Differences between Passive and Active CachingLast reviewed: October 6, 1997Article ID: Q164085 |
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SUMMARYMicrosoft Proxy Server caches data by two different of processes: passive caching and active caching.
Passive CachingPassive caching, also referred to as on-demand caching, is the most basic mode of caching. Microsoft Proxy Server interposes itself between the client and local or remote Web and intercepts requests (for example, HTTP GET requests) from the client. Before forwarding the request on to the Web, Proxy Server first calls into its cache (the Urlcache.dll file) to determine if the cache can satisfy the request by using the RetrieveUrlFile API. If the data is in the cache and has not expired, it is returned immediately to the client by using the Windows Sockets TransmitFile API. If the object is not cached or if the cached copy of the object has expired, Proxy Server retrieves the object from the Web, returns it to the user, and inserts it into the cache (by using the CreateUrlFile and CacheUrlFile APIs). If the local disk space reserved for the cache is too full to hold the new data, older objects are removed from the cache using a formula that evaluates age, popularity, and size.
Active CachingMicrosoft Proxy Server uses active caching to improve the retrieval performance by increasing the likelihood that a requested object will be found in the cache. Active caching works as a superset to passive caching. Typically, in passive caching, an object is placed in the cache and a Time- To-Live (TTL) expiration value is associated with that object. During this TTL, all requests for the object are serviced from the cache without generating traffic back to the upstream Web server. After the TTL has expired, subsequent client requests for the object will generate traffic to and from the Web server. The response from the server will be stored in the cache and a new TTL will be calculated. Active caching augments this system by having the server automatically generate requests for a specified subset of objects. Proxy Server optimizes the choice of objects for active caching on the basis of the following qualities:
MORE INFORMATIONFor more information on caching, please see the Proxy Server on-line documentation.
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