Proxy server caching is used to help decrease the time it takes for users on computer networks to access content on the Internet. When network administrators enable caching, the proxy server will save frequently accessed content so it does not have to download the content from the Internet each time a user requests it. When a user types a web address in a web browser, the request goes to the proxy server, which looks for the specified material in its cache. If the requested information cannot be found, it sends a request to the Internet. This can save bandwidth and decrease the time the user has to wait for content.
Proxy servers can use both passive and active proxy server caching to cache content. When a proxy server cannot find a requested website in its cache and then downloads a new copy from a web server, it is performing passive caching. Active caching occurs when the proxy server is idle and caches websites that users on the network frequently access. By enabling active caching, network administrators can ensure the proxy server has a fresh copy of the content. A downside of active caching is that it can require more bandwidth and may not be suitable for servers that have bandwidth limitations or required downtime.
The data stored from proxy server caching is not permanent and must be regularly updated to ensure that users get the latest version of the content. Although cached objects need to be updated when the website content changes, they also have an expiration time. When an item expires, the proxy server checks with the web server to get information on the latest version of the content. If there's a change, it will download a fresh copy; otherwise, it does not need to download the content again and just updates the expiration time. Proxy servers maintain a log of these actions so administrators can view how often the server is grabbing new content and how often users access content directly from the cache.
Proxy server caching does have a drawback in that it can sometimes harm network performance. If the network administrator allocates too little space on the server for caching, the server will need to keep removing items from the cache. Items that are the least used or soonest to expire will usually be removed first to make room for more items when the need arises.