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Encoder USS Code 39 in Font CACHING AND PERFORMANCE TUNING

CHAPTER 12 CACHING AND PERFORMANCE TUNING
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Figure 12-11. The WAS report after running the script Next, you can have a closer look at how each individual page performed, by examining the Page Data node of your report. By briefly examining the data for each page, you may find that the results for Users.aspx differ significantly from the rest. Here are the figures that I got in my test:
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Page
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Default.aspx Login.aspx Users.aspx
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Average TTFB (milliseconds)
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7.03 7.22 490.84
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Average TTLB (milliseconds)
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7.27 8.19 833.67
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Downloaded Content Length (bytes)
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2360 3040 1054113
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For some reason, my server is taking about 0.5 second in serving the first byte of that page, and then almost another 0.35 second (TTLB minus TTFB) to finish serving it up. Moreover, the output of Users.aspx is 1MB long! These two results probably help to explain why the server is handling so few requests per second (6.50 requests/sec) on average. Why is Users.aspx causing that much work on the server It s probably because the page contains a full-fledged DataGrid control, loaded with the details of 1,000 users, just for display purposes. In the next section, you will make use of caching techniques to save your server the immense work of needing to process this DataGrid control on every request, and see how you can tweak it to produce smaller-sized pages.
Caching
A cache is an area of memory that stores recently accessed data and resources, so that subsequent requests can reuse them without having them regenerated. When a web page is requested for the very first time, the page must be completely generated from scratch by the server and
CHAPTER 12 CACHING AND PERFORMANCE TUNING
sent back to that user in the HTTP response. But at the same time, the server can also arrange for the freshly generated material in that page to be put aside into cache areas, so that it can be used to serve subsequent requests from the same user or other users. So, how does a web application benefit from caching One benefit from caching is that you can eliminate the need to regenerate the same content many times (because you can generate it once, and then cache it in the appropriate location). Also, depending on the nature of the content, you can try to cache it as close to the client as possible. These two benefits manifest themselves in three immediately obvious ways: If content is cached close to the client, you achieve a reduction in latency (that is, apparent inactivity) and hence improve the user experience. If content is cached close to the client, you also achieve a saving in bandwidth (because the route from the client to the cache is shorter than the route from the client to the web server). In particular, if a resource is cached on the browser itself, network resources are eliminated altogether for that resource! By reusing generated content many times, you can vastly reduce the workload on your server, which can now just generate the material once and cache it (instead of regenerating it for each page request). The reduced demand on the server s resources will improve the server s overall performance.
Caching Overview
If you understand the implications of caching correctly, you can use it in your web applications to gain significant performance improvements. In particular, you need to consider what content you can cache, where you can cache it, and for how long. While the first consideration is application-specific, the other two are not.
Cache Content Expiration and Priority
Most items of content have a natural lifetime. In other words, at the time the content is generated, it s usually possible to specify a date and time at which that freshly generated content will become out-of-date. For example, on a site that publishes stock market shares, much of the data changes every few minutes, so you would not want cached data to last for more than a few minutes (otherwise users will almost always be looking at outdated stock prices). By contrast, you can safely cache the pages of a web site that shows the results of sports events, because those results don t change once they ve happened. You don t want cached data to hang around in the cache after it has become out-of-date. Therefore, any cached item has a expiry date that determines its life span that is, for how long it will stay cached before it is considered invalidated and taken out of the cache. There are essentially two ways to set the expiry date: Absolute expiration allows you to set the exact date and time when the cached content will expire. Sliding expiration is a time interval, and it dictates for how long the cached item is permitted to live in the cache after the time it was last accessed.
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