Abstract:
Disclosed is a system and method for load balancing on a computer network that utilizes two levels of addressing abstraction—logical and physical. Logical processes are mapped to physical processes using a logical interface and may be done in a one-to-one, one-to-many, or many-to-one fashion. The mapping is dynamic in the sense that mapping decisions may include selection functions that can be changed on the fly so that servers can be added or removed in a manner that is relatively transparent to the client. The system and method are also applied to the World Wide Web so that web sites can also dynamically distribute processes over a plurality of servers.

Description:
RELATED APPLICATION 
     This application is related to a separately filed U.S. patent application Ser. No. 08/772,705, entitled “System and Method for Locating Resources in Distributed Network” filed Dec. 23, 1996, now U.S. Pat. No. 6,058,423. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to computer networks, and more particularly, the present invention relates to a system and method for performing load balancing on a computer network. 
     BACKGROUND OF THE INVENTION 
     In today&#39;s highly networked computer environments, distributed computing is critical because it allows the resources of relatively small computers to be combined to provide a more powerful overall system. Distributed computing allows data and programs to be spread out, or distributed, across networks of heterogeneous computers, in a manner that is transparent to the user. Thus, data that is too large to fit on a single computer may be broken into smaller pieces and stored on several computers, or processes that service a high volume of users may be “duplicated” on many computers to reduce traffic on a single machine. 
     Most networks utilize a client-server model. Generally speaking, a client is a computer system or process that requests a service of another computer system or process (i.e., a server). A server is the software or system responsible for making local documents, files or processes available to clients. For example, a workstation requesting the contents of a file from a file server is a client of that file server. There are numerous network configurations, such as local area networks (LANs), which may include small intranets, and wide area networks (WANs), which may include the Internet or large intranets. Communicating and data transferring amongst clients and servers within a network is regulated by various protocols. For example, TCP/IP, which stands for Transmission Control Protocol over Internet Protocol, provides the “connectivity” rules for systems on the Internet. HTTP, which stands for HyperText Transmission Protocol, is a data transfer protocol used by the World Wide Web (WWW) that sits on top of TCP/IP. 
     In distributed computing, clients traditionally identify servers by name, using a “name server” to map between the name and a currently active communications address for the server process. For example, the Internet utilizes the domain name system (DNS) to map from a domain name for a machine (e.g., abc.xyz.com) to an Internet Protocol (IP) address. Unfortunately, the name server approach suffers from various weaknesses that exist both on the Internet and on smaller networks. First, the mapping between the name by which the client knows the server and the process that implements the server is typically one-to-one. That is, only one server can service the client when a server name is requested by the client. Second, the communication address of the server cannot be changed without changing at least some of the clients. Thus, if a server is overloaded, it is not straightforward to divide its responsibility among two machines without modifying some of the clients. Third, the client must pay the overhead of contacting the name server each time the client executes. That is, it cannot directly code the address of the server. Fourth, under many networking systems, the physical address of a server process can change each time a server starts to execute (e.g., after recovering from a machine failure), invalidating any physical addresses previously obtained by a client for a naming server. 
     While there have been numerous attempts made to provide improved load balancing on computer networks, none address all of the above-mentioned problems. For example, distributed computing, or load balancing, on the Internet is typically done at the name server level by mapping a single machine name to multiple IP addresses. Each time a domain name is looked up by the name server (DNS), a different IP address can be returned in a round-robin fashion, thereby allowing a limited form of load balancing among a set of functionally identical machines all having the same DNS name. This system also supports coalescing of multiple machine names to have a single IP address. DNS is limited in that it supports load balancing only in a round-robin fashion and requires the servers to be homogeneous. In particular, it does not allow partitioning of data (e.g., a set of very large files containing movies among a set of servers). Moreover, because the load balancing is done at the level of requests to the name server, and given the heavy use of caching in DNS (i.e., clients “remember” or store IP addresses if recently used to avoid future look-ups), the possibility of unbalanced loads is substantial. 
     Another known method of performing load balancing on computer networks involves the use of TCP/IP routers which allow a front end node or gateway to direct communications to one or more homogeneous back end nodes. Thus, all communications with the cluster are addressed to a single computer, and all communications with the back end nodes are transparent to the user. U.S. Pat. No. 5,371,852, issued to Attanasio et al. on Dec. 6, 1994 and assigned to IBM Corp. discloses such a system and method. In particular, the system includes a message switch for changing the information in the message header based upon a specific routing function that is selected using port and protocol information in port type messages. Unfortunately, this system is limited because, among other things, the nodes within the cluster must be homogeneous and located proximate to each other. 
     Load balancing is of critical importance on the World Wide Web (hereinafter, the “Web”), where explosive growth has, and will continue to occur. The Web is a distributed information system comprising a network of computers located throughout the world that communicate using the Internet. The Web allows users to create, browse or edit hypertext documents in massive client-server environment. Two primary components of the Web are Web Browsers (i.e., the software, associated with the client, designed to browse the Web), and Web Pages (i.e., the documents or processes, associated with the server, available via the Web Browsers). Each Web Page has a specific Web address known as a Uniform Resource Locator (URL). URL&#39;s provide not only the location of a file in a directory on a particular machine, but can also point to some other type of service and then further determine how the file/service is to be served. However, because URL&#39;s include a domain name, the Web is faced with the same limitations as those presented above. Namely, a name server (i.e., DNS) must be utilized, thereby limiting the ability to perform any meaningful load balancing. 
     Another area where improved load balancing is required is on intranet systems, which are typically used within large organizations to provide features similar to those found on the Internet. Such systems allow for efficient and varied inter-organizational communications, but are subject to the same limitations as those mentioned above. 
     Thus, without a better way of load balancing, performance of computer networks will be impaired. All of the above references are hereby incorporated by reference. 
     SUMMARY OF THE INVENTION 
     The present invention provides load balancing in a distributed network system by providing server processes that include two levels of abstraction, logical and physical. Physical processes, by assumption, have addresses supported by the transport mechanism while logical processes do not. Each logical process is implemented by a set of physical processes and the system maintains the mapping between the two. The system also provides interfaces that enable a client to send a message to a logical process, automatically redirecting the message to an appropriate physical process. 
     The mapping from logical to physical processes can be one-to-one, many-to-one, or one-to-many. If highly available processes are desired, each physical process can be replicated. The mapping between logical and physical processes is dynamic and may be changed by the administrator. By defining two layers, logical and physical processes, two seemingly conflicting goals can be achieved simultaneously. First, a low-level, compact address may be used in a persistent fashion for each logical process. Thus, a bulky inefficient to use, character string based name is not required. However, the flexibility of dynamic configuration greater than that presently provided by name servers can be achieved. Since logical processes do not consume resources (beyond their logical address), they can exist forever without preventing dynamic reconfiguration of the system. At any point in time, the logical server may be implemented by different numbers of physical processes. If a particular server becomes a bottle neck, (i.e., there are many requests to the server), the administrator may decide to partition the server&#39;s work across several physical processes. Alternatively, if there are few requests to the server, it can be coalesced with other servers which can be jointly implemented by a single physical process. 
     To manage the mapping between logical and physical servers, a sub-set of the machines are implemented as mapping servers. These can be the same machines running name servers or some other sub-set of machines. The mapping servers will be contacted, in a manner transparent to the client, to map from logical to physical addresses when a client attempts to send a message to a server using a logical address. To improve performance, clients can cache this mapping. Moreover, the cache can be managed either with an active or lazy cache invalidation policy. If it is managed with a lazy cache invalidation policy, clients will need the ability to contact the mapping server if a mapping obtained from the cache turns out to be invalid. 
     The mapping servers and client-side cache will be hidden from the application. The system will provide the application a set of interfaces that support sending a message to the address of a logical process and the implementation of these interfaces will be responsible for redirecting the message to an appropriate physical process. 
     When a logical process is created, it is assigned a universally unique, persistent identifier. There are several options for assigning universally unique identifiers, including using a central authority or delegating to each mapping server responsibility for assigning unique identifiers to other servers by allocating to each mapping server a range of identifiers. 
     As described above, the mapping from logical to physical processes can be one-to-one, many-to-one or one-to-many. This mapping is dynamically maintained by the mapping servers with each mapping server typically knowing the entire mapping. The method and system described herein allow a single logical process to be implemented by multiple physical processes. In order to achieve this, the system must first determine to which physical process to send a message (e.g., a random choice may often suffice if all of the physical processes are identical). In certain cases however, it may be desired not only to partition the work load assigned to a physical process but also the data stored by the server. For example, on the Web, where several servers may be available to service a unique client request, hashing based upon a cookie (i.e., a piece of information exchanged between the server and the client on each request) could be used to partition data or share functions among URL&#39;s. 
     One method of partitioning among the physical processes implementing a given logical server is to assign each logical process a class (typically this would be done by the system&#39;s administrator). Then, a selection function associated with each class may be used for mapping a message that is sent to a logical server process to some integral value. When the mapping between logical and physical processes is set up, a range of integral values will be associated with each physical process implementing a given logical process. The entire range of values that can be returned by the selection function for the given class must be covered. Then, when a mapping process needs to determine the address of a physical process for implementing the logical process, it will apply the selection function to the message to be sent, and choose the physical process whose associated range includes the result of applying the selection function. These systems and methods are described in more detail under the detailed description of the preferred embodiments section. 
     In accordance with the above, it is an advantage of the present invention to provide two levels of abstraction in the server process, logical and physical. 
     In accordance with the above, it is a further advantage of the present invention to allow clients the ability to perform load balancing via a selection function stored within their cache or on a mapping server. 
     In accordance with the above, it is a further advantage of the present invention to provide a means of accessing data or resources that have been partitioned among a plurality of heterogeneous computers. 
     In accordance with the above, it is a further advantage of the present invention to provide the ability to dynamically and transparently repartition the work load of a server among a variable number of processes and machines. 
     In accordance with the above, it is a further advantage of the present invention to allow clients the ability to use persistent addresses for processes that include efficient, compact, numerical addresses. 
     In accordance with the above, it is a further advantage of the present invention to provide a system in which the logical address of a server process does not change even if the server is moved to execute on a different computer. 
     In accordance with the above, it is a further advantage of the present invention to provide a method and system for load balancing on the World Wide Web. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a block diagram of a typical client server system in which a name server is utilized. 
     FIG. 2 is a block diagram that depicts a logical interface to perform load balancing in accordance with a preferred embodiment of the present invention. 
     FIG. 3 depicts a block diagram of a system in which a plurality of logical interfaces are used to perform load balancing in accordance with a preferred embodiment of the present invention. 
     FIG. 4 is a block diagram that depicts a system in which the mapping function is downloaded into the client cache in accordance with a preferred embodiment of the present invention. 
     FIG. 5 depicts a mapping table for mapping from logical to physical processes in accordance with a preferred embodiment of the present invention. 
     FIG. 6 depicts a modified mapping table for mapping from logical to physical processes in accordance with a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, a typical client server system is shown  10 . In this traditional system, there exists a plurality of clients  12 , a plurality of servers  20 , and a name server  14 . When a client needs to interact with a particular server process, the client must first contact the name server  14  with a server name  16 . The name server  14  then returns to the client the server address  18 . For example, in the case of the Internet, a client must give the name server (i.e., DNS) a domain name and the name server then returns an IP address to the client. The IP address can then be used to contact a specific server. 
     Under this traditional system, if any load balancing is to be done, it is typically done at the name server  14  level. Thus, if servers  1 - 3  are each configured to include the same physical process desired by the client, the name server may be used to direct the client to server  1 , server  2  or server  3  to execute the desired process. However, when a systems administrator decides to add or remove servers to improve load balancing, the name server must be modified, along with some of the clients that may have invalid server addresses in their cache. Additionally, because caching of addresses is commonplace in such systems, a small number of highly “active” clients may dominate and overload a single server, while letting other servers go under utilized. 
     Referring now to FIG. 2, a block diagram is depicted showing a preferred embodiment of this invention. Here, the name server may be eliminated and the client may interact directly with a logical processing system  33 . The logical process system may be implemented in software and reside on any recordable media on some computer in the network. Here, the client sends a logical process or request  26  to a logical interface  24 . This process is in form of an actual address rather than a string-based name. The logical interface  24  then uses a mapping server that includes a selection function  34  to select a physical process  28 ,  30  or  32  associated with server most suited for handling the requested process. The selection function may be programmable so that the systems administrator can tailor how to load balance. In this case, if servers  1 - 3  each include the ability to handle a requested process, the selection function would dynamically choose an appropriate server. Note that the process on each server would include its own unique physical address  28 ,  30  or  32  that is transparent to the client. The client need only know the logical address of the logical process  26  to have its process serviced. Thus, pursuant to this embodiment, the load balancing is actually done at the client level (particularly where the selection function is stored in the client&#39;s cache) rather than the server level, as with previous methods. This system and method provides increased flexibility in that adding servers or replicating physical processes can all be done within the logical interface system  34  totally transparent to the client  22 . 
     Referring now to FIG. 3, the method and system of the present invention are extended to a more complex network such as the Internet or World Wide Web. On the Web, there exist thousands of Web sites (i.e., servers) each with their own unique address. The ability to load balance at a particular Web site in the past has been limited to what the name server (DNS) could provide. Pursuant to this embodiment, clients  36  are in communication with a plurality of logical server processes or Web sites  38  and  40 . Each logical server process includes its own logical interface system  42 ,  44  that is accessed by a unique logical address. Moreover, each logical interface system would include its own selection function  45 ,  47  that would dynamically map logical processes to physical processes. The selection function may be programmable, meaning that the system&#39;s administrator or web master may tailor the functionality in any fashion. Thus, any Web site that needs to distribute processes among more than one server would use its own selection function to balance the load among servers. For example, if a client  36  wanted to access data on a server owned by logical server process  38  (e.g., SERVER IC), it would not need to know the exact physical address of the server but rather would only need to know the logical address of the logical server process  38 . The logical server interface  42  would then map the logical process address to a physical address. Here again, load balancing would be done at the client level (either by the mapping server or the client cache) rather than at the server level, thereby allowing the ability to dynamically distribute work among several servers. 
     FIG. 4 depicts a block diagram showing how a client&#39;s cache  48  may be utilized to further enhance, rather than interfere with, the efficacy of this system. When the client  49  selects a logical process, as discussed above, a mapping server  56 , which is part of the logical interface system, may be utilized to download the selection or mapping function  50  into the client&#39;s cache  48 . In the case of a relatively small network, this would only be required as often as the selection was modified. In the case of the Web, the selection function could be downloaded each time a new Web sited is contacted (e.g., via a Java applet). Then, the client could dynamically map the logical process  52  to a physical process  54  to increase overall performance by now directly addressing the desired physical process in a manner totally transparent to an end user utilizing a Web Browser. 
     It is of critical importance to note that the selection function  50  allows for dynamic mapping between logical and physical processes. That is, the systems administrator for a network or for a Web site can use any type of mapping or heuristic process that will best allocate the load among the network server. Moreover, if the selection function is downloaded into a client&#39;s cache, it need not merely contain a physical address of a particular server, but rather, may contain any means for dynamically choosing a server address. Thus, a single client with multiple users sharing a single cache (e.g., a large corporation with many users) will not be locked into a single address for a given process because that address is stored in the cache. Instead, the selection function will decide which physical process to use each time a logical process is requested. 
     In addition, this system and method can be adapted to perform any type of load balancing and is not just limited to the case where functionally equivalent processes are “duplicated” among more than one server. Other examples include distributed shared memory, where a block of memory is distributed among several servers, straight forward file servers where there exists a one-to-one correspondence between logical and physical processes, and the web where requests may be partitioned among a set of servers increasing the likelihood that the requested resource is cached in memory. It is understood that other types or classes of logical processes also fall within the scope of this embodiment. Some of these “classes” of logical processes are discussed in further detail below with regard to FIGS. 5 and 6. 
     FIGS. 5 and 6 depict mapping tables for handling different classes of logical processes. Shown in these two tables are four logical server processes characterized into three different classes: DSM-LLSP, FS, and NS. These are examples of possible classes, however, it is recognized that any number of additional classes may be defined and utilized. DSM-LLSP is a distributed shared memory, logical location server process. It returns information on the location of a page of distributed shared memory given an eight byte address for the page of memory. In this example, it is assumed that the work load on the logical server  1  is too great to be implemented by a single physical process and therefore the responsibility of the logical location server process is sub-partitioned. Each physical process that implements a logical location server process is responsible for the sub-range of the address space. The selection function will extract the address in question from the message, and use this address to select the physical process in question. Thus, if a message were sent to logical server process  1 , asking about page 0XFFF065490000000, the selection function would lead the message being sent to the physical process whose address page is shown as p3@host3. 
     FS, which is the class of the second and third logical process, is a file server. In this case, two logical processes have been coalesced to be implemented by a single physical process, p5@host5. Since there is no choice to be made, the selection function will select the single physical process when mapping from logical to physical process in this case. The final class of logical process shown in this example is NS, that is a name server. In this case, it is assumed that the work load of the server process requires partitioning and that any of such physical processes are capable of answering any question (e.g., all three physical processes have complete access to the name server data base). In this case, the selection function may implement a heuristic that ignores its input and performs a fair toss of a three-sided coin (i.e., randomly choosing one of the physical processes). It is noted that any type of heuristic algorithm could also be used. 
     The mapping from logical to physical processes, and thus the information in the mapping table, can change dynamically. As stated above, this mapping from logical to physical processes is duplicated at each mapping process. To maintain the mapping, any known system may be used including a variant of the TreeCast algorithm (see A. Teperman, M. Bach, Y. Moatti and D. Allon, “A Scalable Load Balancing Algorithm for Large Networks of Computers” IBM Isreal Science and Technology, Technology report TR88.313, December 1991 (incorporated herein by reference)) which is a fault-tolerant scalable algorithm for distributing information over large networks. Generally, the algorithm is used to dynamically and fault-tolerantly organize the physical processes by implementing the mapping processes into a spanning tree. Once organized into a spanning tree, information can be “tree cast” to all of the processes on the tree. The organization into a tree is dynamic in the sense that processes can join and leave the tree as the system is running. If a non-leaf node leaves the tree, the tree is automatically reconfigured. In addition, trees can be merged and split. 
     FIG. 5 depicts a second mapping table that shows how the logical process mechanism can be used to dynamically reconfigure server processes. This figure shows the mapping table after the work for logical process  1 , of class DSM-LLSP, is redistributed among four physical processes (as opposed to two shown in FIG.  5 ). A client of a logical server need not be aware of this reconfiguration. It will continue to address its messages to logical process  1 . However, the selection function will now automatically redirect this message to one of four physical processes as opposed to one of two. 
     The embodiments and examples set forth herein were presented in order to best explain the present invention and its practical application and to thereby enable those skilled in the art to make and use the invention. However, those skilled in the art will recognize that the foregoing description and examples have been presented for the purposes of illustration and example only. The description as set forth is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in the light of the above teaching without departing from the spirit and scope of the following claims.