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The Cache Service: An Introduction


What is a proxy cache?

In the absence of a proxy cache server, each request from a web browser goes directly to the web site and is returned directly to the browser. If you reconfigure your browser to use a proxy cache, your request goes to the proxy.

If the page is stored locally by the proxy then it is sent back to you without referring back to the originating site.

If not, then the proxy fetches the page for you. Once it has retrieved the page and passed it back to you, it keeps a copy in its cache space so that other requests for that document can be handled quickly.

Why does Cardiff have one?

  There are several very good reasons for operating a cache service:

  • Reduced network usage - the primary reason for using a cache service is to reduce the amount of network bandwidth that your web browsing consumes. The Cardiff cache service satisfies just over 50% of requests for documents held on external web servers without referring back to the originating server. In addition This frees up the network for other uses - which is of general benefit to the rest of the internet community.
  • Improved access speed - web pages from commonly accessed sites on the internet are very likely to be in the proxy server's cache and will therefore not have to be fetched from the originating website. Access to these resources will be much quicker if there is no need to go outside Cardiff.

Are there any disadvantages?


Yes, there are two disadvantages of caching of which users of the service should be aware:

  • There is a possibility that you could retrieve an old copy of a document which has subsequently been updated. When the cache stores a document it makes an estimate of how long the original is likely to remain unchanged. During this period the cache will issue its copy of the document without reference to the originating site. If the original does in fact change, the cache may return a copy which is out of date.With most browsers the user may use the 'reload' command to force the cache to retrieve the original again, but this eliminates much of the value of caching.
  • Reduced performance for some requests - Requests that are not found either in the local or national cache are then retrieved by the national cache and returned to the browser. There is a slight overhead in processing requests via the cache and performance will be somewhat slower than if the page had been returned directly. However, the performance of the servers and network is such that this difference is barely perceptible under normal circumstances.

Your session is logged


All requests to websites that pass through the proxy cache servers are logged and the logs are retained for a limited period of time.

This is mainly to allow us to spot problems with the caches and to monitor cache usage so that we can optimise the performance. You should be aware that it is possible to extract a list of all the URLs that you have visited via the cache.

We DO NOT do this routinely, but in case of serious abuse of the network we might be called upon to do so. You have been warned ...