Abstract:
An improved load balancing method, system, and computer program product handles popular object requests using a front-end cache, and hashing is applied only to the requests in the stream that were not handled by the front-end cache. A cache (e.g., a web proxy cache) is placed in front of a Level 7 switch, such that the cache services the popular requests from the cache based on the content of the request (e.g., based on the portion of an HTTP request following the domain name). The remaining requests are hashed and then routed to the back-end server. This allows the requests that make it past the cache to still be routed to the back-end server and take advantage of the efficiencies provided therefrom.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an improved system, method, and computer program product for providing load balancing. A more particular aspect is related to a system, method, and computer program product for hierarchical load balancing, wherein frequently requested objects (e.g., web objects) are handled by a front end cache and objects not in the cache are routed to back-end servers. The popular requests are serviced directly from the cache and the remaining requests are “URL hashed” to determine the destination server in a known manner. 
     2. Background of the Invention 
     The traffic on the World Wide Web (“The Web”) is increasing exponentially. The Web is used for a multitude of tasks, including information gathering, electronic commerce, communication, and news dissemination. As a result of this high traffic volume, systems have been developed to distribute Web traffic to minimize waiting time for users. 
     Many of today&#39;s web sites are hosted on server farms, where a number of servers are grouped together to serve web requests as they arrive. To avoid overloading individual servers within the farm, load balancing techniques balance the load across servers so that the best total throughput of the farm and smallest response delay for the user is achieved. Typically, a “server switch” performs request distribution for the server farms, utilizing various techniques for determining the destination server to handle the request. One such technique, called Server Load Balancing (SLB), monitors at short, periodic intervals the load of the servers in the farm and distributes incoming requests to the least loaded server. 
     Content Based Routing (CBR) takes advantage of information in the request to assist in the server selection. The term “hashing” is used throughout the present application generally to CBR and specifically to any form of routing which examines part or all of the content of a request and then routes the request based on the content. “URL hashing” is one form of hashing which exploits the “locality” of the request stream by examining the request information and sending requests to a server that has previously served this request. While this may result in sending the request to a server that is not the least-loaded server, it may require less overall work for the entire server farm. 
     The term “URL” stands for “Universal Resource Locator” and is a method for naming web objects in the Internet. Using a URL, a user of the Internet can connect to a file on any computer connected to the Internet anywhere in the world. A typical URL is a string expression conforming to the following convention: protocol://host name/folder or directory on host/name of file or document. For example, the URL “http://www.ibm.com/products” is parsed as follows. The “http” stands for the “HyperText Transport Protocol”. This directs the browser (e.g., Internet Explorer or Netscape) to use the http protocol when accessing the document. The “www.ibm.com” is the host name for the IBM main website. As is well-known, each host name is associated with an IP address via the Domain Name System (DNS), which returns an address corresponding to the host name. For example, an IP address associated with www.ibm.com is “0.1.0.7”. 
     The “/products/” means that there is a folder or subdirectory on the IBM website called “Products”. Although not shown, within that folder there may be multiple file names, and by adding one of these file names to the URL the computer inputting the URL will be directed to that file. 
     When using URL hashing, the URL is hashed to give it a unique numerical value, which values are assigned to the URL and stored in a table. Each incoming URL is hashed and sent to a particular server and the identification of that server is stored in the table with the hash value; when the hashed value of an incoming URL matches that of a stored hash value in the table, the request is sent to the same server that it was previously sent to. 
       FIG. 1  illustrates a load balancing system  100  in accordance with the prior art. A network  105  of computer work stations  110   a ,  110   b ,  110   c , and  110   d  are connected to a network connection  112  (e.g., the Internet) in a known manner. It is understood that although a network  105  of four computer work stations  110   a - 110   d  are shown in  FIG. 1 , a single computer work station connected to the Internet or many more computer work stations than the four shown in  FIG. 1  may be utilized to equal effect. 
     A URL hashing switch  114  (e.g., a hashing switch from the “ServerIron” family of switches manufactured by Foundry Networks) is coupled between the network connection  112  and a server farm  116 . In the example shown in  FIG. 1 , the server farm  116  comprises plural servers  118   a ,  118   b ,  118   c , and  118   d . In accordance with this prior art system, when a user of the computer network  105  inputs a URL into a web browser executing on, for example, work station  110   a , the URL is transmitted over the Internet in a known manner and is received by URL hashing switch  114 . In accordance with this prior art technique, URL hashing switch  114  hashes the URL and stores the URL in a table. Using the system of  FIG. 1 , the URL hashing switch  114  “decides” which server in server farm  116  will handle each incoming URL, based on its hash value. The URL hashing switch  114  may be pre-configured to direct certain hash values to certain servers, or the hash values can be assigned to servers as the requests arrive based on standard SLB techniques. 
     Some known load balancing methods involve placing a front-end processor before the server farm to distribute the load among the back-end servers by keeping track of the load of the back end servers and routing requests to the least-loaded server, while also exploiting the locality of the request stream by routing repeat requests to the same server. Locality-Aware Request Distribution (LARD) is one such system. Other methods have focused on front-end processors that perform level 4 switching (TCP level switching) to balance the load at back-end servers using a round robin technique. These systems may also store load information about the back-end servers and use this load information to improve upon the round robin scheduling (which would otherwise not consider the load of the servers). The IBM Network Dispatche™ is one such system implemented in software. Other vendors implement these types of systems in switches. Level 4 switching techniques do not attempt to take advantage of the locality of the request stream, meaning that requests that may already have been processed by one server for a client may be sent to a different server for a different client. 
     While each of the above methods operate reasonably well, each method involves sending the request through a routing switch for determining to which server to send the request. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved load balancing method, system, and computer program product in which popular requests are handled by a front-end cache, and hashing is applied only to the requests in the stream that were not handled by the front-end cache. 
     In a preferred embodiment, a web proxy cache is placed in front of a Level 7 switch, such that the web proxy cache services the popular requests from the cache based on the URL, i.e., based on the portion of the HTTP request following the domain name. The remaining requests are URL-hashed and then routed to the backend server. This allows the requests that make it past the web proxy cache to still be routed to the back-end server cache and take advantage of the efficiencies provided therefrom. Preferably the web proxy cache and the Level 7 hashing switch are combined as a single functional element, either in hardware or software. 
     In a more preferred embodiment, a Level 4 switch is placed in front of a plurality of web proxy caches, each of which are in turn placed in front of (or combined with) a respective Level 7 switch, each of which are connected to a respective server farm, so that incoming web requests are handled on a round robin basis before being sent to the web proxy cache, thus improving the throughput from the server farms to the requesting clients. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a load balancing system in accordance with the prior art; 
         FIG. 2  is a block diagram of an improved load balancing system in accordance with the present invention; 
         FIG. 3  is a block diagram of an example of a Caching/Hashing Switch  214  in accordance with the present invention; 
         FIG. 4  is a block diagram of a hierarchical load balancing system in accordance with the present invention; and 
         FIG. 5  is a flow chart illustrating the basic steps carried out in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  is a block diagram of an improved load balancing system in accordance with the present invention. Referring to  FIG. 2 , a group of clients  205 , comprising for example, plural workstations  210   a ,  210   b ,  210   c , and  210   d  is connected via network connection  212  (e.g., the Internet), to a “Caching/Hashing Switch” (CHS)  214 , which is in turn connected to a server farm  216  comprising, for example, plural web servers  218   a ,  218   b ,  218   c , and  218   d.    
     In accordance with the present invention, CHS  214  performs both a caching and switching function. It is understood, however, that the same novel functionality can be accomplished using a cache situated in front of a separate hashing switch. When a request is received for the first time by CHS  214 , the requested object will not be found in the cache. The term “object” as used herein refers generally to files, HTML responses, or any other content that a client might request from a server. The request is then hashed using known hashing techniques and sent to the destination server. The requested object is delivered to the CHS  214  where it is stored, and a copy of the requested object is sent on to the requesting client. 
     For subsequent requests for objects that have previously been requested, they will be either served directly from the cache if they are still present (e.g., if the storage period for the cache, relative to the request, has not expired) or they are hashed again and sent to the same server that served them the first time. 
       FIG. 3  is a block diagram of an example of a combined CHS  214  in accordance with the present invention. Referring now to  FIG. 3 , CHS  214  comprises a single caching/hashing element, i.e., it is a single functional element having a built-in web caching and URL hashing function. How to build these combined functions into a single functional element will be apparent to those skilled in the art. 
     CHS  214  includes a web proxy cache  330  and a URL-hashing switch  332 . The web proxy cache  330  is coupled to the client side via the network connection and is also coupled to the URL-hashing switch  332 . URL-hashing switch  332  is in turn connected to the server farm via connection  336 . By configuring the CHS  214  to first satisfy any web requests from the web proxy cache  320  (by looking for the requested object in the cache) and, if a “cache-miss” occurs, only then sending the request on to the URL-hashing switch  332  via connection  334 , previously requested popular web requests are satisfied without having to burden the server farm, thereby increasing the efficiency of the overall operation of the system. Configuration of these functions can be accomplished by any known methodology; for example, it is well within the ordinary skill of a programmer to be able to write software code to control the CHS  214  so that stored objects for which requests are received are satisfied by return of the cached object, and to hash the request and forward it to the server farm to be satisfied only if the requested object is not found in the cache. 
     In a preferred embodiment, a hierarchical system is employed as shown in  FIG. 4 . The system illustrated in  FIG. 4  will help minimize server farm throughput “bottlenecks” that may occur due to having a proxy cache at the front end of the system. As shown in  FIG. 4 , a Level 4 switch  440  is employed between the client devices  405 /Network Connection  412  and multiple CHS&#39;s  414  and server farms  416  as shown. Level 4 switch  440  performs round robin routing (or any other SLB technique) to the CHS&#39;s  414 . A benefit of this approach is that the Level 4 switches can be bought “off the shelf” and combined with the CHS of the present invention (or a functional equivalent thereof) to provide greater scalability. The system illustrated in  FIG. 4  is arbitrarily scalable because more CHS elements may be added between the Level 4 switch and the server farms (and thus additional server farms may also be added). 
       FIG. 5  is a flowchart illustrating the basic steps of operation of the present invention. At step  502 , a request for a web object is received by the CHS ( 214  in  FIG. 2 ) and at step  504 , the web proxy cache of the CHS is searched to determine if the requested object is stored therein (step  506 ). If the object is already stored in the web proxy cache, at step  510  the object is immediately returned to the client directly from the web proxy cache, with no need to burden the server with fulfilling the request, and the process ends at step  516 . 
     On the other hand, if, at step  506  it is determined that the object is not in the web proxy cache, at step  508  the URL of the object is hashed in a known manner and, at step  512 , the hash value is sent to the destination server. At step  514 , upon receipt of the object request, in the form of the hash value, the server returns the object to the web proxy cache, stores the object in the web proxy cache for future reference, and then sends the requested object to the requesting client. Finally, at step  516  the process ends. 
     The process utilized by the hierarchical system illustrated in  FIG. 4  is essentially identical, except that, prior to delivery of the web object request from the client, the requests pass through Level 4 switch  440  which performs conventional round robin routing to the individual cache/switches  414 . 
     Using the approach of the present invention, client connections are always terminated in the web proxy cache and the URL hashing may be performed on each HTTP request not found in the web proxy cache for determining which back end server to route the request to. This means that even with HTTP 1.1, it is possible to balance the load at the http request level and to take advantage of the locality of each request. HTTP 1.1 added extensions to the HTTP protocol, one of which allowed multiple requests to flow over a single connection from client to web (or web proxy) server. This invention allows requests within the single connection to be routed independently. 
     By handling “popular” web requests using a front-end cache and then utilizing hashing techniques for the un-cached requests, the load-balancing for the remaining requests can be maximized, and the system overall is arbitrarily scalable since, as described above, more CHS&#39;s may be added behind the Level  4  switch and more back-end servers may be added behind the additional CHS&#39;s. 
     Although the present invention has been described with respect to a specific preferred embodiment thereof, various changes and modifications may be suggested to one skilled in the art and it is intended that the present invention encompass such changes and modifications as fall within the scope of the appended claims.