Document caching

Three operations in the retrieval of a document may take significant time:

Format conversion by the server, including version regeneration
Data transmission across the network
Format conversion by the browser

At each stage, the server (in the first case) or browser (in the other cases) may decide to keep a temporary copy of the result. This copy should ideally be common to many browsers.

Automatic caching relieves the user of having to explicitly save things which may be referred to again. It also relieves the system of keeping multiple copies (one for each user who has read the document). It allows local disk space to used optimally. Cache management takes into account such factors as

expiry date
file size
time taken to get the file
frequency of access
time since access

Expiry date

As a guide to help a cache program optimise the data it caches, it is useful if a document is transmitted with an estimate by the server of the lengt of time the data may be kept for. This allows fast changing documents to be flushed from the system, preventing readers from being mislead. (I would not propose any notification of document changes to be distributed to cache managers automatically). For example, an RFC may be cached for years, while the state of the alarm system may be marked as valid for only one minute.

Window-oriented browsers effectively cache documents when they keep several at a time in memory, in different windows. In this case, for very volatile data, it may be useful to have the browser automatically refresh the window when its data expires.

( design issues )_________________________________

Tim BL