Abstract:
A system and method for identifying network endpoints that provide a service of interest orders endpoints according to their network proximity to a requesting client. The requesting client may then contact the closest available providing endpoint, thus increasing the efficiency of usage of network resources while decreasing latency and enhancing reliability. In an embodiment of the invention, when a connection to the nearest providing endpoint cannot be established, a mechanism for locating a next-closest alternative endpoint is provided.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to network service discovery and, more particularly, relates to a system and method for network service discovery using end-point network locality. 
     BACKGROUND 
     Computer networks have become commonplace and are used to create value and increase productivity and customer satisfaction in many business enterprises, and are also productively used in many private settings. The increase in network usage has led to an abundance of remote storage mechanisms. That is, data and/or services needed at a particular computer or site may be stored at and retrieved from a remote location. For example, a domain name server acts as a remote storage location of IP addresses for a client machine. Generally, servers, directories, databases and other repositories provide remote storage facilities that may be accessed over a network connection for any number of reasons. 
     Typically, data and services that may be widely and frequently accessed by different machines in a dispersed network are replicated over the network. That is, for example, a service is identically duplicated to a widely distributed plurality of repositories accessible to clients via the network. A few of the primary benefits provided by data replication are decreased response time for most requests, decreased network utilization per request on average, generally increased capacity and generally increased reliability. 
     With respect to decreasing response time, replication increases the probability that a client machine will be able to access a nearby repository, and thus, especially when large quantities of data are involved, the response time to transmit the data from the repository to the requesting client may be reduced. Similarly, with respect to decreased network utilization, if clients resort to nearby repositories for the information they need, then the sum of network traffic relative to all replicas is less than would be the case if a single repository were utilized instead of replicas. The aforementioned general increase in capacity due to replication comes about because of the ability of multiple replicas to serve multiple clients at the same time without impacting each other. Finally, increased reliability is generally realized in a replicated system due to the ability of each site or replica to serve clients whose preferred replica or site is not functioning. 
     Replication does, however, bring with it certain costs above and beyond the cost of replication itself. For example, a client machine is not inherently aware of which replica or site it should use to access the required data or service (hereinafter collectively referred to as “service”). Typically, in order to reap the benefits of decreased response time and decreased network utilization, client machines need to access the closest available replica. Usually this requires management or administrative personnel to manually configure each client. Not only does this increase management costs and the number of opportunities for error, but in addition, companies and institutions often are reluctant to deploy new replicas because such could require a manual reconfiguration of some or all of the relevant clients. 
     BRIEF SUMMARY OF THE INVENTION 
     In various embodiments, the invention provides a mechanism for assuring that a requesting client in a distributed network is automatically informed of the closest available endpoint for accessing a particular service. In particular, according to an embodiment of the invention a nearest endpoint locator first determines the network location of the requesting client. Subsequently, the nearest endpoint locator identifies network endpoints for accessing the service requested by the client. For each such endpoint, the nearest endpoint locator determines the network location of the endpoint, and then consults an endpoint repository to determine the network proximity of the endpoint to the requesting client. 
     After determining how close each endpoint is to the requesting client, the nearest endpoint locator sorts all or some of the endpoints in order of their proximity to the requesting client. After the sorted list is returned to the client, the client is then able to contact the nearest endpoint currently available to access the requested service. When a listed endpoint is unavailable, the client is able to identify the next closest endpoint by simply choosing the next endpoint in the ordered listing. 
     Additional features and advantages of the invention will be made apparent from the following detailed description of illustrative embodiments which proceeds with reference to the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the appended claims set forth the features of the present invention with particularity, the invention, together with its objects and advantages, may be best understood from the following detailed description taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a block diagram generally illustrating an exemplary device architecture in which embodiments of the present invention may be implemented; 
         FIG. 2  is a schematic diagram of an operating environment within which an embodiment of the invention may be implemented; 
         FIG. 3  is a schematic diagram of the nearest endpoint locator and its environment and interfaces according to an embodiment of the invention; 
         FIG. 4A  is a schematic diagram of the nearest endpoint locator and its environment and interfaces according to a further embodiment of the invention; 
         FIG. 4B  is a schematic diagram of the nearest endpoint locator and its environment and interfaces according to yet another embodiment of the invention; 
         FIG. 5A  is a flow chart illustrating steps taken in accordance with an embodiment of the invention to facilitate service location based on network proximity of a providing endpoint to a requesting client; 
         FIG. 5B  is a flow chart illustrating further steps taken in accordance with an embodiment of the invention to facilitate service location based on network proximity of a providing endpoint to a requesting client; 
         FIG. 6A  is a schematic illustration of a data structure in accordance with an embodiment of the invention listing endpoints that provide a service of interest; and 
         FIG. 6B  is a schematic illustration of a data structure in accordance with an embodiment of the invention listing endpoints that provide a service of interest in order of their network proximity to a requesting client. 
     
    
    
     DETAILED DESCRIPTION 
     Turning to the drawings, wherein like reference numerals refer to like elements, the invention is illustrated as being implemented in a suitable computing environment. Although not required, the invention will be described in the general context of computer-executable instructions, such as program modules, being executed by a computer. Generally, program modules include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including hand-held devices, multi-processor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. The invention may be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
     This description begins with a description of a general-purpose computing device that may be used in an exemplary system for implementing the invention, after which the invention will be described in greater detail with reference to  FIG. 2  and subsequent Figures. Turning now to  FIG. 1 , a general purpose computing device is shown in the form of a conventional computer  20 , including a processing unit  21 , a system memory  22 , and a system bus  23  that couples various system components including the system memory to the processing unit  21 . The system bus  23  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory includes read only memory (ROM)  24  and random access memory (RAM)  25 . A basic input/output system (BIOS)  26 , containing the basic routines that help to transfer information between elements within the computer  20 , such as during start-up, is stored in ROM  24 . The computer  20  further includes a hard disk drive  27  for reading from and writing to a hard disk  60 , a magnetic disk drive  28  for reading from or writing to a removable magnetic disk  29 , and an optical disk drive  30  for reading from or writing to a removable optical disk  31  such as a CD ROM or other optical media. 
     The hard disk drive  27 , magnetic disk drive  28 , and optical disk drive  30  are connected to the system bus  23  by a hard disk drive interface  32 , a magnetic disk drive interface  33 , and an optical disk drive interface  34 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer readable instructions, data structures, program modules and other data for the computer  20 . Although the exemplary environment described herein employs a hard disk  60 , a removable magnetic disk  29 , and a removable optical disk  31 , it will be appreciated by those skilled in the art that other types of computer readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, random access memories, read only memories, storage area networks, and the like may also be used in the exemplary operating environment. 
     A number of program modules may be stored on the hard disk  60 , magnetic disk  29 , optical disk  31 , ROM  24  or RAM  25 , including an operating system  35 , one or more applications programs  36 , other program modules  37 , and program data  38 . A user may enter commands and information into the computer  20  through input devices such as a keyboard  40  and a pointing device  42 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  21  through a serial port interface  46  that is coupled to the system bus, but may be connected by other interfaces, such as a parallel port, game port or a universal serial bus (USB) or a network interface card. A monitor  47  or other type of display device is also connected to the system bus  23  via an interface, such as a video adapter  48 . In addition to the monitor, computers typically include other peripheral output devices, not shown, such as speakers and printers. 
     The computer  20  preferably operates in a networked environment using logical connections to one or more remote computers, such as a remote computer  49 . The remote computer  49  may be another personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computer  20 , although only a memory storage device  50  has been illustrated in  FIG. 1 . Several examples of network types and remote computer types will be discussed additionally with reference to  FIG. 2  below. The logical connections depicted in  FIG. 1  include a local area network (LAN)  51  and a wide area network (WAN)  52 . Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet. 
     When used in a LAN networking environment, the computer  20  is connected to the local network  51  through a network interface or adapter  53 . When used in a WAN networking environment, the computer  20  typically includes a modem  54  or other means for establishing communications over the WAN  52 . The modem  54 , which may be internal or external, is connected to the system bus  23  via the serial port interface  46 . Program modules depicted relative to the computer  20 , or portions thereof, may be stored in the remote memory storage device if such is present. It will be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used. 
     In the description that follows, the invention will be described with reference to acts and symbolic representations of operations that are performed by one or more computers, unless indicated otherwise. As such, it will be understood that such acts and operations, which are at times referred to as being computer-executed, include the manipulation by the processing unit of the computer of electrical signals representing data in a structured form. This manipulation transforms the data or maintains it at locations in the memory system of the computer, which reconfigures or otherwise alters the operation of the computer in a manner well understood by those skilled in the art. The data structures where data is maintained are physical locations of the memory that have particular properties defined by the format of the data. However, while the invention is being described in the foregoing context, it is not meant to be limiting as those of skill in the art will appreciate that many of the acts and operations described hereinafter may also be implemented in hardware. 
     Prior to a further detailed discussion of embodiments of the invention, it may be useful to more expressly define and discuss a number of terms and concepts for the convenience of the reader. The concepts and terms to be discussed include the concept of network well-connectedness, as well as the terms and phrases subnet, site, site repository, network location, network proximity, endpoint, service repository, and nearest endpoint locator. 
     Two or more computers that can intercommunicate via one or more network connections with high bandwidth, low latency, and high reliability, and where the incremental cost of communication is low are said to be “well-connected.” An example of well-connected computers is a group of two or more computers connected via a local area network. On the other hand, two computers are not well-connected as that term is used herein when the sole or fastest connection between them is a dial-up connection to the Internet. 
     In the first example, the computers can communicate at any time over the local network with high bandwidth and reliability, as well as low latency, and the incremental cost of communication is low in that there is no extensive set-up or delay entailed to communicate after a substantial period of silence. However, in the latter case the dial-up connection causes the two computers to lack well-connectedness since there is a substantial incremental cost of communication, e.g. the dial-up connection must be reestablished to initiate communications after a substantial period of silence, and in addition, a dial-up connection is typically associated with relatively low bandwidth and reliability and high latency. 
     Moving on to a discussion of other terms and phrases, the term “subnet” as used herein refers to a logical range of network addresses, in the same manner as that term is generally understood by those of skill in the art. The term “site” refers to a collection of network addresses, such as comprising a set of one or more subnets, wherein any computer with a network address within this collection is well-connected to any other computer whose network address is also in the collection. A “site repository” as that term is used herein is a repository holding information about one or subnets, and/or one or more sites, and the interconnections between one or more sites and/or one or more subnets. An exemplary technology for implementing a site repository is the Active Directory® directory service produced by Microsoft Corporation of Redmond Wash., although those of skill in the art will appreciate the applicability within embodiments of the invention of many other suitable data storage and access technologies as well. 
     The phrase “network location” as used herein refers to the extent of a network containing a particular machine, and may refer to networks at various levels of granularity including a network address, a network subnet, a network site, and so on. The phrase “network proximity” refers herein to a measure of the nearness of two computers to each other in terms of network distance. Typically, although not necessarily, the network proximity is a relative value that may be unitless, and that may have meaning only when compared to one or more other network proximities. In an embodiment of the invention, the nearness of two computers or locations to each other is measured by the incremental cost of generating network traffic between them, with a lower cost indicating closer network proximity. Network proximity is derived in an embodiment of the invention from a combination of the network addresses of the relevant computers and the information retained in the site repository, e.g. information about subnets and/or sites, and the interconnections there between. 
     The term “endpoint” is sometimes used herein to refer to an instance of a particular service. As discussed elsewhere herein, endpoints offering the same service (e.g. DNS, authentication, etc.) may be located at different sites to increase the availability of the service and to generally provide the benefits afforded by replication as discussed above. A related phrase sometimes used herein is the phrase “service repository,” which refers to a repository of information regarding services and the endpoints that offer those services. Examples of potential service repositories include LDAP directories and UDDI servers. 
     Turning to  FIG. 2 , an operating environment usable to implement an embodiment of the invention is shown schematically. The environment  201  of  FIG. 2  includes a requesting client A  203  that seeks to access a service and/or data provided by a service of interest. The service provided by the service of interest is replicated to various endpoints in the environment  201 , including for example Replica A  207 , Replica B  209 , and Replica C  205 . It will be appreciated that replicas such as Replica A  207 , Replica B  209 , and Replica C  205  typically reside at different respective physical machines in the environment  201 . Thus, the number of different network locations from which a particular service is available will typically be equal to the number of replicas. 
     In greater detail with respect to the environment  201  of  FIG. 2 , the requesting client A  203 , a requesting client B  211 , the Replica A  207 , the Replica B  209 , and the Replica C  205  are interconnected via one or more networks or network connections such as local network  213  and WAN  215 . A local network such as network  213  may be connected directly to one or more elements in the environment  201  and indirectly, such as via the WAN  215 , to one or more others. In the example environment  201  of  FIG. 2 , the local network  213  is connected to the WAN  215  via a gateway  217 . 
     The WAN may be or comprise the Internet but such is not required. Although local network  213  and WAN  215  are illustrated, it will be understood that any number of network types in addition to or instead of those shown may be used and that the number of networks or network types is not significant, with a lesser or greater number of networks and network types also being usable to implement embodiments of the invention. In addition to the elements described above, a site repository  219  is also included within environment  201 . The interaction of the site repository  219  with the other elements of the environment  201  will be described briefly hereinafter, and then in greater detail with respect to  FIGS. 5A and 5B . 
     Briefly, in the illustrated example, the requesting client B  211  can access the site repository  219  via the connection sequence of the network  213 , the gateway  217 , and the WAN  215 . Similarly, the requesting client B  211  can access the service of interest  205  and Replica A  207  via the connection sequence of the network  213 , the gateway  217 , and the WAN  215 , while Replica B  209  can be accessed directly from the requesting client B  211  via local network  213 . In contrast, in the example of  FIG. 2 , requesting client A  203  does not have direct access to any other element of the environment  201 , but can access the site repository  219 , the service of interest  205  and Replica A  207  via the WAN  215 , and can access Replica B  209  via the connection sequence of the WAN  215 , the gateway  217 , and the network  213 . Thus, it is likely that with respect to requesting client B  211 , the closest instance of the service and/or data corresponding to the service of interest  205 , e.g. the replica having the closest network proximity to requesting client B  211 , is Replica B  209 . With respect to requesting client A  203 , on the other hand, it is likely that the closest instance of the service and/or data corresponding to the service of interest  205  in terms of network proximity is either the service of interest  205  itself or Replica A  207 . It is possible that two or more instances of a service of interest are at the same network distance from a particular requesting client, as will be discussed later. 
     In overview, according to certain embodiments of the invention, a nearest endpoint locator, to be described in greater detail with respect to  FIG. 3  and  FIGS. 4A and 4B , ascertains the network location of the requesting client, for example requesting client A  203 . Once the network location of the requesting client has been determined, the nearest endpoint locator identifies all network endpoints that are capable of providing the service or data requested by the client, such as for example Replica A  207 , Replica B  209 , and the service of interest  205 . For each endpoint that is capable of providing the relevant service or data, the nearest endpoint locator determines the network location of the providing endpoint, and consults the site repository  219  to determine the network proximity of the endpoint to the requesting client. Having determined how close each providing endpoint is to the requesting client, the nearest endpoint locator sorts all or some of the providing endpoints in order of their proximity to the requesting client and transmits the sorted list to the requesting client. The requesting client is then able to connect to the nearest providing endpoint to access the service required. 
     Although the example operating environment  201  is illustrated in some detail, those of skill in the art will appreciate that the exact environment shown is not required for implementation of embodiments of the invention. Thus, although requesting client A  203  and requesting client B  211  are illustrated as ordinary computers, such as the computer  20  described above with respect to  FIG. 1 , such is not required. In particular, all or some requesting clients may be any other type of computing device capable of attaining access to a network to locate a service of interest on the network. Furthermore, the number of requesting clients may be greater or less than is shown. Moreover, the site repository  219  is illustrated as a database, but may be any other data repository capable of storing information used in embodiments of the invention as will be apparent from the teachings herein. 
       FIG. 3  and  FIGS. 4A and 4B  show the nearest endpoint locator in greater detail. As discussed above, the nearest endpoint locator is a software component that interfaces with data sources and performs calculations in order to identify the closest endpoint, with respect to network proximity, to a given requester. In an embodiment of the invention, the initial inputs taken by the nearest endpoint locator are an identity of the client and an identity of the requested service.  FIG. 3  illustrates schematically the nearest endpoint locator  301  and its logical connectivity according to an embodiment of the invention. In particular, a requesting client  303  accesses the nearest endpoint locator  301  either directly or via a server, proxy, or other intermediary or intermediaries, and conveys to the endpoint locator  301  its identity and the identity of the service of interest. The nearest endpoint locator  301  accesses the site repository  305  and the service repository  307  to determine which endpoints provide the service of interest and to determine the endpoint having the closest network proximity to the requesting client  303 . The information which the nearest endpoint locator  301  provides to the client will be discussed in greater detail with respect to  FIGS. 5 and 6 . 
     Although the foregoing example illustrates the nearest endpoint locator  301  as a separate entity, the nearest endpoint locator  301  is not restricted in location and may be located at any network entity accessible to the requesting client  303 .  FIGS. 4A and 4B  illustrate other possible placements for the nearest endpoint locator  301  in further embodiments of the invention. In particular, as shown in  FIG. 4A , the nearest endpoint locator  401  may reside as an application on the requesting client  403  itself. In this case, the nearest endpoint locator  401  accesses the site repository  405  and service repository  407  from the requesting client  403  using the network connections of the requesting client  403 . 
     An alternative architecture usable in an embodiment of the invention is illustrated schematically in  FIG. 4B . In the illustrated architecture, the nearest endpoint locator  411  is provided and exposed by the service repository  417 . In this configuration, the requesting client  413  accesses the nearest endpoint locator  411  by connecting to the service repository  417 . The nearest endpoint locator  411  thereafter uses the network connections of the service repository  417  to access the site repository  415  and uses information from both the service repository  417  and the site repository  415  to identify a nearest endpoint or endpoints as will be described in greater detail hereinafter with respect to  FIGS. 5A-5B . It will be appreciated that the architectures shown in  FIGS. 3 ,  4 A, and  4 B are merely exemplary. For example, in an alternative embodiment of the invention, the site repository  305 , the nearest endpoint locator  301 , and the service repository  307  are all located on one machine, and in a further embodiment of the invention all are also part of the same service. 
     Referring now to  FIGS. 5A-5B , an exemplary procedure for facilitating location by a requesting client of a nearest endpoint according to an embodiment of the invention is set forth. In discussing the flow charts of  FIGS. 5A-5B , the data structures of  FIGS. 6A-6B  will be referenced as well for the convenience of the reader. While the flow charts of  FIGS. 5A-5B  assume an architecture as shown in  FIG. 3 , it will be appreciated that the steps of the procedure may be easily modified by those of skill in the art, given the teachings herein, to accommodate any suitable architecture including the architectures shown in  FIGS. 4A and 4B  as well as others. 
     At step  501  of the procedure illustrated in  FIG. 5A , the requesting client  303  sends a request to the nearest endpoint locator  301  requesting an identification of endpoints providing a service of interest. As discussed above, the request preferably includes the identity of the requesting client  303  as well as the identity of the service of interest. In the request to the nearest endpoint locator  301 , the network address of the requesting client  303  is usually apparent since the nearest endpoint locator  301  would generally need to know that information in order to send any information to the client  303 . However, the requesting client  303  may be separated from the nearest endpoint locator  301  by a proxy such as an HTTP proxy server or otherwise. In this case, it is preferable that the request from the client  303  also explicitly identifies the network location of the requesting client  303 . 
     A number of mechanisms may be used to allow the client  303  to locate the appropriate nearest endpoint locator  301 . In an embodiment of the invention, the client uses a nearest endpoint locator in the site in which it resides, or in a site designated by an administrator. At step  503 , the nearest endpoint locator  301  contacts the service repository  307  with a request that identifies the service of interest and requests an enumeration of all endpoints that provide that service. In step  505 , the service repository  307  responds to the nearest endpoint locator  301  with a list enumerating the endpoints that provide the service of interest. As used herein, an endpoint is said to provide the service of interest if the endpoint has been identified as hosting the service of interest regardless of whether the endpoint has ever actually provided the service of interest to a requesting client or otherwise.  FIG. 6A  illustrates schematically a data structure  601  corresponding to an enumeration of endpoints providing the service of interest as may be provided by the service repository  307 . Each entry  603  in the enumeration identifies an endpoint. However, the endpoints are typically not in order of proximity to the requesting client  303 . Example proximities are shown in dashed outline  605  for the convenience of the reader although such are not present in the data structure  601 . 
     Subsequently at step  507 , the nearest endpoint locator  301  determines the network location of each listed endpoint. In this connection, the list  601  provided by the service repository  307  preferably identifies for each listed endpoint  603  a network location and/or a network name of the endpoint  603 . If only the network name is identified, then the nearest endpoint locator  301  preferably resolves the network name of each listed endpoint to an appropriate network location, such as via a DNS query to retrieve a network address for each such endpoint. 
     With respect to steps  501 - 507 , in an alternative embodiment of the invention the requesting client  303  enumerates the endpoints that provide the service of interest. Thus, the request of the requesting client  303  to the nearest endpoint locator would include a list of such endpoints and a network location and/or a network name of the endpoint for each listed endpoint. If the list of endpoints from the requesting client  303  does not identify the network location of each endpoint, then the nearest endpoint locator  301  resolves the network name of each listed endpoint to an appropriate network location as discussed above. In an alternative embodiment of the invention, the requesting client  303  specifies a list only of endpoints in which the client  303  is interested, the listed endpoints being a subset of all endpoints providing the service of interest. In this case, the nearest endpoint locator would only return proximity data on the listed endpoints. This embodiment is particularly useful if the client  303  has an additional filtering mechanism and has already eliminated some possible endpoints. Moreover, the nearest endpoint locator  301  can return information saying why certain endpoints were or were not considered as suitable. For example, if a client requests four endpoints, the nearest endpoint locator  301  may return three in sorted order, as well as an indication that the fourth endpoint no longer is defined in the service repository  307 . A requesting client  303  may also specify a time limit on the request to the nearest endpoint locator  301 . Certain clients may be time sensitive, and the calculation of the nearest endpoint list may take a substantial amount of time in certain degenerate circumstances. The ability to specify an upper limit on the time to complete the request allows clients with hard deadlines to know how fast they can respond. 
     At step  509 , the nearest endpoint locator  301  determines the network proximity of each endpoint to the requesting client  303 . In an embodiment of the invention, this step is accomplished by sending a request to the site repository  305  for either the desired network proximity values or information from which the network proximities can be derived. In an embodiment of the invention, the information from which the network proximities can be derived comprises an identification of subnets, sites, subnet-to-site associations, and site-to-site connections. The identification of site-to-site connections may further comprise a “cost” value. The cost of using a replica can be ascertained by summing the costs of the site-to-site connections along the least cost site path between the client and the replica. 
     Next, the nearest endpoint locator sorts the endpoints that provide the service of interest in order of network proximity to the requesting client  303  at step  511 . The list may be sorted in ascending or descending order. In the event that the respective proximities of two or more endpoints to the requesting client  303  are the same, the ordering of such endpoints may be executed either randomly or via additional criteria. For example, the endpoints having identical proximities may be ordered in the same order as they appeared in the enumeration prior to ordering, or they may be ordered using their respective IP addresses, etc. The ordering of endpoints having identical proximities is not critical, and they may be ordered in any suitable manner, including those mentioned above as well as any other. 
     In further embodiments of the invention, caching is used to speed the process of nearest endpoint identification. For example, in an embodiment of the invention the nearest endpoint locator  301  contacts endpoints and filters them out if they are not reachable or available. The resultant information may be cached to reduced response time when the nearest endpoint locator  301  is contacted by a requesting client. Other opportunities for caching are exploited as well in embodiments of the invention. For example, the location of the nearest endpoint locator could be cached by a requesting client. Moreover, the mapping of endpoints to network locations, and the mapping of network locations to proximities can be cached for higher performance as well. 
     According to further embodiments of the invention, recovery paths are provided in case one or more needed components should be unavailable. In particular, in the same manner that the requesting client  303  contacts subsequent endpoints until an available endpoint is found, other components may be duplicated as well, with duplicates being tried on a nearest first basis in an embodiment of the invention. Duplicated components include, for example, the site repository  305 , service repository  307 , nearest endpoint locator  301 , as well as any other component for which a failsafe is desired. 
     In the case of failure of a component such as the nearest endpoint locator  301 , a duplicate component may be reached in any of a number of ways. In an embodiment of the invention, the contactor, such as the requesting client  303 , elects to contact a particular replacement component. In an alternative embodiment of the invention, the replacement component selects and advertises itself as a replacement upon detecting that the primary component is unavailable. Note that the site repository  305  and service repository  307  provide services and may be replicated in the classic sense in the same manner as the service of interest is replicated. 
       FIG. 6B  illustrates schematically an exemplary sorted list  611 , having the entries  613  arranged in order of proximity to the requesting client  303 . The proximities are inherent in the data structure itself in an embodiment of the invention, and thus are not expressly listed. However, for the convenience of the reader, the proximities are listed within the dashed outline  615 . In an alternative embodiment of the invention, the ordering of endpoints is done using a proximity field that identifies the relative proximity of each endpoint to the requesting client. In the latter embodiment, the endpoint proximities are not inherent in the physical order of entries. Instead, the ordering is apparent in the express proximity field. Herein, a listing is said to be ordered if the entries are physically ordered or alternatively if a field or other data accompanies the listing from which an ordering can be ascertained. 
     At step  513 , shown in  FIG. 5B , the sorted list of endpoints providing the service of interest is provided to the requesting client  303 . The sorted list of endpoints may be transmitted to the requesting client  303  in any suitable form, but in an embodiment of the invention the list is returned in sequential pages. Those of skill in the art will be familiar with the use of pages in the context of query results. The use of pages is especially beneficial when the sorted list is very large and the probability of a requesting client using information not on the first page is remote. 
     After receiving the sorted list of endpoints, the requesting client  303  can establish a connection with the closest available endpoint. In particular, at step  515 , the requesting client  303  attempts to contact the nearest endpoint on the sorted list. If the list is in ascending order of proximity, then the first entry on the list identifies the nearest endpoint, whereas if the list is in descending order, then the last entry on the list identifies the nearest endpoint. In the event that each of a plurality of endpoints share the nearest proximity, then such endpoints will be adjacent in the list, and the step of contacting the nearest endpoint on the sorted list comprises contacting the first such endpoint on the list in the direction in which the requesting client  303  is processing the list. 
     If at step  517  it is determined that the attempt to contact the nearest endpoint on the sorted list was unsuccessful, e.g. a connection with the nearest endpoint on the sorted list could not be established, then at step  519  the requesting client  303  attempts to contact the next nearest endpoint on the sorted list. The process flows between steps  519  and  517  until a connection with an endpoint is successfully established. Once a connection to an endpoint is successfully established with respect to the first or a subsequent attempt, the process terminates at step  521 . 
     It will be appreciated that an improved system and method for identifying service instances over a network based on network proximity have been described. In view of the many possible embodiments to which the principles of this invention may be applied, it should be recognized that the embodiments described herein with respect to the drawing figures are meant to be illustrative only and should not be taken as limiting the scope of invention. For example, those of skill in the art will recognize that some elements of the illustrated embodiments shown in software may be implemented in hardware and vice versa or that the illustrated embodiments can be modified in arrangement and detail without departing from the spirit of the invention. Therefore, the invention as described herein contemplates all such embodiments as may come within the scope of the following claims and equivalents thereof.