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CHAPTER DOWNLOADS Oracle Application Server 10g administration handbook: Cache invalidation John Garmany and Don Burleson 07.19.2004 Rating: --- (out of 5) Digg This!     StumbleUpon     Del.icio.us    The following is the fourth part of a six-part series on Oracle Application Server 10g administration. Each tip is excerpted from the Osborne Oracle Press book, "Oracle Application Server 10g administration handbook," by John Garmany and Don Burleson. Check back frequently for the next installment, or go to the main series page for all installments. [TABLE]

Cache invalidation

With the Internet containing helpful caches throughout its infrastructure, from the browser to the Web site, there must be a way to ensure that caches are not serving old content. Basic caching information is actually sent with the document when it is served.

Are static pages really static?

When you request a Web page, your browser will first look for that page in its own cache. If it finds it in the cache, it will include the Last-Modified time stamp in the HTTP 1.1 Request Header If-Modified-Since. If the page has not been modified, the server will return a 304 code (Not Modified), and the browser can serve the page from the cache.

Oracle's Web Cache will check the Last-Modified timestamp on the page in the cache and either send the fresh page or reply with the Not-Modified code. So how does Oracle's Web Cache know if the page in its cache is fresh? It uses a number of different methods, from monitoring headers like all caches, to being told by the application or database that content has changed. There are a number of HTTP headers in HTTP 1.1 to control caching of Web pages. The two main headers are the Expires and Cache-control. HTTP header information is necessary to define how caches maintain and serve you Web pages, but they are not capable of supporting dynami

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cally generated Web pages.

Caching dynamic content

The Oracle Application Server 10g Web Cache is designed to cache dynamically generated content. When the Oracle HTTP Server (OHS) receives a request for nonstatic data, it passes that request to an application to create and serve the content. This may be a Java servlet that queries a database and formats the results, or a J2EE application.

Once the result is formatted, it is sent back to the requester. The results are cached in the Web Cache. The browser does not know that the page was created on the fly. The Web Cache, however, must have a mechanism to ensure that the page is still valid before re-serving the page to another request. The Web Cache must also ensure that the page is only served to the correct requester. If a brokerage firm has a Web site that displays your current holdings, the site must ensure that it serves your page only to you and not the next requester, which may be your neighbor. The Web Cache has this same problem of what page to serve for which request.

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About the authors

A senior Oracle trainer with Burleson Consulting, John Garmany is also a respected Oracle expert and author and chosen by Oracle Press to write the "officially authorized edition" for the "Oracle Application Server 10g administration handbook." John also serves as a writer for DBAZine, "Oracle Internals" and has authored several popular Oracle books.

Don Burleson is one of the world's top Oracle database experts with more than 20 years of full-time DBA experience. He specializes in creating database architectures for very large online databases and he has worked with some of the world's most powerful and complex systems. Don's professional Web sites include www.dba-oracle.com and www.remote-dba.net.

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