Abstract:
A system and method for hierarchically caching objects includes one or more level  1  nodes, each including at least one level  1  cache; one or more level  2  nodes within which the objects are permanently stored or generated upon request, each level  2  node coupled to at least one of the one or more level  1  nodes and including one or more level  2  caches; and a storage control device for storing, in a coordinated manner, one or more objects in at least one level  1  cache and/or at least one level  2  cache, based on a set of one or more criteria. Furthermore, in a system for receiving requests for objects from one or more clients, the system having a set of one or more level  1  nodes, each containing at least one level  1  cache, a method for managing a level  1  cache includes the steps of applying, for part of the at least one level  1  cache, a cache replacement policy designed to minimize utilization of a set of one or more resources in the system; and using, for other parts of the at least on level  1  cache, one or more other cache replacement policies designed to minimize utilization of one or more other sets of one or more resources in the system.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of system caching. Specifically, the present invention provides a system and method for coordinated hierarchical caching in a client/server environment, particularly on the World Wide Web. Furthermore, the present invention provides a cache replacement method which adapts to changing system resource bottlenecks. 
     Glossary of Terms 
     While dictionary meanings are also implied by terms used here, the following glossary of some terms may be useful. 
     Client 
     A client is a computer which typically issues commands to one or more servers which perform the task associated with the command. 
     Server 
     Any computer that performs a task at the command of one or more computers is a server. A Web server typically supports one or more clients. 
     World Wide Web (Web) 
     The Internet&#39;s application that lets people seeking information on the Internet switch from server to server and database to database. Users can click on highlighted words or phrases of interest (hyperlinks), click on graphics such as applets or image maps, fill in forms, and enter URLs. An Internet Web server supports clients and provides information. The Web can be considered as the Internet with all of the resources addressed as URLs and which uses HTTP protocols to transfer information between computers and which uses HTML (among other formats) to display the information corresponding to URLs and provide a point-and-click interface to other URLs. 
     Universal Resource Locator (URL) 
     A way to identify or address information on the Internet. 
     HyperText Markup Language (HTML) 
     HTML is a standard device-independent representation of content on Web servers. Web servers may deliver content (including hyperlinks) to clients in HTML with confidence that the client will choose an appropriate presentation. 
     Hypertext Transfer Protocol (HTTP) 
     HTTP is an example of a stateless protocol, which means that every request from a client to a server is treated independently. Clients send request to servers and servers respond using this protocol. 
     Internet Browser or Web Browser 
     A graphical interface tool that runs Internet protocols such as HTTP, and displays results on the customer&#39;s screen. The browser can act as an Internet tour guide, complete with pictorial desktops, directories and search tools used when a user “surfs” the Internet. In this application, the Web browser is a client service which communicates with the Web servers. 
     Object 
     An object is data which can be stored in a cache. 
     Cache Replacement Policy 
     A cache replacement policy is an algorithm for determining which objects should be placed in a cache when the cache contains insufficient space to contain all objects. 
     BACKGROUND OF THE INVENTION 
     Caching is commonly used for improving performance on computer systems. By caching an object, the cost for fetching or creating the object is only incurred once. Subsequent requests for a cached object can be satisfied from the cache, a process which incurs significantly less overhead than regenerating the object or fetching it from a remote location. 
     Systems on the World Wide Web (“Web”) today often contain browser and proxy caching (see “Caching Proxies: Limitations and Potentials” by M. Abrams et. al., “Fourth International World Wide Web Conference Proceedings,” December 1996, pp. 119-133 and “World Wide Web Proxies” by A. Luotonen and K. Altis in “Computer Networks and ISDN Systems,” vol. 27 (1994), pp. 147-154) to improve performance. While these caching systems are sometimes hierarchical in nature, normally caching at one level is not coordinated with caching at the next level. In other words, each cache makes decisions independently of what another cache is doing. 
     In U.S. Pat. No. 5,924,116 entitled “Collaborative Caching” by Yu et al., filed Apr. 2, 1997, a method for collaborative caching in a hierarchy of proxy caches is disclosed. The disclosed system bases a client&#39;s or proxy&#39;s caching decision on whether the object is cached at another proxy location in the hierarchy closer to the Web server which stores or generates the object. No cache is disclosed to be local to the Web server and the client or proxy must rely on header information contained in the object and caching status information provided by the other proxy location for its caching decision. While this system addresses general network access (browsing) speed, it does not address the problem of slow service of Web pages (such as dynamic pages) to clients from a particularly hot Web server site. The Yu system would have to be implemented across the internet&#39;s multitude of proxy servers to provide improved service from a particular Web site. Finally, it would be practically impossible to update, with the Yu system, dynamic pages (those generated “on the fly”) in view of network bottlenecks and the decoupled nature of the proxies to the Web server. Thus, there is a need for a client/server system that implements a true hierarchy of caches from a Web server level to the client browser level, supports dynamic pages and coordinates the actions of the caches so that superior performance can be attained. 
     Level  1 , level  2 , etc. caches in uniprocessors are well known to those skilled in the art. However, while level  1  and level  2  processor caches are often coordinated to some extent (for example, the decision to place an object in one processor cache might be made depending on whether the object is contained in another processor cache), the client/server environment has characteristics which pose different problems to a caching system. First, in the client/server environment, there can be multiple level  2  (server) caches located behind a level  1  (e.g., router) cache, all with different contents. In contrast, a uniprocessor contains, at most, one level  2  cache. Second, compared with processor caches, there are more constraints over what can be stored in these router and server caches. For example, in a Web-based client/server environment, it may be necessary to keep some URLs out of a router cache in order to force a particular request to be directed to a server. It is also undesirable to cache objects (e.g., static pages) in server caches which are inexpensive for the Web server to generate. This would not be the case for processor caches because level  2  caches are generally faster than main memory. Third, the parameters affecting admittance to the cache and cache replacement are significantly different for the two environments. In processor caching, least recently used (LRU) information generally suffices. In Web-based client/server systems, considerations such as the expected number of accesses per unit time, expected lifetime, object size and time to create or fetch the object are important to consider. 
     Another prior art system is the Parallel Sysplex in the System 390 (Special Issue on S/390 Parallel Sysplex Cluster, IBM Systems Journal, Vol. 36, No. 2, 1997). The Parallel Sysplex has level  1  database caches in each of the processing nodes and a level  2  cache in the coupling facility, which is shared among the nodes. While the level  1  and level  2  caches in the Sysplex are coordinated, there are several differences between the Sysplex and a client/server system. 
     In the Sysplex, the database system runs at the same node as the level  1  cache, the level  2  cache is on a shared coupling facility and the permanent data is located on shared disks accessed from the node at which the level  1  caches reside. In a client/server system, however, remote clients may have to make requests which initially go to the level  1  cache, while the permanent locations of the data are at the nodes at which the level  2  caches reside. Furthermore, the criteria for placing objects in the level  1  or level  2  caches are very different for the two systems. In the Sysplex, hot shared records/objects, especially those frequently updated, are placed in the level  2  coupling facility cache. In the client/server case, there is no concept of sharing (i.e. programs at different Sysplex nodes that access and update the same object), and the criteria for caching objects should include the size of the objects, the hotness (desirability measured by hits) of the objects, the time to create or fetch the objects, and the update rate. Therefore, there is a need for a coordinated hierarchical caching system directed to the client/server environment where a level  2  cache is located where the objects are permanently stored or generated. 
     In many cases, caches do not have sufficient memory to store all cacheable objects at one time. Selectivity must be applied in determining which objects should be cached when the cache is full or almost full. To address this problem, a number of cache replacement algorithms exist in the prior art such as that disclosed in U.S. patent application Ser. No. 08/958,506 entitled “A New Algorithm for Cache Replacement” by Challenger et al., filed Oct. 27, 1997 and herein incorporated by reference. These algorithms, however, have been designed to address the problem of a single system bottleneck such as a system processor. Networked systems generally may suffer from at least one of several potential bottlenecks at any given time. In addition, the bottleneck(s) having the dominant impact on performance may change over time. 
     Therefore, there is a need for an algorithm that can handle situations where one of several resources (e.g., server CPU, router CPU, network bandwidth) might create bottlenecks and where the resource which is causing the bottleneck (if any) may change dynamically. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system for hierarchically caching objects including one or more level  1  nodes, each including at least one level  1  cache; one or more level  2  nodes within which the objects are permanently stored or generated upon request, each level  2  node coupled to at least one of the one or more level  1  nodes and including one or more level  2  caches; and storing device for storing, in a coordinated manner, one or more objects in at least one level  1  cache and/or at least one level  2  cache, based on a set of one or more criteria. 
     The set of one or more criteria preferably include at least one of: a size of one or more of the objects, a storage space available in one or more of the caches, a CPU load on one or more of the level  1  nodes, a CPU load on one or more of the level  2  nodes, a pattern of data accesses to one or more of the objects, a frequency of update of one or more of the objects, a time to create or fetch one or more of the objects, and network traffic between one or more of the level  1  nodes and one or more of the level  2  nodes. 
     The level  1  nodes can be Web server accelerators, routers or Web servers. The nodes are preferably disposed within an encapsulated cluster. 
     The system of the present invention can include one or more level  3  nodes coupled to at least one of the one or more level  2  nodes and wherein the storing device comprises a device for storing one or more objects in at least one level  3  cache and/or at least one of the at least one level  1  cache, the at least one level  2  cache and the at least one level  3  cache. 
     Finally, the present invention also provides a system for hierarchically caching objects including one or more level I nodes, each including at least one level I cache, for all integers I such that L&gt;=I&gt;0 where L&gt;=3, wherein the objects are permanently stored or generated on at least one of the nodes; and a storing device for storing, in a coordinated manner, one or more of the objects in at least one level j cache and/or at least one level k cache where L&gt;=k&gt;j&gt;0, based on a set of one or more criteria. 
     The present invention also provides a method for caching objects including the steps of: providing one or more level  1  nodes, each including at least one level  1  cache; providing one or more level  2  nodes within which the objects are permanently stored or generated upon request, each level  2  node coupled to at least one of the one or more level  1  nodes and including one or more level  2  caches; and storing one or more objects in at least one level  1  cache and/or at least one level  2  cache, in a coordinated manner based on a set of one or more criteria. 
     Preferably, the set of one or more criteria includes at least one of: a size of one or more of the objects, a storage space available in one or more of the caches, a CPU load on one or more of the level  1  nodes, a CPU load on one or more of the level  2  nodes, a pattern of data accesses to one or more of the objects, a frequency of update of one or more of the objects, a time to create or fetch one or more of the objects, and network traffic between one or more of the level  1  nodes and one or more of the level  2  nodes. 
     The storing step preferably includes the step of maintaining information that one or more objects should not be cached in at least one level  1  cache. 
     The method of the present invention preferably further includes the steps of: receiving, at the one or more level  1  nodes, a request for an object from a client; determining whether the requested object is in the at least one level  1  cache; transmitting, in response to the object being found in the at least one level  1  cache, the object from the at least one level  1  cache to the client; and attempting, in response to the object not being found in the at least one level  1  cache, to satisfy the request from the at least one level  2  cache. The attempting step preferably includes the steps of: forwarding the object to at least one of the one or more level  2  nodes; determining whether the object is in at least one level  2  cache corresponding to the at least one of the one or more level  2  nodes; and sending, in response to the object being found in the at least one level  2  cache, the object from the level  2  cache to the client. Preferably, the method further includes the steps of identifying the object as being sufficiently hot; and moving the object or a copy of the object to a level  1  cache. 
     Preferably, at any specific time, an object is stored in, at most, one of the level  2  caches. 
     The method of the present invention preferably further includes the step of preventing the caching of an object in a level  2  cache where a cost of providing the object from the level  2  cache is high relative to a cost of providing the object from a level  2  node corresponding to the level  2  cache. The cost of providing the object from the level  2  cache preferably includes a cost of at least one invalidation and updating of the object in the cache after its value changes. 
     The method of the present invention preferably further includes the step of caching, in response to a level  1  cache being full, an object in a level  2  cache. 
     The method of the present invention preferably further includes the step of preventing an object from being cached in a level  1  cache. It is also preferable that the method further includes the step of allowing the object to be cached in at least one level  2  cache. The preventing step is made necessary due to a difficulty of maintaining sufficiently current values of the object in the level  1  cache or because the request for the object causes a side effect on a level  2  node. 
     The storing step preferably includes the step of determining the object to be a general cache candidate. The determining step can include the step of checking a text string or header information associated with the object, the step of applying a function to the object (such as determining the size of the object or determining the expected lifetime of the object). 
     The storing step preferably includes the step of identifying the object to be a level  1  cache candidate. The identifying step can include the step of determining the size of the object, determining any limits in logging facilities of the associated level  1  node or determining sufficient space in the level  1  cache. 
     The storing step preferably includes the step of identifying the object to be a level  2  cache candidate. The identifying step can include the step of determining the object not to be a level  1  cache candidate, the step of determining the size of the object, or the step of determining sufficient space in the level  2  cache. 
     The storing step preferably includes the step of applying a cache replacement policy. The applying step can include the steps of applying, for part of the at least one level  1  cache, a cache replacement policy designed to minimize utilization of a set of one or more resources in the system; and using, for other parts of the at least one level  1  cache, one or more other cache replacement policies designed to minimize utilization of one or more other sets of one or more resources in the system. 
     The present invention also provides a method for caching objects comprising the steps of: providing one or more level I nodes, each including at least one level I cache, where L&gt;=3 and I is an integer such that L&gt;=I&gt;0, wherein the objects are permanently stored or generated on at least one of the nodes; and storing, in a coordinated manner, one or more objects in at least one level j cache and/or at least one level k cache where L&gt;=k&gt;j&gt;0, based on a set of one or more criteria. The set of one or more criteria preferably includes at least one of: a size of one or more of the objects, a storage space available in one or more of the caches, a CPU load on one or more of the level I nodes, a pattern of data accesses to one or more of the objects, a frequency of update of one or more of the objects, a time to create or fetch one or more of the objects, and network traffic between one or more of the level I nodes, the level j nodes and/or the level k nodes. 
     Another aspect of the present invention is the novel cache replacement method used by the Web server accelerator (e.g., router). This method for determining which objects should be placed in a cache is particularly useful when multiple parameters affecting the desirability of caching objects are non-uniform across objects. Such parameters include the frequency with which an object is accessed, object size, the time to calculate an object or fetch it from a remote location, and the lifetime (i.e. time between updates) of an object. 
     Using the method of the present invention, a router applies a combination of three cache replacement algorithms to manage different parts of its cache. One replacement algorithm is designed for optimal performance when server CPUs are the bottleneck. Another replacement algorithm is designed for optimal performance when the router CPU is the bottleneck. The third algorithm is designed for optimal performance when the network between the router and the server(s) is the bottleneck. When one of the three aforementioned resources becomes a bottleneck, the router increases the amount of cache storage space managed by the replacement algorithm designed to minimize consumption of the bottleneck resource. Another technique of the present invention which is sometimes used to improve performance when a resource becomes a bottleneck is to vary the percentage of requests which bypass the router&#39;s cache and are sent directly to a server. 
     Particularly, the present invention provides, in a system adapted to receive requests for objects from one or more clients, the system having a set of one or more level  1  nodes, each containing at least one level  1  cache, a method for managing a level  1  cache including the steps of: applying, for part of the at least one level  1  cache, a cache replacement policy designed to minimize utilization of a set of one or more resources in the system; and using, for other parts of the at least one level  1  cache, one or more other cache replacement policies designed to minimize utilization of one or more other sets of one or more resources in the system. 
     The cache replacement policy is preferably designed to minimize resources on a level  1  node containing the level  1  cache. Here, the cache replacement policy can estimate a desirability of caching an object from an expected number of hits to the object per unit time if the object is cached divided by a size of the object. Preferably, the cache replacement policy estimates the desirability of caching the object by calculating or estimating the quantity (1/a−p/u)/s, where a is the expected time between successive requests for the object, u is the expected time between successive updates of the object, p is the probability that the object will be accessed between successive updates to the object, and s is the size of the object. 
     Where the system further has one or more level  2  nodes coupled to one or more of the level  1  nodes, the cache replacement policy is preferably designed to minimize resources on at least one level  2  node. Here, the cache replacement policy can estimate the desirability of caching an object from the expected number of hits to the object per unit time if it is cached multiplied by the expected processing time on one or more of the level  2  nodes to fetch or calculate the object divided by the size of the object. The cache replacement policy preferably estimates the desirability of caching the object by calculating or estimating the quantity (ta−t*p/u)/s, where a is the expected time between successive requests for the object, t is the expected processing time on one or more of the level  2  nodes to fetch or calculate the object, u is the expected time between successive updates of the object, p is the probability that the object will be accessed between successive updates to the object, and s is the size of the object. 
     Where the system further has one or more level  2  nodes and one or more networks located between the one or more level  1  nodes and the one or more level  2  nodes, the cache replacement policy is preferably designed to minimize traffic on one or more of the networks. Here, the cache replacement policy can estimate the desirability of caching an object from the expected number of hits to the object per unit time if the object is cached. Preferably, the cache replacement policy estimates the desirability of caching the object by calculating or estimating the quantity 1/a−p/u, where a is the expected time between successive requests for the object, u is the expected time between successive updates of the object, and p is the probability that the object will be accessed between successive updates to the object. 
     It is preferable that the method of the present invention also include the step of dynamically varying the parts of the level  1  cache managed by at least one of the cache replacement policies. 
     Preferably, the method further includes the steps of determining one or more resource bottlenecks corresponding to one or more resources; and dynamically varying, in response to the determining step, the parts of the level  1  cache managed by at least one of the cache replacement policies. The dynamically varying step preferably includes the step of increasing, in response to the determining step, the part of the level  1  cache managed by a cache replacement policy designed to minimize utilization of the one or more resources. 
     The system can further have one or more level  2  nodes and one or more networks between the one or more level  1  nodes and the one or more level  2  nodes, wherein the one or more resources becoming a bottleneck contain at least one of: processing power of the level one node containing the level  1  cache, processing power of at least one of the level  2  nodes and bandwidth of at least one of the networks. 
     In a system adapted to receive requests for objects from one or more clients, the system having one or more level  1  nodes, each containing at least one level  1  cache, and having one or more level  2  nodes coupled to one or more of the level  1  nodes, a method for handling requests in the system includes the steps of: determining a level  1  node to be a bottleneck in the system; and sending, in response to the determining step, one or more requests to one or more level  2  nodes without first attempting to satisfy the one or more requests from a level  1  cache on the level  1  node. 
     In a system adapted to receive requests for objects from one or more clients, the system having a set of one or more level  1  nodes, each containing at least one level  1  cache, one or more level  2  nodes, and one or more networks between the one or more level  1  nodes and the one or more level  2  nodes, a method for handling requests in the system includes the steps of: determining at least one of the networks or at least one of the level  2  nodes to be a bottleneck in the system; and reducing, in response to the determining step, the number of requests sent to the one or more level  2  nodes without first attempting to satisfy the request from a level  1  cache. 
     Preferably, where the system further includes one or more level  2  nodes coupled to one or more level  1  nodes, the method further includes the step of: determining a level  1  node to be a bottleneck in the system; and sending, in response to the determining step, one or more requests to one or more level  2  nodes without first attempting to satisfy the one or more requests from a level  1  cache on the level  1  node. 
     Preferably, where the system further comprises one or more level  2  nodes and a network between the one or more level  1  nodes and the one or more level  2  nodes, the method further includes the steps of: determining at least one of the network or the level  2  nodes to be a bottleneck in the system; and reducing, in response to the determining step, the number of requests sent to one or more level  2  nodes without first attempting to satisfy the request from a level  1  cache. 
     Finally, the method of the present invention preferably includes the steps of determining, in response to the object not being found in the at least one level  1  cache, whether the requested object should be cached in any level  1  cache and satisfying, in response to the determination that the object should not be cached in any level  1  cache, the request in an optimized fashion. The satisfying step preferably includes the step of returning a response to the client from a level  2  node through at least one of the one or more level  1  nodes, bypassing the at least one level  1  cache or the step of returning a response to the client from a level  2  node, bypassing at least one of the one or more level  1  nodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will be understood by reference to the drawing, wherein: 
     FIG. 1 is a schematic diagram of an embodiment of a coordinated hierarchical caching system according to the present invention; 
     FIG. 2 is a block diagram of a method for serving objects from caches to clients according to an embodiment of the present invention; 
     FIG. 3 is a block diagram of a method for caching objects according to an embodiment of the present invention; 
     FIG. 4 is a block diagram of an algorithm for dynamically varying cache replacement policies used by routers according to an embodiment of the present invention; 
     FIG. 5 is a block diagram of an algorithm for moving objects from one or more server caches to a router cache according to an embodiment of the present invention; and 
     FIG. 6 is a schematic diagram of an embodiment of the hierarchical caching system of the present invention with more than two levels in the hierarchy. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 depicts an embodiment of a coordinated hierarchical caching system according to the present invention. As shown, a client  10  communicates requests over a network link  55  to an encapsulated cluster  70  comprised of a Web server accelerator or router  20  (level  1  node) and two Web servers  40  (level  2  nodes). An encapsulated cluster is described in U.S. patent application Ser. No. 08/947,361 entitled “Affinity-Based Router and Routing Method” by Devarakonda et al., filed Dec. 23, 1996 (provisional effective filing date), which is hereby incorporated by reference. Other embodiments of the present invention can include the level  1  and level  2  nodes without the encapsulated cluster  70 . 
     Within the encapsulated cluster  70 , the two servers  40  communicate with the Web server accelerator  20  over network links  58 . The Web server accelerator (hereinafter “router”)  20  may actually be any processor system which can communicate with other nodes over the network links  58 . One embodiment of the system of the present invention includes a TCP router as a Web server accelerator (see Attanasio, Clement R. and Smith, Stephen E., “A Virtual Multi-Processor Implemented by an Encapsulated Cluster of Loosely Coupled Computers”, IBM Research Report RC 18442, 1992; Attanasio, Clement R. and Smith, Stephen E., “Method and Apparatus for Making a Cluster of Computers Appear as a Single Host”; U.S. Pat. No. 5,371,852, Dec. 6, 1994; and Dias, D. M., Kish, W., Mukherjee, R., and Tewari, R., “A Scalable and Highly Available Web Server”, Proc. 41st IEEE Computer Society Intl. Conf. (COMPCON) 1996, Technologies for the Information Superhighway, pp. 85-92, February 1996), such as an IBM 2210 or 2216, which routes requests to multiple processors, each running a Web server. Although two servers  40  are pictured in FIG. 1, according to the present invention, there can be one or more servers in this architecture with one or more network links. In order to take advantage of the coordinated hierarchical structure of the present invention, it is preferable that all requests transmitted from the client  10  to the encapsulated cluster  70  initially are directed to the router  20 . However, one skilled in the art could appreciate that some requests to the encapsulated cluster  70  can be transmitted directly to the Web server(s)  40 . Furthermore, the present invention can be implemented with multiple network links connecting the router  20  to one or more servers  40 . 
     When the router  20  receives a request from the client  10 , the router  20  might route the request to a server without looking at the request. The router  20  can choose the appropriate server  40  to route the request to using a variety of methods including, but not limited to, a round robin method or schemes which take the load of the servers  40  into account (see “Network Dispatcher: a connection router for scalable Internet services” by G. Hunt et al. in Proceedings of the 7th International World Wide Web Conference, April 1998). In this situation, when the router  20  has not examined the request&#39;s contents, the server  40  handling the request can respond directly to the client  10  over network link  59 . The response to the request does not have to go through the router  20 . 
     Alternatively, to take advantage of the system of the present invention, the router  20  might examine the request upon its receipt. By examining the request, the router  20  can determine whether the request can be satisfied from data in its level  1  cache  30 . If not, the request is routed to a server  40  over network link  58 . The method for routing the request to a particular server  40  may be, but is not limited to, one of the following: a round robin method, a scheme which takes the load of the servers  40  into account or a content-based approach which bases routing decisions on the contents of the request. In any case, the router  20  must close its connection with the client  10  in order to examine the request in this embodiment. After the request is examined, the router  20  functions as a proxy and establishes a new connection with a server  40  over a network link  58  in order to satisfy the request. The server  40  sends a response to the request back over the network link  58  to the router  20  which subsequently forwards the response back to the client  10  over the network link  55 . One skilled in the art could adapt the present invention to systems in which the servers  40  could send results to the client  10  directly, over network links  59  without going through the router  20  even if the router  20 , examined the contents of the request. 
     Coordinated caching between the router  20  and servers  40  is necessary due to the characteristics of the client/server system of the present invention. The router (level  1 ) cache  30  can serve Web data an order of magnitude faster than the Web servers  40  can. In order to achieve fast performance, all objects cached in the router cache  30  must be cached in memory. Since the router memory is limited in size, it is often not possible to cache all hot objects at the router. Each Web server (level  2 ) cache  50  is significantly slower than the router cache  30 , but has more memory. The server caches  50  are generally used to cache dynamic pages which are expensive to create. In many cases, no performance advantage is conveyed by caching static pages at the servers  40  because static pages often can be delivered via the Web servers  40  themselves more cheaply than via their associated server caches  50 . By contrast, the router cache  30  can generally deliver static pages significantly more cheaply than a Web server  40  can. Therefore, the router cache  30  is much better suited for caching static pages than any of the server caches  50  are. In addition, due to the router&#39;s superior performance, it is also beneficial to cache hot dynamic pages in the router  20 . 
     According to the present invention, the router cache  30  is used to store the hottest objects. Objects which cannot fit in the router cache  30  may instead be stored in a server cache  50 . Some objects can be cached at the router  20 , but not at the servers  40 . Examples of such pages are static pages for which little, if any, CPU time would be saved by caching at a server  40 . Furthermore, some pages are not permitted to be cached at the router  20 . Examples of such pages are dynamic pages for which it is not feasible to maintain current copies in the router cache  30  and dynamic pages which cause side-effects at the servers  40 . 
     FIG. 2 shows how a preferred embodiment of the system of the present invention handles client requests. In step  200 , the encapsulated cluster  70  performs conventional operations such as routine computations. In step  205 , the encapsulated cluster  70  receives a request from a client  10 . In step  210 , the router  20  determines whether the requested object is stored in its level  1  cache  30 . If it is stored as such, the object is transmitted from the level  1  cache  30  to the client  10  over network link  55  in step  220  and the process continues in step  200 . If the requested object is not stored in the level  1  cache  30 , the request is routed to a server  40  in step  230 . As described hereinabove, the method for routing the request to a server  40  may be, but is not limited to, one of the following: a round robin method, a scheme which takes the load of the servers into account or a content-based approach which bases routing decisions on the request contents. 
     In step  240 , the server  40  receiving the forwarded request looks for the object in its level  2  cache  50 . If the object is located in the level  2  cache  50 , the cached object is returned to the client  10  in step  220  and system operation continues in step  200 . Preferably, if the system determines that the object is sufficiently hot, and the object is identified as being cacheable (see description associated with FIG. 3 hereinbelow) at the router  20 , the object may be moved from the level  2  cache  50  to the level  1  cache  30  (step not shown). The system can optionally retain a cached copy of the object in the level  2  cache  50  after moving it to the level  1  cache  30 . If it is determined, in step  240 , that the object is not contained in the level  2  cache  50 , the server  40  fetches or generates the page in step  250 . Alternatively, in step  240 , the system examines multiple level  2  caches for the requested object. While some (static) objects are fetched from the file system, other (dynamic) pages are created by programs which execute on the server  40  which could potentially invoke programs on other computers besides the server  40 . 
     In step  260 , the fetched/generated object is transmitted to the client  10  and the system optionally caches the object. If the object was generated by a request which caused a side-effect (changed a state, such as modifying a file) on the server  40 , the object likely would not be cached for practical purposes. Furthermore, if the server  40  determines that caching the object would not lead to significant savings in a limiting resource or that the cost and/or difficulty of keeping a sufficiently updated copy of the object would be prohibitive, the object likely would not be cached. However, if it is determined that the object should be cached, the method depicted in FIG. 3 is, preferably, applied to place the object in zero or more caches as appropriate. 
     FIG. 3 shows a preferred method of caching objects according to another aspect of the present invention. Upon pre-fetching objects or after transmitting a fetched/generated object to a client, the system determines, in step  303 , whether an object O is, generally, a cache candidate. This step can be handled in a number of ways. For example, O might have data associated with it (e.g. a text string or a header such as a HTTP header) which indicates whether or not it should be cached. Alternatively, a function could be applied to determine whether O should be cached. For example, the function could determine the size of O and determine that O should not be cached if its size is too large. As another example, the function could determine the expected lifetime of the object and determine that O should not be cached if its expected lifetime is too short. One skilled in that art will appreciate that various other schemes could be applied for determining whether O is a cache candidate. 
     If it is determined that the object O is a cache candidate, the system determines, in step  309 , whether O is a cache candidate at the level  1  cache  30 . Some of the criteria used for this determination are the size of the object and any limits in the logging facilities of the level  1  node. If it is determined that O is not a cache candidate at the level  1  cache  30 , processing continues at step  313 . If it is determined that O is a cache candidate at the level  1  cache  30 , the system determines whether sufficient space exists in the cache for O, in step  317 . If sufficient space exists in the cache for O, O is added to the level  1  cache  30  in step  328 . If it is determined that O is a cache candidate at the level  1  cache  30  but that insufficient space exists in the cache for O, a cache replacement algorithm is used, in step  319 , to determine whether O should replace one or more other objects in the level  1  cache  30 . If it is determined that O should replace one or more other objects in the level  1  cache  30 , the system, in step  328 , caches O and removes one or more objects identified by the cache replacement algorithm in step  319  in order to make room for O. Furthermore, one or more of the objects removed from the level  1  cache  30  as a result of the replacement algorithm may be cached at one or more level  2  caches  50  in step  328 . While it is possible to use other cache replacement algorithms in step  319 , the preferred cache replacement algorithm is depicted in FIG.  4 . 
     If it is determined, in step  319 , that O should not replace one or more other objects in the level  1  cache  30 , the system determines, in step  312 , whether O is a cache candidate at a level  2  cache  50 . In a preferred embodiment, each object is stored or generated and cached at a specific level  2  node. One skilled in the art will appreciate that, in another embodiment, the system could allow an object to be cached at multiple level  2  nodes. In this case, steps  312 ,  313 ,  314  and/or  315  could be applied to multiple level  2  caches for the same object. 
     If it is determined, in step  312 , that O should not be cached at a level  2  cache  50 , processing continues at step  200 . If it is determined that O is a cache candidate at a level  2  cache  50 , the system determines whether sufficient space exists in the level  2  cache  50  for O, in step  313 . If sufficient space exists in the cache for O, O is added to the level  2  cache  50  in step  314 . If it is determined that O is a cache candidate at the level  2  cache  50  but that insufficient space exists in the cache for O, a cache replacement algorithm is applied, in step  315 , to determine whether O should replace one or more other objects in the level  2  cache  50 . If it is determined that O should replace one or more other objects in the level  2  cache  50 , the system, in step  314 , also removes one or more objects identified by the cache replacement algorithm in step  315  in order to make room for O. An extension of this algorithm which could be performed by one skilled in the art is to cache O at multiple level  2  caches  50  in step  315 . While it is possible to use other cache replacement algorithms, the preferred cache replacement algorithm applied in step  315  is described in Challenger et al. Finally, if it is determined that O should not replace one or more other objects in the level  2  cache  50 , the system continues normal operation in step  200 . 
     Another aspect of the present invention is the preferred method for handling cache replacement at the level  1  cache  30 . This method can be used with any system employing a cache similar to the cache on a Web server accelerator such as the router  20  and is not limited to systems employing hierarchical caching and/or multiple servers. A key feature of the method is that it optimizes performance, whether the performance bottleneck of the system is router processing power, server processing power, or the network between the server(s) and the router. 
     Suppose that server processing power is the bottleneck in the system. The desirability of caching an object O at the level  1  cache  30  is proportional to the expected number of hits to O per unit time if it is cached multiplied by the expected server processing time to fetch or calculate O divided by the size of O. One formula for estimating the caching desirability is: 
     
       
           d   1 ( O )=( t/a−t*p/u )/ s   
       
     
     where a is the expected time between successive requests for O, t is the expected server processing time to fetch or calculate O, u is the expected time between successive updates of O, p is the probability that O will be accessed between successive updates to O, and s is the size of O. One skilled in the art will appreciate that other formulas for estimating the desirability of caching O can be used. The patent application, Challenger et al., describes the preferred method for estimating a, p and u. One skilled in the art will also appreciate that other methods for estimating these quantities could be used. 
     Let r 1  be a cache replacement policy designed to minimize the consumption of server processing resources. The preferred embodiment of r 1  is described in Challenger et al. Other embodiments of r 1  can be used in the present invention as well. 
     Suppose that, instead, router processing power is the bottleneck in the system. The desirability of caching an object O at the level  1  cache  30  is proportional to the expected number of hits to O per unit time if it is cached divided by the size of O. One formula for estimating the desirability is: 
     
       
           d   2 ( O )=(1 /a−p/u )/ s.   
       
     
     One skilled in the art will appreciate that other formulas could be used for estimating the desirability of caching O as well. 
     Let r 2  be a cache replacement policy designed to minimize the consumption of router processing resources. The preferred embodiment of r 2  is to use the approach contained in Challenger et al., but to use d 2 (O) to estimate the desirability of caching O instead of d 1 (O). Other embodiments of r 2  can be used in the present invention as well. 
     Suppose that, instead, the network between the router  20  and the server(s)  40  is the bottleneck in the system. The desirability of caching an object O at the level  1  cache  30  is proportional to the expected number of hits to O per unit time, if it is cached. One formula for estimating the desirability is: 
     
       
           d   3 ( O )=1/ a−p/u.   
       
     
     One skilled in the art will appreciate that other formulas can be used for estimating the desirability of caching O. 
     Let r 3  be a cache replacement policy designed to minimize network traffic between the router and the server(s). The preferred embodiment of r 3  is to use the approach contained in Challenger et al., but to use d 3 (O) to estimate the desirability of caching O instead of d 1 (O). Other embodiments of r 3  can be used in the present invention as well. 
     FIG. 4 depicts the preferred algorithm for handling cache replacement at the level  1  cache  30  according to the present invention. The basic approach is to use a combination of different cache replacement policies, each designed to minimize consumption of a different resource. When a particular resource becomes a bottleneck, the proportion of the cache managed by the policy designed to minimize consumption of that resource is increased. 
     In step  400 , the level  1  cache  30  is initialized to handle replacement by managing x % of the bytes in the cache using r 1 , y % using r 2 , and z % using r 3  where 100&gt;=x,y,z&gt;=0 and x+y+z=100. The values of x, y, and z are determined from default initialization parameters. 
     In step  410 , the level  1  cache  30  performs traditional cache operations such as insertions, invalidations and lookups. It is preferable that the system concurrently checks for bottlenecks using methods known in the prior art. If the router  20  is not a system bottleneck, the router  20  will often try to satisfy all requests from its cache  30 . If, however, the router  20  starts to become the sole bottleneck, it may try to route some requests directly to the server(s)  40  without examining its cache. Routing requests to servers  40  without trying to perform a lookup in the router cache  30  entails less router overhead. 
     If at some point, the system recognizes that some of the servers  40  (but not all) have become bottlenecks (step  431 ), load balancing is used to remove the server bottlenecks in step  428 . 
     If, at some point, the system recognizes that all of the servers  40  become bottlenecks (step  490 ), the system determines, in step  480 , if the entire cache  30  is managed using r 1 . If the answer is no, the amount of cache  30  managed using r 1  is increased in step  470 . If, on the other hand, the answer is yes, the system determines, in step  413 , whether the router  20  is also a bottleneck. If so, there is nothing obvious that can be done to improve system throughput and processing returns to step  410 . If the answer is no, the system, in step  416 , decreases the proportion of requests which bypass the router cache  30  (if nonzero). 
     If, at some point, the system recognizes that the router  20  becomes a bottleneck (step  420 ), the system determines, in step  430 , whether the entire cache  30  is managed using r 2 . If the answer is no, the amount of cache  30  managed using r 2  is increased in step  440 . If, on the other hand, the answer is yes, the system determines, in step  450 , whether the server(s)  40  or the network between the router and server(s)  58  is also a bottleneck. If so, there is nothing obvious that can be done to improve system throughput and processing returns to step  410 . If the answer is no, the system, in step  460 , increases the proportion of requests which bypass the router cache  30  (if less than 1). 
     If, at some point, the system recognizes that the network  58  between the router and server(s) becomes a bottleneck (step  425 ), the system determines, in step  422 , whether the entire cache  30  is managed using r 3 . If the answer is no, the amount of cache  30  managed using r 3  is increased in step  419 . If, on the other hand, the answer is yes, the process continues to step  413  and the system proceeds as described hereinabove. 
     Another feature of the present invention is depicted in FIG.  5 . When sufficient space becomes available in the router cache  30  after a period when objects had to be rejected or replaced from the cache  30  due to the cache  30  being full (step  500 ), the system may attempt to move or copy one or more objects from one or more of the server caches  50  to the router cache  30  in step  510 . The system then proceeds with general operations in step  200 . 
     It is possible to have more than two levels in the caching hierarchy according to the present invention. FIG. 6 depicts a system which has an encapsulated cluster  75  with a level  3  node  62  having an accompanying level  3  cache  63 . There can be more than one level  3  nodes  62  in this embodiment. Here, requests are sometimes directed to the level  3  nodes  62  if they cannot be handled by the level  2  nodes  40 . In general, it is possible to have n levels in the hierarchy for any finite positive integer n. 
     In another feature of the present invention, it may be desirable to not cache some hot or other objects in the router cache  30 . For example, though hot, an object may be too large, and may be better cached in the server cache  50 . The overhead for serving such objects can be lowered if it is determined, at the router  20 , that the object will not be cached, before sending the request to the server  40 . If this is done, one can avoid examining and starting to cache the object on the return path from the server  40 . Those skilled in the art will readily appreciate that the response can be directly and efficiently sent back to the client  10 . Alternatively, the request can be handed off to the server  40  and returned directly to the client  10 , bypassing the router  20  (or handed off to a different router to handle on the return path to the client  10 ). 
     In order to determine whether an object requested by the client  10  is to be cached at the router cache  30 , statistics of (hot) data not in the router cache  30  are also preferably maintained at the router cache  30 . This can be accomplished by maintaining a list, ordered by LRU information, of items that should not be cached at the router cache  30 , due to size or other specific reasons. Configuration at the router  20  may also include specification of some such objects, classes of objects, or directories of objects that should not be cached in the router cache  30 . 
     Now that the invention has been described by way of a preferred embodiment, various modifications and improvements will occur to those of skill in the art. Thus, it should be understood that the preferred embodiment is provided as an example and not as a limitation. The scope of the invention is defined by the appended claims.