LOAD BALANCING COOPERATING CACHE SERVERS

Abstract: In a system including a collection of cooperating cache servers, such as proxy cache servers, a request can be forwarded to a cooperating cache server if the requested object cannot be found locally. An overload condition is detected if for example, due to reference skew, some objects are in high demand by all the clients and the cache servers that contain those hot objects become overloaded due to forwarded requests. In response, the load is balanced by shifting some or all of the forwarded requests from an overloaded cache server to a less loaded one. Both centralized and distributed load balancing environments are described.

Description: LOAD BALANCING COOPERATING CACHE SERVERS Field of the Invention -»- The present invention is related to load balancing among cooperating cache servers and in particular to load balancing based on load conditions and a frequency that requests are forwarded from cooperating cache servers . Background The growth in the usage of the World Wide Web has been increasing exponentially. As a result, response times for accessing web objects can become unsatisfactorily slow. One approach to improving web access time is to employ one or more proxy cache servers between browsers and the originating web servers. Examples of proxy cache servers include a cluster of PC servers running Microsoft's Windows NT TM , such as the NETFINITY TM servers from IBM; and workstation servers running IBM' s AIX TM operating system, such as the IBM RS/6000 TM or SP/2 TM . In fact, more and more organi∑iations, such as Internet Service Providers (ISPs) and corporations, are using a collection of cooperating proxy cache servers to help improve response time as well as reduce traffic to the Internet. A collection of cooperating cache servers have distinct advantages over a single cache server in terms of reliability and performance. If one fails, requests can still be serviced by other cooperating cache servers. Requests can be distributed among the servers, thus increasing scalability. Finally, the aggregate cache size is much larger so that it is more likely that a requested object will be found in one of the cache servers. ith cooperating cache servers, a request that cannot be serviced locally due to a cache miss can be forwarded to another cache server storing the requested object. As a result, there are two kinds of requests that can come to a cache server: direct request and forwarded requests. Direct request are those that are received directly from clients. Forwarded requests are those that come from other cooperating cache servers on behalf of their clients due to cache misses on the cache servers. With requests forwarded among the cache servers, a cache server can easily become overloaded if it happens to contain in- demand (or "hot") objects that most clients are currently interested in, creating uneven workloads among the cache servers. Uneven workloads can create a performance bottleneck, as many of the cache servers are waiting for the same overloaded cache server to respond to requests forwarded to it. Therefore, there is a need for a way to perform dynamic load balancing among a collection of proxy cache servers. The present invention addresses such a need. Load balancing is traditionally done by a front-end scheduler which "evenly distributes" incoming direct requests among the cache servers. For example, load balancing can be done at the DNS level ^y manipulating a mapping table, such as is done by the NETRA^ proxy cache by Sun Microsystems ("Proxy Cache Server, Product Overview", white paper, Sun Microsystems, http://www.sun.com/). Load balancing among a cluster of servers can also be done with a front -end router, such as the NETDISPATCHER TM offered by IBM (see e.g., G. Goldszmidt and G. Hunt, "NetDispatcher: A TCP Connection Router," IBM Research Report, RC 20853, May 1997) . Here, incoming requests are distributed by the NETDISPATCHER TM to the least loaded server in the cluster. However, these traditional approaches distribute only "direct requests" and do not address a load imbalance problem resulting from too many requests for hot objects being simultaneously forwarded to the same proxy server. The present invention addresses such a need. Cooperative caching, or remote caching, has been used in distributed file systems to improve system performance (see "Cooperative caching: Using Remote Client Memory to Improve File System Performance," by M. D. Dahlin et al., Proc. of 1st Symp. on Operating Systems Design and Implementation, pp.1-14, 1994). Here, the file caches of a collection of workstations distributed on a LAN are coordinated to form a more effective overall file cache. Each workstation caches not only objects referenced by local requests but also objects that may be referenced by requests from a remote workstation. Upon a local cache miss, a local request can be sent to other client workstations where a copy can be obtained, if found. Otherwise, the object is obtained from the object server. The emphasis here is mainly how to maintain cache coherency in the face of updates and how to maintain cache hit ratios by moving a locally replaced object to the cache memory of another workstation. There is no dynamic load balancing. Cooperative caching is also used in collective proxy cache servers to reduce the access time. Upon a cache miss, instead of going directly to the originating web server potentially through a WAN, a cache server may forward the request to obtain the object from a cooperating cache server in a LAN or a regional area network. For example, upon a local cache miss in the SQUID system, a cache server multicasts a request (using the Internet Cache Protocol (ICP)) to a set of other cache servers ( see "Squid Internet Object Cache", by D. Wessels et al . , http://squid.nlanr.net/). If their caches contain the requested object, these cooperating cache servers reply with a message indicating such. The requested object is then obtained from the cooperating cache server which responded first to the request, instead of from the original web server on the Internet. However, if none replies after a time-out period, then the requested object will be fetched from the originating web server. Load imbalances can occur at a cache server due to forwarded requests. Instead of multicasting, the CRISP system uses a logical central directory to locate an object cached on another proxy server (see "Directory Structures for Scaleable Internet Caches", S. Gadde et al., Technical Report CS- 1997 -18, Dept. of Computer Science, Duke University, 1997) . Here, upon a cache miss, a cache server asks the directory server for the object, with central knowledge of the caches object storage, the directory server sends such a request to the server whose cache includes the object. If found, the object is then sent to the requesting server while the original server continues to cache the object. If no cache has a copy of the requested object, the requesting server obtains the object from the originating web server through the Internet (potentially through a WAN) . Again, this can create a load imbalance at the cache server due to subsequent requests forwarded to this cache server. Yet another way to locate an object on a cooperating cache server is through a hash function. An example is the Cache Array Routing Protocol (CARP) (see V. Valloppillil and . w. Ross, "Cache Array Routing Protocol vl.O," Internet Draft, http://ircache.nlanr.net/Cache/ICP/draft-vinod-carp-vl-03.txt, Feb. 1998) . In CARP, the entire object space is partitioned among the cooperating cache servers, with one partition for each cache server, when a request is received by a cache server from a configured client browser, a hash function is applied to a key from the request, such as the URL or the destination IP address, to identify the partition. If the hash partition is the assigned to requesting cache server, then the request is serviced locally. Otherwise, it is forwarded to the proper cache server in the identified partition. SQUID, CRISP and CARP use the caches of other proxy servers to reduce the possibility of having to go through the WAN for a missed object. They differ in the mechanism for locating a cooperating cache server whose cache may contain a copy of the requested object. Each cache server services two kinds of requests: direct requests and forwarded requests . Direct requests are those made directly from the browsers connected to the proxy server. Forwarded requests are those made by cooperating cache servers whose caches do not have the requested objects. In any event, depending on the types of objects a proxy server caches at a given moment, its CPU could be overloaded because it is busy serving both direct and forwarded requests. Summary of the Invention In accordance with the aforementioned needs, one aspect of the present invention provides a cache server load balancing method, comprising the steps of: receiving forwarded requests from a cooperating cache server in response to a cache miss for an object on the cooperating cache server; and shifting one or more of said forwarded requests for the object between cooperating cache servers based on a load condition and a forwarding frequency for the object. Another aspect of the present invention provides a method of load balancing in a collection of cooperating cache servers, where each cache server can receive direct requests and forwarded requests, and upon a cache miss, a request can be forwarded to an owning cache server caching said object, the method comprising the steps of: monitoring a load condition and a forwarding frequency for said cooperating cache servers; and shifting one or more forwarded requests from one cooperating cache server to a second cooperating cache server based on a change in the load condition and the forwarding frequency. For example, in a system including a collection of cooperating proxy cache servers, a request can be forwarded to another cooperating server if the requested object cannot be found locally. Instead of fetching the object from the originating web server through the Internet, a cache server can obtain a copy from a cooperating cache server in a local area network or an intranet. The average response time for access to an object can be significantly improved by the cooperating cache server. However, due to reference skew, some objects can be in high demand by all the clients. As a result, the proxy cache servers that contain those hot objects can become overloaded by forwarded requests coming from other proxy cache servers, creating a performance bottleneck. According to the present invention, we propose a load balancing method for a collection of cooperating proxy cache servers by shifting some or all of the forwarded requests from an overloaded cache server to a less loaded one. An example of a cache server load balancing method in accordance with the present invention includes the steps of: receiving forwarded requests from a cooperating cache server in response to a cache miss for an object on the cooperating cache server; and shifting one or more of the forwarded requests for the object between cooperating cache servers based on a load condition and a forwarding frequency for the object. The present invention also includes features for periodically monitoring the load condition on and the forwarding frequency to the owning cache server; and proactively shifting one or more subsequent forwarded requests for the cached object from the owning cache server to one or more of the cooperating cache servers, in response to the monitoring. Alternatively, the shifting step further includes the step of checking the load condition and forwarding frequency, in response to the receipt of a forwarded request. In one example, the load condition of the cooperating cache server is a weighted sum of a count of said forwarded requests, and a count of direct requests to said cooperating cache server. In another example, the cache information is maintained at: each object level; or a partition of objects level. The present embodiment includes various implementations for performing the load balancing, including both centralized and distributed environments and various hybrids thereof. For example, a distributed load monitor can be used for monitoring and maintaining a local load condition, the forwarding frequency and ownership information for cached objects on each cooperating cache server. The cooperating cache servers can periodically exchange and maintain one or more of: the load condition information; the forwarding frequency; and the ownership information. For example, the cooperating cache servers can exchange information by piggybacking one or more of: the load condition information; the forwarding frequency; and the ownership information, with one or more of the forwarded requests and responses. In another example, an overloaded cooperating cache server can identify a less loaded cooperating cache server; and communicate a shift request and a copy of the cached object to the less loaded cooperating cache server (which then caches the object), so that subsequent requests for the object will not be forwarded. Alternatively, an overloaded cooperating cache server can communicate the shift request to the less loaded cooperating cache server, which then obtains a copy of the object from an originating object server, in response to the shift request. In yet another alternative, the owning cache server can multicast the shift request message to one or more of the other cooperating cache servers so that subsequent forward requests will be shifted. In a fully distributed implementation of the present invention, the cooperating cache servers can each include a distributed load monitor for monitoring and locally maintaining load conditions, and also can maintain • the forwarding frequency and ownership information in a local copy of a caching table or by means of a hashing function. The cooperating cache servers can modify the ownership information by means of the local copy of the caching table or the hash function. The present embodiment includes still other features for modifying the ownership for the object to a shared ownership between at least two of the cooperating cache servers and forwarding subsequent object requests to one or more less loaded shared owners of the object. If a decrease in the load condition for a shared object is detected, the shared ownership can be merged, in response to the decrease in the load condition. In yet another example, the shifting of one or more of the forwarded requests based on the load condition an the forwarding frequency can be accomplished by communicating a copy of the object from the owning cache server to one or more of the cooperating cache servers, so that subsequent requests will not be forwarded (as long as the object remains in the recipient's cache). An example of a centralized environment in accordance with the present embodiment includes: a centralized logical load monitor for maintaining the forwarding frequency and the load condition for the cooperating cache servers. The load monitor can include a logical directory server for maintaining a load table for monitoring the load on the cache servers and a caching table (or hash function) for monitoring the forwarding frequency and locating objects. The directory server receives requests for object locations in other cache servers for a locally missed object and forwards requests for locally missed objects. The directory server load balances requests among the cooperating cache servers by manipulating the caching table based on the load and the forwarding frequency for a given object, in response to the requests for object locations. Brief Description of the Drawings These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description, appended claims, and accompanying drawings wherein: Figure la shows an example of a system in a block diagram form employing a collection of proxy cache servers, wherein a centralized load balancing logic according to the present invention can be applied; Figure lb shows another example of a system in a block diagram form employing a collection of proxy cache servers, where a distributed load balancing logic according to the present invention can be applied; Figures 2a-b show examples of data formats for two tables that can be maintained by the load monitor depicted in Figures la-b; Figure 3 shows an example of a logic flow for the load monitor in response to a request from a cache server because of a cache miss; and Figure 4 shows an example of a logic flow for a cache server in response to a request for an object. Detailed Description Examples of the load balancing logic of the present embodiment will be described for both centralized and distributed architectures. Figure la shows an example of a block diagram of a system employing a collection of proxy cache servers, where a centralized load balancing logic proposed in this invention can be applied. As depicted, the system includes a collection of proxy cache servers 150. Although only a single level of cache server is depicted, there could be a hierarchy of cache servers 150. As is conventional, these proxy cache servers are connected with each other through a local area network (LAN) or a regional area network or intranet 140. Each cache server 150 is also connected to a wide area network (WAN) or the Internet 110. Through the WAN, these proxy cache servers can reach 115 the originating web servers for objects that cannot be found locally on their own caches. According to the present embodiment a logical load monitor 120 includes a load balancing logic 130 for monitoring the load conditions and forwarding frequency (Fig. 2a) of the cooperating cache servers 150 and provides load balancing for them. As will be described below, various load monitor 120 features can: reside in one or more of the cache servers; be duplicated and distributed among the cache servers; or reside in another dedicated system such as a personal computer (PC) server or workstation. In a centralized system configuration, the load monitor 120 can perform a central directory function in directing forwarded requests 125 to the cache servers. One or more browsers 160 can be configured to connect to each cache server 150. Direct requests 155 are sent from the clients such as computers running conventional browsers 160 to the configured cache server 150. If the requested object can be found locally, then it is returned to the browser. Otherwise, the cache server 150 communicates a message to the load monitor 120. various example implementations of the load monitor 120 will be described in more detail below. If no load imbalance condition or trend exits, the load monitor 120 then forwards the request 125 to the cache server 150 that owns the requested object. The owning cache server then sends the requested object to the requesting cache server, e.g., via the LAN 140. If an actual load imbalance is identified, or predicted based on a loading trend, the load monitor 120 initiates a shifting of forwarded requests from the overloaded cache server to one or more underloaded (or less loaded) servers. As will be described in more detail below, the shifting of ownership can be based on the load condition of the servers 150 and the forwarding frequency, as well as other factors. Figures 2a-b shows examples of data formats of two tables maintained by the load monitor. As depicted, the tables include a load table 102, and a caching table 101. One skilled in the art will appreciate that a single table, or various other data structures could alternatively or equivalently be used. The load table 102 includes the load condition 1021 of each (A,B,C ... 1022) cache server 150 so that overloaded and underloaded servers can be identified. As is conventional, load conditions 1021 can be updated periodically by probing each cache server. The load of a cache server can be a weighted sum of the number of forwarded requests and the number of direct requests. An overloaded cache server 150 can be identified by any conventional techniques, e.g., the load monitor can compute the mean load of all proxy cache servers in past intervals. Overloaded cache servers can be those with loads exceeding a threshold above the mean load. According to the present embodiment, load balancing takes into account the amount of overloading as well as the load due to the forwarding frequency 1011 of the cached objects. This way, the load monitor can decide whether or not to continue shifting some or all forwarded requests from an overloaded cache server C 10213 to an underloaded server A 10211. The caching table 1010 includes the forwarding frequency 1011 and ownership 1012 information of an object or a partition of objects. As will be discussed below, the ownership can be single as in A 10122, or shared 10121, 10123 among two or more cooperating cache servers. The forwarding frequency 1011 represents the number of times a request for an object has been forwarded through the load monitor. In addition to the forwarding frequency 1011, the caching table 101 can also maintain a timestamp 1013, indicating the most recent time a request for an object was forwarded. Further, the caching information for an object or a partition 1010 can include a forwarding frequency over a given time period (count/time) for the object ID or partition ID 1010 through the load monitor 120. Object partitions 1010 can alternatively be based on a hash function on object identifiers, or can be based on the directory structures that objects are organized by on the web servers. In the case of a partition, any object belonging to a partition will be forwarded by the load monitor. The shifting of ownership can be based on the load condition of the servers, the forwarding frequency 1011 and other information such as the time stamp information. Figure 3 shows an example of a logic flow for steps taken by the load monitor 120 in response to a request 125 from a cache server 150 because of a cache miss. As depicted, in step 201, it checks to see if the requested object/partition can be found in the caching table. If not, in step 202, a new entry is created for the object/partition and a cache server is assigned as its owner. After the entry is located in the caching table, in step 203, the forwarding frequency 1011 is updated, e.g., incremented by 1. The load monitor then examines the load table 102 to see if the owner is currently overloaded (and that the forwarding frequency 1011 is a significant contributor thereto), in step 204. If yes, in step 205, the load monitor finds an underloaded (or less loaded) cache server and assign it as the new 10122 (or shared) owner 10122 of the requested object. The ownership information 1012 for the object in the caching table 101 is updated accordingly. Those skilled in the art will appreciate that the logic flow could comprise a shared 10123 or hierarchical ownership 1012 in the caching table 101 or other data structure employed. The request (possibly with a copy of the requested object) can then be forwarded 125 to a new sole 10122 (or shared 10123) owner, in step 206. Alternatively, the new owner can be requested to obtain 115 an object copy from the originating object server, e.g., via the Internet 110. Those skilled in the art will appreciate that the load checking step 204 can be performed proactively, i.e., periodically or in response to an identified overload or overload trend 1021 - due at least in part to a high forwarding frequency 1011 - for a given object id/partition id 1010 and cache server (ownership 1012) . If so, then in step 205, the load monitor finds an underloaded (or less loaded) cache server, assigns it as the new (or shared) owner of the requested object, and possibly sends a copy of the object to the new (or shared) owner as above. Conversely, if a shared ownership model is used, in step 208, when the load condition 10211 and forwarding frequency 10111 for a shared ownership object (p 10101) drops below a predetermined threshold, in step 209, the shared ownership (B, A 10121) can be merged to a single ownership and one of the copies purged from one of the cache servers A 10121, e.g., to make room for another hot obj ect . Figure 4 shows an example of a logic flow for a cache server when a request for an object is received, either directly 155 from a browser 160 or forwarded 125 from the load monitor 120. As depicted, in step 301, it first checks to see if the requested object can be found locally in its cache. If yes, in step 302, it returns the object and the process ends, in step 306. Otherwise, in step 303, it checks to see if the request is a direct request or a forwarded request. If it is a direct request, in step 304, the request is sent to the load monitor and the process ends, in step 306. On the other hand, if the request is a forwarded request, in step 305, the cache server will fetch the object from the originating web server and return the object. The process then ends, in step 306. Referring now to Figures la and 2a-b, assume for example, a browser 160 connecting to a cache server C 10223 requests 155 an object p 10101. From the caching table 101, it can be seen that object p 10101 is not cached on server C, but it is cached on ("owned" by) cache server B (assuming B, A 10121 is initially only solely designated by B) . In response to a cache miss on object p, server C 10223 sends a request to the load monitor 120 for object p. Depending on the load condition 10212 and forwarding frequency 1011 of requests for p 10101 on server B, the load monitor may forward the request to server B, asking it to send a copy of object p to server C. Or, if server B is currently overloaded or is trending as such, the load monitor might shift the forwarded request by finding an underloaded (or less loaded) server to serve as a new (or shared as in B, A 10121) owner of object p. The request is then forwarded to the new (or shared e.g., A) owning server for the object. Note that even after the transfer of ownership, a copy of object p is still on server B's cache and can still serve direct requests coming to server B. However, in this example, all future forwarded requests for object p (or perhaps some, in the case of a shared ownership) will be shifted to server A. Alternatively, in the case of shared ownership B, A 10121, future forwarded requests for object p 10101 can be sent to the less loaded server. Now that a load balancing method according to the present embodiment has been described for a collection of proxy cache servers where a logical central directory is used for locating an object, various alternatives will be considered. The present invention can be adapted to achieve load balancing for these systems as well. For example, the present invention can be configured to perform load balancing for a collection of cooperating proxy cache servers where each cache server 150 multicasts to a list of cooperating cache servers to locate a copy of a locally missed object. In this case, no specific ownership information need be maintained anywhere in the system. However, there is also no guarantee of finding an object from the cooperating cache servers, either. Assume that a logical load monitor 120 is used to maintain the load conditions 1021 of all proxy cache servers and share this information with each cache server 150. The load balancing can be achieved by excluding overloaded servers from the list of cooperating servers to which a cache server multicasts its request (also called a shift request) . As a result, only less loaded cache servers will receive forwarded requests 125. Another alternative is a load balancing method for a collection of cooperating proxy cache servers where a hash function is used to locate a copy of a locally missed object. In this case, the object space can be partitioned among the cooperating proxy cache servers 150, with one partition for each cache server. In order to achieve load balancing by shifting forwarded requests, one can change the hash function so that forwarded requests will not go to overloaded servers. One preferred approach is to hash the object space into a large number of buckets, much larger than the total number of proxy cache servers. These hash buckets are then assigned to the cache servers, with the goal of balancing the loads among them. Periodically, one can move one or more hash buckets from one overloaded server to an underloaded server, effectively changing the hash function. In either case, the load condition of the cooperating cache server can factor in the forwarding frequency directly into the calculated load condition. For example, the load condition can be a weighted sum of a count of said forwarded requests, and a count of direct requests to said cooperating cache server. Alternatively, the load monitor could separately maintain the overall forwarding frequency for each cooperating cache server. Referring now to Figures lb and 2a-b, yet another alternative is a load monitor 120 that is distributed, i.e., wherein some or all the load monitor is duplicated across the cache servers 150. In one example, the distributed load monitor includes local load condition information 1021 (and as described below, possibly the load conditions of all (A, B, C, ... 1022)) of the cooperating cache servers 150. The distributed load monitor 120' preferably also includes the caching table 101 with the forwarding frequency 1011 and ownership 1012 information for each object id/partition id 1010. Alternatively, a hashing function, for example as described above, could be distributed and stored in the cache servers. Load condition information 1021 and/or caching information 101: can be exchanged periodically; when there is a change in status (ownership or significant change in load condition) ; or piggybacked with cache forwarding requests and responses. Load condition 1021 information could also have a time stamp (not shown) associated with it for tracking or other purposes . Here, if a cache server 150 has a cache miss, the local load monitor 120' looks up the ownership of the requested object in its local caching table 101 and forwards the request, to the owning cache server. Alternatively, the hash function could be applied to a key from the request, such as the URL or the destination IP address, to identify the partition and the request then forwarded to the correct cache server. When the forwarded request (i.e., from a cache server who had a cache miss) is received, the owning cache server identifies it as a forwarded request (e.g., by identifying it as from another cache server as opposed to a client) and updates its forwarding frequency 1011 information as applicable (Fig. 3, step 203). If an overload trend or condition is indicated (step 204) , the owning cache server can respond to the requesting cache server with a shift request and a copy of the cached object. Alternatively, the requesting cache server can obtain a copy from the originating object server via an intranet, WAN or Internet 110. In either case, when the forwarding server caches a copy of the object, this server will no longer issue forward requests (steps 301, 302) as long as it remains in the cache, thus proportionally reducing the load on the owning server. In addition, the owning cache server can multicast a shift request message to one or more of the other cooperating cache servers 150 so that subsequent forward requests will be shifted, e.g., by updating their local copy of the caching table or modifying the hash function (step 205) . At this point, other cache servers can forward their requests to the new owner (or to the lease loaded owner of two or more cache servers 150 if ownership is shared) as indicated in their local copy of the caching table 101. When the original cache owner's load has decreased to an acceptable level (step 204), e.g., as indicated by a threshold, the shared ownership information can be merged to its original state (e.g., B,A 10121 --> B) . In the case that the load condition information 1021 for all cache servers ( A,B,C ... 1022) is fully distributed, the requesting cache server could proactively check the load condition (and associated time stamp) of the owning server (step 204), i.e., before forwarding the request. If overloaded, the requesting server could request a copy of the object from the owning server (or from the originating server via the intranet or Internet 110) and possibly a load condition confirmation. The owning cache server could update its caching table 101 or modify the hash function to indicate the new shared ownership (step 205) . The requesting server (or the owning server) could then multicast a message to all other cache servers 150 indicating the new shared ownership of the object and possibly include an updated load condition. At this point, other cache servers would update their caching tables 101 or modify the hash function to indicate the new shared ownership (step 202) , and can forward their requests (step 206) to the least loaded owner of two or more cache servers 150 sharing ownership as indicated in their local copy of the caching table 101. when a shared cache owner's load has decreased to an acceptable level (steps 204 and 208), e.g., as indicated by a threshold, the ownership information can be merged to its original state, in step 209. A preferred embodiment of the present invention includes features that can be implemented as software tangibly embodied on a computer program product or program storage device for execution on a processor (not shown) provided with cache server 150 or other computer embodying the load monitor 120, such as in the centralized model described. For example, software implemented in a popular object-oriented computer executable code such as JAVA provides portability across different platforms. Those skilled in the art will appreciate that many other compiled or interpreted, procedure-oriented and/or object-oriented (00) programming environments, including but not limited to REXX, C, C++ and Smalltalk can also be employed. Those skilled in the art will also appreciate that methods of the present embodiment may be the software may be embodied on a magnetic, electrical, optical, or other persistent program and/or data storage device, including but not limited to: magnetic disks, Direct Access Storage Devices (DASD) , bubble memory; tape; optical disk formats such as CD-ROMs and DVD; and other persistent (also called nonvolatile) storage devices such as core, ROM, PROM, flash memory, or battery backed RAM. Those skilled in the art will appreciate that within the spirit and scope of the present invention, one or more of the components instantiated in the memory of the server 120' could be accessed and maintained directly via disk (not shown) , the network, another server, or could be distributed across a plurality of servers. In summary, in a system including a collection of cooperating cache servers, such as proxy cache servers, a request can be forwarded to a cooperating cache server if the requested object cannot be found locally. An overload condition is detected if for example, due to reference skew, some objects are in high demand by all the clients and the cache servers that contain those hot objects become overloaded due to forwarded requests. In response, the load is balanced by shifting some or all of the forwarded requests from an overloaded cache server to a less loaded one. Both centralized and distributed load balancing environments are described. While we have described our preferred embodiments of our invention with alternatives, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first disclosed.

Claims: CLAIMS 1. A cache server load balancing method, comprising the steps of: receiving forwarded requests from a cooperating cache server in response to a cache miss for an object on the cooperating cache server; and shifting one or more of said forwarded requests for the object between cooperating cache servers based on a load condition and a forwarding frequency for the object. 2. The method of claim 1, said shifting step further comprising the steps of: periodically monitoring the load condition on and the forwarding frequency to an owning cache server; and proactively shifting one or more subsequent forwarded requests for the cached object from the owning cache server to one or more of said cooperating cache servers, in response to said monitoring. 3. The method of claim 1 or 2, said shifting step further comprising the step of checking the load condition and forwarding frequency, in response to the forwarded request. 4. The method of claim 1, 2 or 3, wherein said shifting comprises the step of modifying an ownership for the object to a shared ownership between two or more of said cooperating cache servers. 5. The method of claim 4, further comprising the step of merging said shared ownership in response to change in the load condition. 6. The method of any of claims 1 to 5, further comprising the step of locally monitoring the load on each cooperating cache server. 7. The method of claim 6, further comprising the step of: a distributed load monitor monitoring and maintaining a local load condition, the forwarding frequency and ownership information for cached objects on said each cooperating cache server. 8. The method of claim 7, further comprising the steps of: said cooperating cache servers periodically exchanging and maintaining one or more of: the load condition information; the forwarding frequency; and the ownership information. ^ 9. The method of claim 7, further comprising the steps of: said cooperating cache servers exchanging by piggybacking one or more of: the load condition information; the forwarding frequency; and the ownership information; with one or more of the forwarded requests and responses. 10. The method of any of claims 1 to 9 , further comprising the step of: receiving a forwarded request and updating the forwarding frequency. 11. The method of claim 7, 8, 9 or 10, further comprising the steps of: identifying a less loaded cooperating cache server; and communicating one or more of: a shift request; and a copy of the cached object, to said less loaded cooperating cache server. 12. The method of claim 11, further comprising the steps of: said less loaded cooperating cache server receiving said shift request; and said less loaded cooperating cache server requesting a copy of the object from an originating object server, in response to said shift request. 13. The method of claim 11, wherein the copy is obtained via one or more of an intranet, WAN or Internet. 14. The method of any of claims 1 to 13, further comprising the step of multicasting a shift request message to one or more of the other cooperating cache servers so that subsequent forward requests will be shifted. 15. The method of claim 14, further comprising the step of: the cooperating cache servers maintaining one of a local copy of a caching table and modifying a hash function; and the cooperating cache servers modifying the ownership information by one of: updating a local copy of a caching table; and modifying a hash function. 16. The method of claim 15, further comprising the steps of: modifying the ownership for the object to a shared ownership between at least two of said cooperating cache servers; and said cooperating cache servers forwarding subsequent object requests to one or more less loaded shared owners of the object. 17. The method of claim 16, further comprising the steps of: detecting a decrease in the load condition for a shared object; and merging the shared ownership, in response to the decrease in the load condition. 18. The method of any of claims 1 to 17, wherein said shifting one or more of said forwarded requests comprises the steps of: communicating a copy of the object from an owning cache server to one or more of said cooperating cache servers; and said cooperating cache server receiving and caching the copy of the object. 19. The method of any of claims 1 to 18, further comprising the steps of: calculating the load condition of each cache server in past intervals; computing a mean load of all cache servers in past intervals; and finding the cache servers that exceed a threshold above said mean load. 20. The method of any of claims 1 to 19, wherein the load condition of said cooperating cache server can be a weighted sum of a count of said forwarded requests, and a count of direct requests to said cooperating cache server. 21. The method of any of claims 1 to 20, further comprising the step of maintaining cache information at one or more of: each object level; and a partition of objects level. 22. The method of claim 21, wherein said cache information of said object level or said partition comprises the forwarding frequency associated with the object. 23. The method of claim 22, further comprising the step of: a distributed load monitor monitoring and locally maintaining load conditions, forwarding frequency and ownership information for cached objects on each cache server. 24. The method of claim 23, further comprising the steps of: said cooperating cache servers periodically exchanging one or more of the load condition, the forwarding frequency and the ownership information. 25. The method of claim 22,23 or 24 further comprising the steps of: said cooperating cache servers exchanging by piggybacking one or more of: the load condition; the forwarding frequency; and the ownership information; with one or more of the forwarded requests and responses. 26. A method of load balancing in a collection of cooperating cache servers, where each cache server can receive direct requests and forwarded requests, and upon a cache miss, a request can be forwarded to an owning cache server caching said object, the method comprising the steps of: monitoring a load condition and a forwarding frequency for said cooperating cache servers; and shifting one or more forwarded requests from one cooperating cache server to a second cooperating cache server based on a change in the load condition and the forwarding frequency. 27. The method of claim 26, wherein said step of monitoring the load condition comprises the steps of: calculating the load condition of each cache server in past intervals; computing a mean load of all proxy cache servers in past intervals; and f inding those proxy cache servers that exceed a threshold above said mean load . -»- 28. The method of claim 26 or 27, wherein said shifting step can be performed in response to one or more of: said forwarded requests from said cooperating cache servers; and periodically monitoring the load condition and the forwarding frequency. 29. The method of claim 26 or 27, further comprising the step of a centralized logical load monitor maintaining the forwarding frequency and the load condition for the cooperating cache servers. 30. The method of claim 26, 27, 28 or 29 wherein the load condition of said cache server can be a weighted sum of: a count of forwarded requests; and a count of direct requests to said cache server. 31. The method of claim 26, 27, 28 or 29 further comprising the step of maintaining cache information at each object level or at a partition of objects level. 32. The method of claim 31, wherein said cache information of the object level or the partition level comprises the forwarding frequency of requests through said load monitor to said object. 33. The method of any of claims 26 to 32, wherein said cooperating cache servers comprise cooperating proxy cache servers. 34. The method of any of claims 26 to 32, further comprising the steps of: a logical directory server maintaining a caching table and a load table; said cache servers interrogating said directory server for object locations in other cache servers for a locally missed object; and said directory server load balancing requests among said cache servers by manipulating said caching table, in response to requests for object locations. 35. The method of claim 29, further comprising the steps of: each cache server multicasting to a list of cooperating cache servers to locate a copy of a locally missed object; and said shifting step comprising the step of excluding overloaded cache servers from a subset of neighboring cache servers for multicasting. 36. A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for cache server load balancing, said method steps comprising: receiving forwarded requests from a cooperating cache server in response to a cache miss for an object on the cooperating cache server; and shifting one or more of said forwarded requests for the object between cooperating cache servers based on a load condition and a forwarding frequency for the object.

Inventors: JORDAN KEVIN MICHAEL, WU KUN-LUNG, YU PHILIP SHI-LUNG, JORDAN, KEVIN, MICHAEL, WU, KUN-LUNG, YU, PHILIP, SHI-LUNG
Applicants: IBM, IBM UK, IBM UNITED KINGDOM LIMITED, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1999
Main classification: G06F9
IPC codes: G06F9, G06F12, G06F13