting client PC for transmitting a domain name of a web server client PC to a central server in order to access a web site of the web server client PC and to request data from the web server client PC, receiving actual network access information of the web site corresponding to the domain name from the central server, and accessing the web site using the network access information to receive a web service; the web server client PC including the web site implemented therein, accessing the central server to register the domain name and the actual network access information of the web site, periodically backing up data of the web site on the central server, and providing the web service through the web site upon receiving a web service request from the requesting client PC; and the central server checking, in real time, whether or not the web server client PC can provide the web service, buffering a backup web site received from the web server client PC, receiving the domain name from the requesting client PC, transmitting the network access information of the web server client PC if the web server client PC can provide the web service, and transmitting network access information of the backup web site if the web server client PC cannot provide the web service, wherein, if the web server client PC cannot provide the web service, the central server provides, in real time, a web service, associated with the access to the web site and the request for the data, to the requesting client PC through the buffered web site. In one aspect of the invention, the web server client PC includes a P2P connection unit for logging into the central server and maintaining connection to the central server; a web server unit for providing a web service through a web site; a web server driving unit for automatically driving the web server unit when logging into the central server through the P2P connection unit, and transmitting both a web service availability state of the web server unit and a network access path of the web site of the web server unit to the central server; and a backup unit for automatically accessing the central server to back up data of the web site of the web server unit on the central server. In addition, if the requesting client PC inputs a domain name for accessing the web site of the web server client PC when the web server client PC can provide the web service, the central server transmits an IP address of the web server client PC corresponding to the input domain name or an address/path of the central server having information of the web site of the web server client PC, as network access information, to the requesting client PC, and when the requesting client PC accesses the central server using the address/path of the central server, the central server provides a frame having information of the web site data of the web server client PC, whereby the central server allows the web server client PC to provide the web service through the frame even when the web server client PC is inaccessible with the IP address of the web server client PC alone. In addition, the central server includes a P2P connection processor for processing P2P access requests from a plurality of client PCs; a URL converter for receiving and storing a domain name and actual network access information from the web server client PC, receiving a domain name from the requesting client PC, checking whether or not the web server client PC can provide the web service, and transmitting, in real time, network access information of the web server client PC or network access information of the central server to the requesting client PC according to whether or not the web server client PC can provide the web service; a URL address DB for receiving and storing a domain name and an IP address/path of the web server client PC from the URL converter; a server buffer for buffering backup data of an updated web site received from the web server client PC; a web site backup DB for storing the backup web site data buffered by the server buffer; and a dual web server unit for substituting for the web server client PC and providing the web service through the web site stored in the web site backup DB when the web server client PC cannot provide the web service. In addition, the central server determines that the web server client PC cannot provide the web service when the web server unit of the web server client PC is inoperable or when the web server client PC is powered off or is not accessible due to network failure after the central server receives a domain name of a web site from the requesting client PC and accesses the web server client PC using network access information including an actual address of the web site. In addition, the central server checks, in real time, whether or not the web server client PC can provide the web service, converts the network access information to an address of the backup web site data buffered by the central server if the web server client PC cannot provide the web service, and the central server converts the network access information to an actual address of the web site data of the web server client PC if the web server client PC can provide the web service, whereby, upon receiving a web site data request, including the domain name of the web server client PC, from the web server client PC, the central server provides, in real time, network access information, which is always ready to provide the web service, to the requesting client PC. In another aspect of the present invention, there is also provided a dual web service method in a central server supporting a web service function of individual client PCs in a P2P environment, which includes the steps of: 1), by the central server, receiving and storing actual network access information including an IP address and a domain name from a web server client PC; 2) periodically backing up and buffering updated web site data from the web server client PC; 3) receiving a domain name of a web site of the web server client PC from a requesting client PC desiring to access the web site of the web server client PC; and 4) checking whether or not the web server client PC can provide a web service and transmitting network access information of the web site of the web server client PC or network access information of the backup web site of the central server to the requesting client PC according to whether or not the web server client PC can provide the web service. In one aspect of the invention, the step 2) includes one of the steps of: 2-1), by the central server, backing up and buffering the web site data from the web server client PC each time the web site data is updated in the web server client PC; 2-2) backing up and buffering the web site data from the web server client PC each time the web server client PC logs into the central server; and 2-3) backing up and buffering the web site data from the web server client PC at a fixed time everyday. In addition, the step 4) includes the steps of: 4-1), by the central server, searching for network access information corresponding to the received domain name; 4-2) accessing the web server client PC and checking whether or not the web server client PC can provide the web service; 4-3) transmitting the network access information of the web server client PC to the requesting client PC if the web server client PC can provide the web service; 4-4) transmitting the network access information of the backup web site buffered by the central server to the requesting client PC if the web server client PC cannot provide the web service; and 4-5) providing the web service through the buffered backup web site when the requesting client PC makes a web service request to the central server. In addition, the step 4-3) includes the steps of: 4-3-1) transmitting an address/path of the central server having information of the web site of the web server client PC, as network access information, to the requesting client PC if the requesting client PC inputs a domain name for accessing the web site of the web server client PC; 4-3-2) providing a frame having information of the web site data of the web server client PC when the requesting client PC accesses the central server using the network access information, whereby the central server allows the web server client PC to provide the web service even when the web server client PC is inaccessible with the IP address of the web server client PC alone. In the system and method for providing a dual web service in a web server in a P2P environment using a central server according to the present invention, when the requesting client PC requests access to the web site of the web server client PC through the domain name in the P2P network, the central server determines whether or not the web server client PC can provide the web service, and transmits network access information of the web site of the web server client PC to the requesting client PC if the web server client PC can provide the web service, and transmits network access information of the backup web site of the central server to the requesting client PC if the web server client PC cannot provide the web service. Thus, there is an advantage in that, when the web server client PC cannot provide the web service, the central server provides the web service through the backup web site of the web server client PC buffered in the central server, thereby achieving a stable web service.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features, aspects, and advantages of preferred embodiments of the present invention will be more fully described in the following detailed description, taken accompanying drawings. In the drawings: Fig. 1 illustrates a functional structure of a dual web service system according to an embodiment of the present invention; Figs. 2a and 2b illustrate a functional structure of the dual web service system according to the embodiment of the present invention when a web server client PC can provide a web service; Fig. 3 illustrates a functional structure of the dual web service system according to the embodiment of the present invention when the web server client PC cannot provide a web service; Fig. 4 illustrates the configuration of a web server client PC according to an embodiment of the present invention; Fig. 5 illustrates the configuration of a central server according to an embodiment of the present invention; Fig. 6 is a flow chart illustrating how the central server operates in a method for providing a dual web service according to an embodiment of the present invention; Fig. 7 is a flow chart illustrating how the central server backs up web site data according to an embodiment of the present invention; Fig. 8 is a flow chart illustrating how the central server transmits network access information according to an embodiment of the present invention; and Fig. 9 is a flow chart illustrating how the central server transmits network access information and provides frames according to an embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Fig. 1 illustrates a dual web service system according to an embodiment of the present invention. The dual web service system 1 according to the present invention includes a requesting client PC 2, a web server client PC 3, and a central server 4. The requesting client PC 2 transmits a domain name of the web server client PC 3 to the central server 4, receives network access information, as a response to the transmitted domain name, from the central server 4, and accesses the web server client PC 3 or the central server 4 based on the received network access information to receive a web service through a web site. The network access information is a URL including an IP address for accessing a web site and downloading data therefrom. The web server client PC 3 implements therein a web server containing a web site, transmits a service availability state to the central server 5, and backs up data of a web site of the web server client PC 3 on the central server 5. The central server 4 registers a domain name corresponding to the web site of the web server client server PC 3 received from the web server client PC 3 and actual network access information corresponding to the domain name. The central server 4 determines, in real time, whether or not the web server client PC 3 can provide the web service. If the web server client PC 3 can provide the web service, the central server 4 transmits the network access information of the web server client PC 3 to the requesting client PC 2 in response to the domain name transmission from the requesting client PC 2 so that the web server client PC 3 can provide the web service. If the web server client PC 3 cannot provide the web service, the central server 4 transmits the network access information of the backup web site of the web server client PC 3, which has been backed up and buffered in the central server 4, to the requesting client PC 2 in response to the domain name transmission from the requesting client PC 2 so that the central server 4 can provide the web service. Accordingly, the present invention has an advantage in that the central server 4 can stably maintain the web service of the web server client PC 3 in P2P environments. Fig. 2a illustrates how the central server responds with an IP address of the web server client PC when the web server client PC can provide the web service in the dual web service system according to the embodiment of the present invention. In Fig. 2a, the dual web service system 1 is shown as functioning in the case where the web server client PC 3 is assigned a unique IP address to allow access from the outside. The requesting client PC 2 transmits the domain name of the web server client PC 3 to the central server 4, and the central server 4 determines whether or not the web server client PC 3 can provide the web service. If the web server client PC 3 can provide the web service, the central server 4 responds to the domain name received from the requesting client PC 2 by transmitting an IP address of the web server client PC 3, as network access information, to the requesting client PC 2. The requesting client PC 2 accesses the web site of the web server client PC 3 according to the network access information to receive the web service. Fig. 2b illustrates how the central server responds with its IP address and provides frames when the web server client PC can provide the web service in the dual web service system according to the embodiment of the present invention. In Fig. 2b, the dual web service system 1 is shown as functioning in the case where the web server client PC 3 is connected to a router so that the web server client PC 3 is not accessible from the outside with the IP address alone. The requesting client PC 2 transmits the domain name of the web server client PC 3 to the central server 4, and the central server 4 determines whether or not the web server client PC 3 can provide the web service. If the web server client PC 3 can provide the web service, the central server 4 responds to the domain name from the requesting client PC 2 by transmitting an IP address of the central server 4, as network access information, to the requesting client PC 2. The requesting client PC 2 accesses the central server 4 according to the network access information, and the central server 4 provides frames, contents of which refer to the web site data of the web server client PC 3, to the request client PC 2. The requesting client PC 2 receives the web service from the web server client PC 3 through the frames received from the central server 4. Fig. 3 illustrates how the dual web service system functions when the web server client PC cannot provide the web service according to the embodiment of the present invention. If the central server 4 determines that the web server client PC 3 cannot provide the web service, the central server 4 transmits an address/path of the backup web site data of the web server client PC 3, which has been backed up and buffered in the central server, as network access information, to the requesting client PC 2. The requesting client PC 2 requests, using the received network access information, that the central server 4 provide the backup web site data. In response to this request, the central server 4 temporarily substitutes for the web server client PC 3 and provides a dual web service using the backup web site data buffered in the central server 4. Accordingly, the requesting client PC 2 can receive a stable web service from the backup web site through the central server 4 although the web server client PC 3, which the requesting client PC 2 desires to access, cannot provide the web service. This system can be effectively used for web sites of large corporations, large shopping malls, public offices, governmental agencies, etc. Fig. 4 illustrates the configuration of a web server client PC according to an embodiment of the present invention. The web server client PC 3 according to the present invention includes a P2P connection unit 31, a web server unit 32, a web server driving unit 33, and a web site backup unit 34. When the web server client PC 3 gains access to the central server 4 through the P2P connection unit 31, the web server client PC 3 can share data with other client PCs present on a P2P network implemented with the central server 4. The web server unit 32 includes a web site and provides the web service to requesting client PCs 2 that have accessed the web site. When the web server client PC 3 accesses the central server 4 through the P2P connection unit 31, the web server driving unit 33 automatically drives the web server unit 32 to switch the web server client PC 3 to a state in which it can provide the web service. The web server driving unit 33 responds, in real time, to an inquiry from the central server 4 as to whether or not the web server client PC 3 can provide the web service, and transmits a network access path of the web site of the web server unit 32 to the central server 4. Also when the IP address (for example, a dynamic IP address) of the web server client PC 3 is changed, the changed IP address is reported, in real time, to the central server 4. The web server client PC 3 can provide the web service when the web server client PC 3 remains connected to the central server 4 through the P2P connection unit 31 and the web server unit 32 is running. In this state, the web server client PC 3 itself can process a web site access request received from the requesting client PC 2 and provide the web site to the requesting client PC 2. However, the web server client PC3 cannot provide the web service when the web server client PC 3 is not connected to the central server 4, for example, due to network failure or when the web server unit 32 is not running although the web server client PC 3 remains connected to the central server 4. In this case, the central server 4 substitutes for the web server client PC 3 and provides the web service to the requesting client PC 2. The web site backup unit 34 periodically backs up web site data present in the web server 32 through the P2P connection unit 31 according to a set condition. Thus, the original web site data is stored in the web server client PC 3 and the backup web site data is dually stored in the central server 4. Fig. 5 illustrates the configuration of a central server according to an embodiment of the present invention. The central server 4 according to the present invention includes a P2P connection processor 41, a URL converter 42, a URL address DB 43, a server buffer 44, a web site backup DB 48, and a dual web server unit 46. The P2P connection processor 41 receives connection requests from client PCs and allows them to join the P2P network service through login authentication. First, the URL converter 42 stores a domain name set by the web server client PC 3 and an actual address/path corresponding to the domain name in the URL address DB 43. Upon receiving the domain name from the requesting client PC 2, the URL converter 42 obtains actual network connection information corresponding to the domain name with reference to the URL address DB 43, and accesses the web server client PC 3 corresponding to the network access information and checks whether or not the web server client PC 3 can provide a web service. When the checked result is that the web server client PC 3 can provide the web service, the central server 4 transfers the network access information of the web server client PC 3 to the requesting client PC 2 to allow the requesting client PC 2 to access the web server client PC 3. Alternatively, the central server 4 transmits its network access information to the requesting client PC 2, and provides frames to the requesting client PC 2 when it accesses the central server 4 and then allows the requesting client PC 2 to receive the web service from the web server client PC 3 through contents of the frames. When the web server client PC 3 is inoperable, the web server client PC 3 cannot provide the web server, so that network access information of the backup web site buffered in the central server 4 is transmitted to the requesting client PC 2. The requesting client PC 2 determines that the received network access information is network access information of the server for providing the web site, and requests access to the backup web site of the central server 4. The central server 4 determines that the web server client PC 3 cannot provide the web service when the web server unit of the web server client PC 3 is inoperable or when the web server client PC 3 is powered off or is not accessible due to network failure. The dual web server unit 46 of the central server 4 receives the web site request of the requesting client PC 2, and provides the web service by providing the backup web site of the web server client PC 3, already buffered in the central server 4. Web site data for backup, uploaded by the web site backup unit 34 of the web server client PC 3, is transmitted to the central server 4, and the server buffer of the central server 4 updates backup web site data in the web site backup DB 45 with that received from the web server client PC 3. The dual web service system 1 according to the present invention has been described with reference to the procedure in which the requesting client PC 2 accesses the web site of the web server client PC 3 or the backup web site of the central server 4. However, the dual web service system 1 according to the present invention can also be implemented in a different manner in which the requesting client PC 2 can automatically receive the web service in real time through the web site of the central server 4 when the web server client PC 3 abruptly becomes unable to provide the web service while the requesting client PC 2 is receiving the web service through the web server client PC 3. The real-time dual web service through the change of the server for providing the web site is provided in such a manner that the central server 4 receives the domain name from the requesting client PC 2, checks whether or not the web server client PC 3 can provide the web service, and transmits network access information of the web server client PC 3 or the central server 4 to the requesting client PC 2 according to the checked result. Alternatively, the real-time dual web service can be provided in the following manner. Regardless of the transmission of the domain name from the requesting client PC 2, the central server 4 checks, in real time, whether or not the web server client PC 3 can provide the web service, and converts the network access information to an address of the backup web site data buffered in the central server, so as to provide, in real time, network access information, which is always ready to provide the web service, to the requesting client PC 2. Fig. 6 is a flow chart illustrating how the central server operates in a method for providing a dual web service according to an embodiment of the present invention. At step SlO, which is a process of registering a domain name and network access information, the central server 4 receives and stores actual network access information of the web site and a domain name set by the web server client PC 3. The central server 4 acts as a domain name server. At step S20, which is a process of buffering web site data, the central server 4 periodically receives web site data from the web server client PC 3 so that the central server 4 provides a substitute web service through the backup web site buffered in the central server 4 when the web server client PC 3 cannot provide the web service. At step S30, which is a process of receiving a domain name, the central server 4 receives the domain name of the web server client PC 3 from the requesting client PC 2. At step S40, which is a process of transmitting network access information, the central server 4 accesses the web server client PC 3 and checks whether or not the web server client PC 3 can provide the web service. The central server 4 transmits network access information of the web server client PC 3 to the requesting client PC 2 if the result of the check is that the web server client PC 3 can provide the web service, and transmits network access information of the central server 4 to the requesting client PC 2 if the web server client PC 3 cannot provide the web service. In the case where the central server 4 previously sets network access information by checking, in real time, whether or not the web server client PC 3 can provide the web service, the central server 4 immediately responds with suitable network access information corresponding to the domain name transmitted from the requesting client PC 2. Fig. 7 is a flow chart illustrating how the central server backs up web site data according to an embodiment of the present invention. The user can arbitrarily set the time to back up web site data, for example, as in one of the following steps. At step S21, which is a process of backing up web site data during update, the web server client PC 3 automatically uploads web site data to the central server 4 so that the web site data is backed up on the central server 4 when the user of the web server client PC 3 changes the web site data. At step S22, which is a process of backing up web site data during login, the web server client PC 3 automatically uploads web site data to the central server 4 when the web server client PC 3 accesses the central server 4 and is subjected to a login authentication process. At step S23, which is a process of backing up web site data at a fixed time, the web server client PC 3 automatically uploads web site data to the central server 4 at a specific time set by the user everyday. It is also desirable that the three steps S21 to S23 be used in conjunction to maintain match of the web site data. Fig. 8 is a flow chart illustrating how the central server transmits network access information according to an embodiment of the present invention. At step S41 , which is a process of searching for network access information in the central server 4, the central server 4 searches for and finds network access information corresponding to a received domain name. At step S42, which is a process of checking whether or not the web server client PC 3 can provide the web service, the central server 4 accesses the web server client PC 3 based on the network access information found at step S41, and checks whether or not the web service can be provided through the web site of the web server client PC 3. If the central server 4 can access the web server client PC 3 over the network and the web server unit of the web server client PC 3 is running, the central server 4 determines that the web server client PC 3 can provide the web service, otherwise the central server 4 determines that the web server client PC 3 cannot provide the web service. If the web server client PC 3 can provide the web service, the central server 4 transmits network access information of the web server client PC 3 to the requesting client PC 2 at step S43. The requesting client PC 2 receives the web service through the web site of the web server client PC 3 based on the received network access information. If the web server client PC 3 cannot provide the web service, the central server 4 transmits network access information of a backup web site of the web server client PC, buffered in the central server 4, to the requesting client PC 2 at step S44. Upon receiving a web site access request from the requesting client PC 2, the central server 4 provides the web service through the buffered backup web site at step S45 which is a process of providing the web service. Fig. 9 is a flow chart illustrating how the central server transmits network access information and provides frames according to an embodiment of the present invention. If the web server client PC 3 can provide the web service but the web server client PC 3 cannot be accessed properly with its IP address alone at step S43 which is the process of transmitting network access information,, the central server 4 transmits frames as follows, thereby making it possible to refer to proper data. At step S431 , which is a process of transmitting an address/path of the central server as network access information, the central server 4 transmits its network access information to the requesting client PC 2 in response to the received domain name. The requesting client PC 2 accesses the central server 4 based on the received network access information. At step 432, which is a process of providing frames, the central server 4 provides empty frames containing information of the contents of the web site of the web server client PC 3 to the requesting client PC 2. Web site data is received from the web server client PC 3 to display the contents in the frames. Accordingly, even if it is difficult to determine a proper address/path of the web site of the web server client PC 3 with its IP address alone, for example, even if the web server client PC 3 is assigned the same IP address as other client PCs through a router, the frame transmission makes it possible for the information in the frames to refer to the web site data from the web server client PC 3. The present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. INDUSTRIAL APPLICABILITY In a system and method for providing a dual web service in a central server in a P2P environment using a central server, the central server provides a service to change a domain name to actual network access information for client PCs, so that a specific one of the client PCs serves as a web server having a domain name. If the web server client PC is inoperable, the central server transmits network access information of the central server, as network access information corresponding to the domain name, to the requesting client PC, and provides the web service through a backup web site from the web server client PC, buffered in the central server, thereby achieving a continuous web service through a dual web service operation of the central server responsible for the web server function.