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78 lines
2.7 KiB
78 lines
2.7 KiB
<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta name="generator" content=
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"HTML Tidy for Mac OS X (vers 31 October 2006 - Apple Inc. build 13), see www.w3.org" />
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<title>
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Caching -- /DesignIssues
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</title>
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<nextid />
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</head>
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<body bgcolor="#FFC060" text="#302005">
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<a href="OldDocs.html"><img src=
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"../Icons/WWW/arch1990.gif" /></a>
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<hr />
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<h1>
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Document caching
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</h1>Three operations in the retrieval of a document may take
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significant time:
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<ul>
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<li>
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<a name="3" href="Formats.html#4">Format conversion</a> by
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the server, including <a name="5" href=
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"Versioning.html#3">version regeneration</a>
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</li>
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<li>Data transmission across the network
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</li>
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<li>Format conversion by the browser
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</li>
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</ul>At each stage, the server (in the first case) or browser
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(in the other cases) may decide to keep a temporary copy of the
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result. This copy should ideally be common to many browsers.
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<p>
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Automatic caching relieves the user of having to explicitly
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save things which may be referred to again. It also relieves
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the system of keeping multiple copies (one for each user who
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has read the document). It allows local disk space to used
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optimally. Cache management takes into account such factors
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as
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</p>
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<ul>
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<li>
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<a name="2" href="#1">expiry date</a>
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</li>
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<li>file size
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</li>
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<li>time taken to get the file
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</li>
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<li>frequency of access
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</li>
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<li>time since access
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</li>
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</ul>
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<h2>
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<a name="1">Expiry date</a>
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</h2>As a guide to help a cache program optimise the data it
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caches, it is useful if a document is transmitted with an
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estimate by the server of the lengt of time the data may be
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kept for. This allows fast changing documents to be flushed
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from the system, preventing readers from being mislead. (I
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would not propose any notification of document changes to be
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distributed to cache managers automatically). For example, an
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RFC may be cached for years, while the state of the alarm
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system may be marked as valid for only one minute.
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<p>
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Window-oriented browsers effectively cache documents when
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they keep several at a time in memory, in different windows.
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In this case, for very volatile data, it may be useful to
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have the browser automatically refresh the window when its
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data expires.
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</p>
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<p>
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( <a name="4" href="Overview.html">design issues</a>
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)_________________________________
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</p>
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<address>
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<a name="0" href="../../../WWW/disclaimer.html">Tim BL</a>
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</address>
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</body>
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</html>
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