Abstract:
A facility for determining whether to consistency-check a cache entry is described. The facility randomly or pseudorandomly selects a value in a range. If the selected value satisfies a predetermined consistency-checking threshold within the range, the facility consistency-checks the entry, and may decide to propagate this knowledge to other cache managers. If, on the other hand, the selected value does not satisfy the consistency-checking threshold, the facility determines not to consistency-check the entry.

Description:
TECHNICAL FIELD 
       [0001]    The described technology is directed to the field of processing requests for data. 
       BACKGROUND 
       [0002]    Caching generally refers to the local storage of data obtained from a remote source to streamline access to this data.  FIG. 1  is a network diagram that shows a typical architecture for caching data. It shows a number of data provider nodes such as data providers  110 ,  120 , and  130  that each provide data to a number of data consumer nodes. For example, data provider  110  provides data to consumers including consumers  112  and  113 . Each data provider further has a local cache. For example, provider  110  has local cache  111 . When a data provider receives a primary data request from one of its consumers, it first checks its local cache to see whether the primary data request can be satisfied from the cache. If so, the provider responds to the primary data request with a response that is based upon cache contents; otherwise, the provider (1) issues a secondary data request to an authoritative data source, (2) responds to the primary data request with a response that is based upon data provided by the data source in response to the secondary data request, and (3) stores the data provided by the data source in response to the secondary data request as an entry in its local cache for use to satisfy future primary data requests. In some cases (not shown), multiple providers may share the same cache. In some cases (not shown), a cache may manage itself, or may be managed by an entity other than the corresponding data provider. 
         [0003]    As one example, each data consumer may be a web client requesting web pages or other resources from data providers that are web servers. Data sources may be central data stores or services that provide data that enables the web servers to construct web pages or other resources requested by web clients. Such an architecture employing local caching is described in U.S. patent application Ser. No. 11/177,206, filed on Jul. 8, 2005, which is hereby incorporated by reference in its entirety. Alternatively, data providers may be proxy servers or other intermediate caches—such as caches stored on the same computer system that the web client executes on—that are for one or more web clients and web servers. 
         [0004]    The level of efficiency provided by caching data in an environment such as is described above is heavily dependent on how each cache is managed—in particular, the basis used by the cache and/or its data provider to determine for how long a version of a data object stored in a cache entry should be used to satisfy primary requests. 
         [0005]    Conventional schemes for maintaining the freshness of cache entries have significant disadvantages. A first conventional scheme involves the use of an object lifetime for each cached data object—a maximum amount of time that the data object should be kept in a cache before expiring the entry containing the object (sometimes referred to as the object&#39;s “time to live”). In this scheme, either a data source supplying the data object assigns such an amount of time to the object, or the data provider assigns an amount of time. When a cache entry containing the data object is added to the cache, the assigned amount of time is added to the current time to establish an expiration time for that entry. After the entry&#39;s expiration time passes, the entry is expired, and can no longer be retrieved from the cache, and the space it occupies in the cache can be reclaimed for new cache entries. 
         [0006]    It is often difficult to select object lifetimes that produce good results. On one hand, if an object lifetime is selected that is too long, the cached object could become “stale”—i.e., it no longer matches the object that the data source that provided it would provide in response to a new secondary data request for the object—and it is of less value to requesting data consumers. On the other hand, if an object lifetime is selected that is too short, the likelihood that the cached object will ever be used to satisfy a primary data request is low, and little benefit is derived from caching the object. 
         [0007]    A second conventional scheme for maintaining the freshness of cache entries involves the use of explicit invalidations. Explicit invalidations are notifications sent by a data source to data providers when the data source determines that the version of an object that it would provide in response to a secondary data request has changed, and any cached earlier-provided versions of that object provided by the data source are now stale. When a data provider receives an explicit invalidation notification from a data source, it immediately invalidates the referenced object from its cache, if it is contained there. 
         [0008]    While the explicit invalidation scheme has the capacity to overcome the disadvantages associated with the object lifetime scheme, the explicit invalidation scheme has the disadvantage that it consumes substantial processing and/or communication resources. First, detecting a change in the object and sending explicit invalidations often consumes significant processing power on the part of the data source. Once the data source has detected this change, it can either (1) transmit an explicit invalidation to all data providers, incurring significant network bandwidth to convey the explicit invalidation and significant data provider processing costs to act on the explicit invalidation, or (2) itself spend additional processing and storage resource determining which data providers are most likely to have a stale version of the object cached, and send explicit invalidations to only those data providers. 
         [0009]    An approach to maintaining the freshness of cache entries that overcame these disadvantages of conventional schemes for invalidating cache entries would have significant utility. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a network diagram that shows a typical architecture for caching data. 
           [0011]      FIG. 2  is a block diagram showing some of the components typically incorporated in at least some of the computer systems and other devices on which the facility executes. 
           [0012]      FIG. 3  is a timeline diagram illustrating the operation of the facility. 
           [0013]      FIG. 4  is a flow diagram showing steps typically performed by the facility to manage a cache while responding to primary requests. 
           [0014]      FIG. 5  is a flow diagram showing steps typically performed by the facility as part of the make secondary request routine invoked in  FIG. 4 . 
           [0015]      FIG. 6  is a flow diagram showing steps typically performed by the facility as part of the update cache routine invoked in  FIG. 4 . 
           [0016]      FIG. 7  is a flow diagram showing steps typically performed by the facility as part of the consistency check and peer coordination routine invoked in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    A software facility for performing probabilistic cache entry consistency checking (“the facility”) is described. The facility permits two amounts of time to be established for each entry: a minimum lifetime specifying a minimum amount of time that the entry will be present in the cache and honored—unless explicitly invalidated—and a maximum lifetime specifying a maximum amount of time that the entry will be honored. If a primary data request that can be satisfied using a cache entry is made at a time before its minimum lifetime has expired, then the facility uses the cache entry to satisfy the primary data request. If a primary data request that can be satisfied using a cache entry is made at a time after its maximum lifetime has expired, then the facility issues a secondary data request to a data source to satisfy the primary data request and updates the cache to reflect the result of the secondary data request. If, however, a primary data request that can be satisfied using a cache entry is made at a time after its minimum lifetime has expired but before its maximum lifetime has been reached, then the facility determines probabilistically whether to use the cache entry or to consistency-check the cached object by issuing a secondary data request. In particular, the facility generates a random or pseudorandom number, and compares it to a probability threshold determined for the object. If the random number is above the probability threshold, the facility uses the cache entry, whereas if the random number is not above the probability threshold, the facility issues a secondary data request to a data source to satisfy the primary data request and updates the cache to reflect the result of the secondary data request. 
         [0018]    In some embodiments, where the facility issues a new secondary data request to satisfy the primary data request, the facility compares the new secondary data result to the cached result. If these differ, the facility notifies the managers of other caches to invalidate any entries they have for corresponding objects. In some embodiments, the facility includes the new result in the notification. In some embodiments, the facility includes in the notification a digest for the invalidated cache entry that permits receiving cache managers to efficiently determine whether the sender is invalidating the value it has cached for the object or a different one. 
         [0019]    By managing caches in some or all of the ways described above, embodiments of the facility provide good cache performance without consuming an excessive level of resources. 
         [0020]      FIG. 2  is a block diagram showing some of the components typically incorporated in at least some of the computer systems and other devices on which the facility executes. These computer systems and devices  200  may include one or more central processing units (“CPUs”)  201  for executing computer programs; a computer memory  202  for storing programs and data while they are being used; a persistent storage device  203 , such as a hard drive for persistently storing programs and data; a computer-readable media drive  204 , such as a CD-ROM drive, for reading programs and data stored on a computer-readable medium; and a network connection  205  for connecting the computer system to other computer systems, such as via the Internet. While computer systems configured as described above are typically used to support the operation of the facility, those skilled in the art will appreciate that the facility may be implemented using devices of various types and configurations, and having various components. 
         [0021]      FIG. 3  is a timeline diagram illustrating the operation of the facility. The diagram shows a timeline  300 , in which time progresses to the right. At time zero, an object is added to a cache managed by the facility. At that time, two lifetimes are established for the object: a minimum lifetime  310 —here, 10 minutes—and a maximum lifetime  320 —here, 30 minutes. The particular length of these lifetimes is configurable in a variety of manners, e.g., may be manually set for individual objects, types of objects, all objects, etc.; or may be automatically set based upon programmatic logic. The minimum lifetime is said to expire at a time obtained by adding a minimum lifetime length determined for the object to the time at which the object is added to the cache. Similarly, the maximum lifetime is said to expire at a time obtained by adding a maximum lifetime length determined for the object to the time at which the object is added to the cache. In some embodiments, the facility later extends the object&#39;s two lifetimes in certain situations, such as when a new secondary request returns a version of the object that matches a cached version of the object. 
         [0022]    The diagram indicates that, when a primary request is received during the object&#39;s minimum lifetime, a response to which requires the object, the facility will unconditionally use the version of the object from the cache to respond to the primary request. The figure further indicates that, when such a request is received after the cached object&#39;s maximum lifetime has expired, the facility unconditionally issues a secondary request to obtain a new version of the object. If such a primary request is received after the minimum lifetime has expired but before the maximum lifetime has expired, however, the facility determines probabilistically whether to use the cached version of the object or make a secondary request for the object. In particular, the facility retrieves a configurable probability value in a range representing probabilities between 0% and 100% that the cached object should be consistency-checked, such as 10% in the range between zero and one. It then generates a random number in this range and compares it to this probability threshold. If the random number is below the probability threshold, then the facility consistency-checks the cached object, otherwise it uses the cached object. In some embodiments, the probability threshold is fixed for each object, or across all objects. In some embodiments, the probability threshold for one or more objects is variable, such as increasing over the period from expiration of the object&#39;s minimum lifetime to the expiration of its maximum lifetime, or decreasing over this period. 
         [0023]      FIG. 4  is a flow diagram showing steps typically performed by the facility to manage a cache while responding to primary requests. In step  401 , the facility receives a primary request as described above. In step  402 , the facility identifies objects needed to respond to the primary request received in step  401 . In some cases, only one object is needed to respond to the primary request, which may be explicitly identified within the primary request. In other cases, the facility must use external resources, such as web page templates, to determine which objects are needed to respond to the primary request. In steps  403 - 413 , the facility loops through each object identified in step  402 . In step  404 , if a version of the current object is contained in the cache and this cache entry has an unexpired minimum lifetime, then the facility continues in step  405 , else the facility continues in  406 . In step  405 , the facility uses the cached version of the object in constructing a response to the primary request received in step  401 . After step  405 , the facility continues in step  413 . 
         [0024]    In step  406 , if the cache contains a version of the object in an entry that, while having an expired minimum lifetime, has an unexpired maximum lifetime, then the facility continues in step  407 , else the facility continues in step  409 . In step  407 , the facility determines a random number in a range, such as the range between zero and one. In step  408 , if the random number determined in step  407  is less than or equal to the conditional consistency-checking probability threshold, then the facility continues in step  411  to perform a set of steps that amount to consistency checking the cache entry, else the facility continues in step  405  to use the cached version of the object. 
         [0025]    In step  409 , the facility invokes a make secondary request routine discussed below in connection with  FIG. 5  to retrieve an up-to-date version of the object. In step  410 , the facility invokes an update cache routine discussed below in connection with  FIG. 6  to update the cache to contain the version of the object returned in response to the secondary request. After step  410 , the facility continues in step  413 . 
         [0026]    In step  411 , the facility invokes the make secondary request routine. In step  412 , the facility invokes a consistency check and peer coordination routine discussed below in connection with  FIG. 7  to compare the result of the secondary request made in step  411  to the version stored in the cache and, if different, provide an indication to peer caches, that their cache entries for this object may have expired. After step  412 , the facility continues in step  410 . 
         [0027]    In step  413 , if additional identified objects remain to be processed, then the facility continues in step  403  to process the next identified object, else the facility continues in step  414 . In step  414 , the facility sends a response to the primary request constructed using cached and/or invalidated objects to the sender of the primary request. After step  414 , the facility continues in step  401  to receive the next primary request. 
         [0028]    Those skilled in the art will appreciate that the steps shown in  FIG. 4  and in each of the flow diagrams discussed below may be altered in a variety of ways. For example, the order of the steps may be rearranged; some steps may be performed in parallel; shown steps may be omitted, or other steps may be included; steps may be performed at greater or lesser levels of granularity; etc. 
         [0029]      FIG. 5  is a flow diagram showing steps typically performed by the facility as part of the make secondary request routine invoked in  FIG. 4 . In step  501 , the facility issues a secondary request to the appropriate data source for the object. In step  502 , the facility receives a response to the secondary request issued in step  501  from the data source to which it was sent. In some embodiments, step  502  is performed asynchronously with respect to step  501  in an event-driven manner, and is triggered by the arrival of the response of the secondary request. After step  502 , the make secondary request routine returns. 
         [0030]      FIG. 6  is a flow diagram showing steps typically performed by the facility as part of the update cache routine invoked in  FIG. 4 . In step  601 , the facility removes the entry from the object from the local cache, if it is present there. In step  602 , the facility adds an entry to the local cache for the version of the object contained in response to secondary request. The facility typically establishes new minimum and maximum lifetimes for the added entry that are based on the time at which the entry is added to the cache. In some embodiments, where the object was present in the cache, the facility combines steps  601  and  602  to merely update the existing cache entry, rather than removing it and adding a new entry. In step  603 , the facility uses the version of the object contained in the response to secondary request in constructing a response to the primary request. After step  603 , the facility returns from the invalidate object routine. 
         [0031]      FIG. 7  is a flow diagram showing steps typically performed by the facility as part of the consistency check and peer coordination routine invoked in  FIG. 4 . In step  701 , if the version of the object contained in the response to secondary request matches the cached version of the object, then the facility continues in step  702 , else the facility returns from the consistency check and peer coordination routine. 
         [0032]    In step  702 , the facility sends an invalidation notification to the managers of any peer caches. In some embodiments, the facility sends the invalidation notification from the data provider for the current cache to the data provider for each of the peer caches. In some embodiments, the data provider for this cache sends the invalidation notification to a central clearinghouse for invalidation notifications, which may either forward the invalidation notification to the data provider for each peer cache, or make the invalidation notification available for retrieval by the data providers for any or all of the peer caches. In some embodiments, the invalidation notification contains only information identifying the object, such as an object identifier, other contents of a secondary request used to obtain the object from the appropriate data source, or a cache key used to identify the object within the cache. In such embodiments, a peer cache manager receiving the invalidation notification may remove any matching entry from its own cache, or it may refresh such an entry by issuing its own secondary request for the object to the appropriate data source. In some embodiments, the invalidation notification contains a copy of the new version of the object received in the response to secondary request. In such embodiments, a peer cache manager receiving the invalidation notification substitutes this new version of the object in its own cache, in some embodiments resetting the object lifetimes for the object. In some embodiments, the sent invalidation notification contains a digest representation of the contents of the old version of the object, such as a digest representation generated using a hashing algorithm such as the MD5 hashing algorithm. In such embodiments, peer cache managers receiving invalidation notification only process it if the digest representation of the object contents matches a digest representation of the version of the object that it caches. In some embodiments, the sent invalidation notification contains a digest representation of the new version of the object received in the response to the secondary request. In such embodiments, peer cache managers receiving the invalidation notification only process the invalidation notification in cases where the contained digest representation of the new value does not match a digest representation of the version of the object in its own cache. In some embodiments, the facility varies the particular contents of the invalidation notification based upon the size of the object, or some other attribute of the object. As one example, up to a certain size threshold, the facility may include a complete representation of the object in the invalidation notification, and above that size threshold, include only a digest representation or no representation. After step  702 , the facility returns from the consistency check and peer coordination routine. 
         [0033]    It will be appreciated by those skilled in the art that the above-described facility may be straightforwardly adapted or extended in various ways. For example, the facility may be used to manage a single cache or multiple peer caches in a variety of different configurations, caching data of a variety of types from services or other data sources of a variety of types. Myriad approaches may be used to set minimum and maximum lifetime durations and invalidation probability thresholds. Different techniques may be used to coordinate between peer caches. While the foregoing description makes reference to particular embodiments, the scope of the invention is defined solely by the claims that follow and the elements recited therein.