Caching
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If the user returns to a page fairly soon, it is likely that the
data will not be retrieved from the source web server, as above,
again. By default, browsers cache all web resources on the local
hard drive. An HTTP request will be sent by the browser that asks
for the data only if it has been updated since the last download. If
it has not, the cached version will be reused in the rendering step.
This is particularly valuable in reducing the amount of web traffic
on the internet. The decision about expiry is made independently for
each resource (image, style sheet, JavaScript file etc, as well as
for the HTML itself). Thus even on sites with highly dynamic
content, many of the basic resources are only supplied once per
session or less. |
It is worth any web site designer collecting all the CSS and
JavaScript into a few site-wide files so that they can be downloaded
into users' caches and reduce page download times and demands on the
server.
There are other components of the internet that can cache web
content. The most common in practice are often built into corporate
and academic firewalls where they cache web resources requested by
one user for the benefit of all.
Apart from the facilities built into web servers that can ascertain
when physical files have been updated, it is possible for designers
of dynamically generated web pages to control the HTTP headers sent
back to requesting users, so that pages are not cached when they
should not be - for example internet banking and news pages.
This helps with understanding the difference between the HTTP 'GET'
and 'POST' verbs—data requested with a GET may be cached, if other
conditions are met, whereas data obtained after POSTing information
to the server usually will not.
Web standards
At its core, the Web is made up of three standards:
the Uniform Resource Identifier (URI), which is a universal system
for referencing resources on the Web, such as Web pages;
the HyperText Transfer Protocol (HTTP), which specifies how the
browser and server communicate with each other; and
the HyperText Markup Language (HTML), used to define the structure
and content of hypertext documents.
Berners-Lee now heads the World Wide Web Consortium (W3C), which
develops and maintains these and other standards that enable
computers on the Web to effectively store and communicate different
forms of information. |
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