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Monday, July 31, 2006

ASP Tips: Tip 3: Cache Data and HTML on the Web Server's Disks

Sometimes, you may have too much data to cache in memory. "Too much" is a judgment call; it depends on how much memory you want to consume, as well as the number of items to cache and the frequency of which these items will be retrieved. In any case, if you have too much data for in-memory caching, consider caching data in text or XML files on the Web servers' hard disks. You can combine caching data on disks and in memory to build the optimum caching strategy for your site.

Note that when measuring the performance of a single ASP page, retrieving data on disk may not always be faster than retrieving the data from a database. But caching reduces load on the database and on the network. Under high loads, this will greatly improve overall throughput. Caching can be very effective when caching the results of an expensive query, such as a multitable join or a complex stored procedure, or caching large result sets. As always, test competing schemes.

ASP and COM provide several tools for building disk-based caching schemes. The ADO recordset Save() and Open() functions save and load recordsets from disk. You could use these methods to rewrite the sample code from the Application data caching tip, above, substituting a Save() to file for the code that writes to the Application object.

There are a few other components that work with files:

  • Scripting.FileSystemObject allows you to create, read, and write files.
  • MSXML, the Microsoft® XML parser that comes with Internet Explorer, supports saving and loading XML documents.
  • The LookupTable object (sample, used on MSN) is a great choice for loading simple lists from disk.

Finally, consider caching the presentation of data on disk, rather than the data itself. Prerendered HTML can be stored as an .htm or .asp file on disk; hyperlinks can point to these files directly. You can automate the process of generating HTML using commercial tools such as XBuilder, or Microsoft® SQL Server™ Internet publishing features. Alternatively, you can #include HTML snippets into an .asp file. You can also read HTML files from disk using the FileSystemObject, or use XML for early rendering.

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