Abstract:
Described is a technology by which a web proxy server evaluates its cached objects, and when an object is invalid, performs a freshness check on that object, independent of any client requests. As a result, the cache contains objects that have a greater likelihood of being fresh when requested by a client. By scanning a web cache data structure to determine whether corresponding cached content is still valid, and sending a freshness check to a web server when the content is not valid, the cache is kept up to date. The scanning may be periodic or based upon some other triggering event, and all of the cache&#39;s corresponding entries may be scanned, or some smaller subset of the entries. In one example implementation, a web proxy server that contains the cache includes a freshness check mechanism that scans and keeps the cached objects up to date.

Description:
BACKGROUND 
       [0001]    One type of web proxy product accelerates clients&#39; access to web content via web caching. In general, these products cache web objects that were returned to clients, and use those cached objects for subsequent client requests, thereby saving the expense of making additional calls to the web server that provides the content. 
         [0002]    However, sometimes when a requested object exists in the cache, the object is not valid to be served as a result of it being too old, as indicated by a timestamp. In this manner, users are protected against being served content that is obsolete, as generally determined by the website designer, e.g., a news site may only allow certain content to be considered valid in a cache for a few minutes, whereas a page that is changed weekly may allow its objects to be cached until the next weekly change. 
         [0003]    When an object is too old, the web proxy performs a “freshness” check, by sending a special HTTP request to the web server. If the object is still valid, the server returns a new timestamp for the object, otherwise the server returns the entire object that has changed. The process of freshness checking and possible object downloading to update the cache can be time consuming, particularly in high latency situations in which the connection between the web proxy and remote web server is slow. 
       SUMMARY 
       [0004]    This Summary is provided to introduce a selection of representative concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used in any way that would limit the scope of the claimed subject matter. 
         [0005]    Briefly, various aspects of the subject matter described herein are directed towards a technology by which a web proxy server performs freshness checks on its cached objects, independent of any client requests, whereby the cache contains objects that have a greater likelihood of being fresh when requested by a client. By evaluating data in a web cache data structure to determine whether content in a web cache corresponding to that data is still valid, and sending a freshness check to a web server when the content is not valid, the cache is kept up to date. The scanning may be periodic or on some other triggering event, and all of the cache&#39;s corresponding entries may be scanned, or some smaller subset thereof. 
         [0006]    In one example implementation, a web proxy server that receives requests from a client for content directed towards a web server includes a freshness check mechanism. The freshness check mechanism evaluates the web proxy server&#39;s cached content, and updates the cache with new content (or new freshness data) when invalid content is found in the cache. As a result, the cache, which is used for serving cached content in response to client requests, is updated independent of a pending client request for that content. 
         [0007]    Other advantages may become apparent from the following detailed description when taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention is illustrated by way of example and not limited in the accompanying figures in which like reference numerals indicate similar elements and in which: 
           [0009]      FIG. 1  shows an illustrative example of a network having a web proxy server with proactive freshness checking. 
           [0010]      FIG. 2  is a flow diagram representing example steps taken by a web proxy server to check cached objects for freshness. 
           [0011]      FIG. 3  shows an illustrative example of a general-purpose network computing environment into which various aspects of the present invention may be incorporated. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Various aspects of the technology described herein are generally directed towards increasing useful cache hits in a web proxy server by proactively working to keep cached content valid, rather than reactively in response to a client request. This eliminates or dramatically reduces the number of times the web proxy server needs to perform a freshness check on behalf of a waiting client. 
         [0013]    In one example implementation, a freshness checking mechanism of the web proxy server operates in the background, actively scanning the objects stored in the cache engine looking for invalid objects. However, rather than performing an active scan of all objects, it is alternatively feasible to have other triggers, and/or to configure a scanner in numerous ways. For example, a data structure that contains information on the cached objects may be sorted into an event list, with an event that triggers a freshness check on only those objects that have timestamps indicating a freshness check is needed. Alternatively, the objects may be sorted into subsets that are scanned at different frequencies depending on their timestamps, e.g., check one subset every minute, check another subset every half-hour, check another subset every day. 
         [0014]    Thus, as will be understood, the technology described herein is not limited to any type of configuration, any type of looping model or any type of event driven model. As such, the present invention is not limited to any particular embodiments, aspects, concepts, structures, functionalities or examples described herein. Rather, any of the embodiments, aspects, concepts, structures, functionalities or examples described herein are non-limiting, and the present invention may be used various ways that provide benefits and advantages in computing and accessing network content in general. 
         [0015]    Turning to  FIG. 1 , there is shown an example network configuration in which clients  102   1 - 102   n  issue requests for content to a web server  110 . A web proxy server  120  (e.g., an edge server such as an Internet Security and Acceleration, or ISA Server available from Microsoft Corporation), receives the requests from the clients  102   1 - 102   n . The clients  102   1 - 102   n  may have no knowledge of the presence of the web proxy server  120 , that is, the web proxy server is transparent, although it is feasible to have one or more of the clients  102   1 - 102   n  make requests to the web proxy server  120  to perform some operation on behalf of the clients  102   1 - 102   n . 
         [0016]    When the web proxy server  120  first receives a web request from the client (e.g.,  102   1 ), a request/response handler  122  in the web proxy server  120  searches a local cache  124  data structure  124  to see if the requested content is cached and still valid. If so, the content (e.g., a main page or an embedded object described thereon) is returned from the cache  126 . If not, a freshness check is sent to the web server, to either obtain an updated object or a new timestamp that verifies the object is still valid. This aspect is conventional caching for efficiency purposes. 
         [0017]    Rather than wait for a client request before determining whether requested content is valid, the web proxy server  120  includes a freshness check mechanism  128  that operates (without waiting for a client request) to update any invalid objects in the cache  126 , either with a new object and associated metadata in the cache data structure  124 , or by updating the data structure  124  with changed metadata, including a timestamp indicating the object is still valid. As a result, (and depending on frequency of checking), most objects in the cache  126  are fresh, and can be served from the cache  126  without the need to perform a freshness check while the user is waiting. 
         [0018]    Note that what is considered “invalid” need not be the same as actually invalid. For example, if a scan is performed every five minutes, and an object is going to be invalid before the next scan, that object can be considered invalid for purposes of freshness checking. However, the web server may return the same timestamp, in which event the freshness check request is inefficient, and thus a balance between various factors such as scanning frequency, web request latency, client demands and so forth may help decide on whether to consider an almost invalid object as being invalid with respect to sending a freshness check. 
         [0019]    Turning to  FIG. 2 , the exemplified freshness check mechanism  128  in the web proxy server  120  scans each of the entries (step  202 ) in the cache data structure  124  looking for invalid entries (step  204 ). Note that there may be several data structures and/or ways of viewing the data within such a data structure (e.g., by ordering, filtering and/or sorting) that can make this scanning action more efficient. For example, ordering the data structure from the soonest to expire (first) and the longest to expire (last) will send freshness checks in an order that may be more efficient. As another example, ordering and grouping the entries by timestamp can allow selection of a range or ranges of invalid or possibly invalid entries, eliminating the need to individually check the timestamps of known valid entries. Further, HTTP pipelining techniques or the like may be used to efficiently check the status of several web objects at the same time. 
         [0020]    Once an invalid entry is detected at step  204 , the web proxy initiates a “standard” freshness check at step  206 . If a new object and accompanying metadata is returned (step  208 ), the object is added to the cache at step  210 , and the cache data structure (or possibly multiple data structures) updated at step  214  with the changed metadata. Otherwise metadata alone is returned (step  212 ), whereby the cache data structure is updated at step  214 , including to contain the new timestamp. Note that error conditions are not described herein for purposes of simplicity, however it can be understood that retries may be sent following the “no” branch of step  212 , and objects and/or metadata that are still not found can be removed from the cache. 
         [0021]    Further, it should be noted that the proactive freshness check initiated by the freshness check mechanism  128  is not considered a client request with respect to maintaining the information in the cache. More particularly, because of size limitations, cache management systems remove an object based on when the object was last requested, whereby the cache maintains more recently requested objects over those not requested for some time. Thus, an object request initiated from the freshness check mechanism  128  is not considered as being a client request for that object, otherwise the cache management system would be unable to distinguish which objects are to be kept in the cache based on a recently requested priority. 
         [0022]    Step  216  represents delaying, such as to periodically repeat the scan rather than continuously scan. Depending on the scanning frequency, the background freshness checking mechanism may dramatically reduce the number of times a cache entry is requested but it is found to be invalid. Note that the scanning frequency need not be periodic, but can be repeated on any appropriate basis, such as based upon how many users are presently sending web requests, how many entries are in the cache, how quickly or slowly web requests are being handled, and/or virtually any other measurable criteria. 
         [0023]    Moreover, as described above, all cache entries may be scanned per scanning process, or a scanning process may alternatively only scan a subset of entries. For example, the timestamps may be used to group entries into subsets so that only entries that have a possibility of being invalid during a scan need to be evaluated. 
       Exemplary Operating Environment 
       [0024]      FIG. 3  illustrates an example of a suitable computing system environment  300  on which the web proxy server  120  ( FIG. 1 ) or  121  ( FIG. 2 ) may be implemented, for example. The computing system environment  300  is only one example of a suitable computing environment and is not intended to suggest any limitation as to the scope of use or functionality of the invention. Neither should the computing environment  300  be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary operating environment  300 . 
         [0025]    The invention is operational with numerous other general purpose or special purpose computing system environments or configurations. Examples of well known computing systems, environments, and/or configurations that may be suitable for use with the invention include, but are not limited to: personal computers, server computers, hand-held or laptop devices, tablet devices, multiprocessor systems, microprocessor-based systems, set top boxes, programmable consumer electronics, network PCs, minicomputers, mainframe computers, distributed computing environments that include any of the above systems or devices, and the like. 
         [0026]    The invention may be described in the general context of computer-executable instructions, such as program modules, being executed by a computer. Generally, program modules include routines, programs, objects, components, data structures, and so forth, which perform particular tasks or implement particular abstract data types. The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in local and/or remote computer storage media including memory storage devices. 
         [0027]    With reference to  FIG. 3 , an exemplary system for implementing various aspects of the invention may include a general purpose computing device in the form of a computer  310 . Components of the computer  310  may include, but are not limited to, a processing unit  320 , a system memory  330 , and a system bus  321  that couples various system components including the system memory to the processing unit  320 . The system bus  321  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. By way of example, and not limitation, such architectures include Industry Standard Architecture (ISA) bus, Micro Channel Architecture (MCA) bus, Enhanced ISA (EISA) bus, Video Electronics Standards Association (VESA) local bus, and Peripheral Component Interconnect (PCI) bus also known as Mezzanine bus. 
         [0028]    The computer  310  typically includes a variety of computer-readable media. Computer-readable media can be any available media that can be accessed by the computer  310  and includes both volatile and nonvolatile media, and removable and non-removable media. By way of example, and not limitation, computer-readable media may comprise computer storage media and communication media. Computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules or other data. Computer storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can accessed by the computer  310 . Communication media typically embodies computer-readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Combinations of the any of the above should also be included within the scope of computer-readable media. 
         [0029]    The system memory  330  includes computer storage media in the form of volatile and/or nonvolatile memory such as read only memory (ROM)  331  and random access memory (RAM)  332 . A basic input/output system  333  (BIOS), containing the basic routines that help to transfer information between elements within computer  310 , such as during start-up, is typically stored in ROM  331 . RAM  332  typically contains data and/or program modules that are immediately accessible to and/or presently being operated on by processing unit  320 . By way of example, and not limitation,  FIG. 3  illustrates operating system  334 , application programs  335 , other program modules  336  and program data  337 . 
         [0030]    The computer  310  may also include other removable/non-removable, volatile/nonvolatile computer storage media. By way of example only,  FIG. 3  illustrates a hard disk drive  341  that reads from or writes to non-removable, nonvolatile magnetic media, a magnetic disk drive  351  that reads from or writes to a removable, nonvolatile magnetic disk  352 , and an optical disk drive  355  that reads from or writes to a removable, nonvolatile optical disk  356  such as a CD ROM or other optical media. Other removable/non-removable, volatile/nonvolatile computer storage media that can be used in the exemplary operating environment include, but are not limited to, magnetic tape cassettes, flash memory cards, digital versatile disks, digital video tape, solid state RAM, solid state ROM, and the like. The hard disk drive  341  is typically connected to the system bus  321  through a non-removable memory interface such as interface  340 , and magnetic disk drive  351  and optical disk drive  355  are typically connected to the system bus  321  by a removable memory interface, such as interface  350 . 
         [0031]    The drives and their associated computer storage media, described above and illustrated in  FIG. 3 , provide storage of computer-readable instructions, data structures, program modules and other data for the computer  310 . In  FIG. 3 , for example, hard disk drive  341  is illustrated as storing operating system  344 , application programs  345 , other program modules  346  and program data  347 . Note that these components can either be the same as or different from operating system  334 , application programs  335 , other program modules  336 , and program data  337 . Operating system  344 , application programs  345 , other program modules  346 , and program data  347  are given different numbers herein to illustrate that, at a minimum, they are different copies. A user may enter commands and information into the computer  310  through input devices such as a tablet, or electronic digitizer,  364 , a microphone  363 , a keyboard  362  and pointing device  361 , commonly referred to as mouse, trackball or touch pad. Other input devices not shown in  FIG. 3  may include a joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to the processing unit  320  through a user input interface  360  that is coupled to the system bus, but may be connected by other interface and bus structures, such as a parallel port, game port or a universal serial bus (USB). A monitor  391  or other type of display device is also connected to the system bus  321  via an interface, such as a video interface  390 . The monitor  391  may also be integrated with a touch-screen panel or the like. Note that the monitor and/or touch screen panel can be physically coupled to a housing in which the computing device  310  is incorporated, such as in a tablet-type personal computer. In addition, computers such as the computing device  310  may also include other peripheral output devices such as speakers  395  and printer  396 , which may be connected through an output peripheral interface  394  or the like. 
         [0032]    The computer  310  may operate in a networked environment using logical connections to one or more remote computers, such as a remote computer  380 . The remote computer  380  may be a personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computer  310 , although only a memory storage device  381  has been illustrated in  FIG. 3 . The logical connections depicted in  FIG. 3  include one or more local area networks (LAN)  371  and one or more wide area networks (WAN)  373 , but may also include other networks. Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet. 
         [0033]    When used in a LAN networking environment, the computer  310  is connected to the LAN  371  through a network interface or adapter  370 . When used in a WAN networking environment, the computer  310  typically includes a modem  372  or other means for establishing communications over the WAN  373 , such as the Internet. The modem  372 , which may be internal or external, may be connected to the system bus  321  via the user input interface  360  or other appropriate mechanism. A wireless networking component  374  such as comprising an interface and antenna may be coupled through a suitable device such as an access point or peer computer to a WAN or LAN. In a networked environment, program modules depicted relative to the computer  310 , or portions thereof, may be stored in the remote memory storage device. By way of example, and not limitation,  FIG. 3  illustrates remote application programs  385  as residing on memory device  381 . It may be appreciated that the network connections shown are exemplary and other means of establishing a communications link between the computers may be used. 
         [0034]    An auxiliary subsystem  399  (e.g., for auxiliary display of content) may be connected via the user interface  360  to allow data such as program content, system status and event notifications to be provided to the user, even if the main portions of the computer system are in a low power state. The auxiliary subsystem  399  may be connected to the modem  372  and/or network interface  370  to allow communication between these systems while the main processing unit  320  is in a low power state. 
       CONCLUSION 
       [0035]    While the invention is susceptible to various modifications and alternative constructions, certain illustrated embodiments thereof are shown in the drawings and have been described above in detail. It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention.