Firefox Hacks [Electronic resources] نسخه متنی

Hack 54. Turn Off Absolutely All Caching

Be sure that the web page
you're viewing is freshly retrieved from across the
network.

You can clean up Firefox so that no old data is hanging around [Hack #8], but sometimes you might be
looking for a more permanent solution. This brief hack explains how
to achieve
that.

5.12.1. Disable Browser Caching

To turn off the memory and disk browser caches completely, set these
preferences:

browser.cache.memory.enable /* set to false, default = true */
browser.cache.disk.enable   /* set to false, default = true */

Turning these preferences off guarantees that Firefox will make an
HTTP request for every URL it is asked to display.

Firefox does not use HTTP HEAD requests, at least
not for the retrieval of web pages; it uses GET
requests. Every URL can therefore be made to return a full copy of
the resource (the web page) in question. Embedded images and other
URLs are, of course, retrieved in separate GET
requests. It is possible that web pages or Firefox extensions might
issue their own HEAD requests, though.

GET requests never add any HTTP
If-Modified-Since headers if local caching is
turned off. Such headers are only sent when local caching is enabled
(the default) and if a currently cached copy of
the requested page was originally delivered with a
Last-Modified header.

The golden rule of standard Firefox URL loading is this:

Everything is always refreshed from the Web if local caching is
disabled or if the Reload (bypass cache) command
is run.

Reload (bypass cache) is run with Shift-F5
(Shift and click Reload on the Macintosh). Just left-clicking the
Reload button is not enough, because Reload obeys HTTP caching
protocols (see the next section). Clicking Reload is enough if local
caching is turned off.

Even less reliable is pressing Enter or Return in the Location bar.
In that case, if local caching is still on, Firefox might not issue
an HTTP request of any kind. Change this preference to 1 (one) to fix
that:

browser.cache.check_doc_frequency /* 1 = everytime. default = 3 = as needed */

There is, however, an exception to the golden rule. The exception
applies to uses of datasources, an advanced XUL programming topic:

The content of datasources persists until they are destroyed or until
Firefox shuts down.

Datasources are an internal feature commonly exploited in XUL
applications. Datasources are also used deep inside bookmarks and
elsewhere.

Switching Firefox into Offline mode also affects page caching. If
Firefox is in offline mode, it will attempt to use cached copies of
web pages, even if the network connection is still available and it
is only Firefox that is offline, not the network.

5.12.2. Disable HTTP Caching

Even when Firefox is trying to do the right thing, web pages might
still be cached by HTTP caching mechanisms. In that case, web server
proxies (such as squid) between Firefox and the
ultimate web server can hold temporary copies of a cached file, and
Firefox is denied a taste of the absolutely latest thing. For best
results, make sure web pages are sent with all these HTTP headers
intact:

Expires: Thu, 01 Jan 1970 00:00:00 GMT
Last-Modified: Thu, 01 Jan 1970 00:00:00 GMT
Pragma: no-cache
Cache-Control: max-age=0, no-cache, must-revalidate

Don't set these headers in <META> tags; intervening proxies will not notice them. Instead, set them as part of the HTTP message itself.

To see the cache-related information that Firefox sends to a web
server when it makes requests, just examine the HTTP headers in the
browser request [Hack #51] .

If intermediate web caches and local caches are a concern, then web
servers should be instructed to never return an
HTTP 304 response code. The 304 code tells the browser that the
server has nothing new to offer. The browser will conclude that any
cached copy it has is therefore good enough.

If 304 responses can't be stopped, then ensure that
no Last-Modified header line is ever sent by the
server. It's that header that stimulates the browser
to add If-Modified-Since headers to future
requests for that page, if it's allowed to cache the
page. The same argument applies to server-generated
Etag headers. The existence of such headers can
prompt the browser to add If-None-Match headers,
which again can generate a 304 response.

5.12.3. Disable XUL Caching

By default, Firefox includes a copy of the
XUL.mfl (Windows),
XUL.mfasl (Unix), or XUL FastLoad
File (Macintosh) file. This is the Mozilla FastLoad cache
that resides in the profile area. You can set the following
preference to prevent it from being
used:

nglayout.debug.disable_xul_fastload /* default = false, set to true */

When FastLoad is at work, Firefox reads files
directly from the chrome only if it needs to, which saves disk access
time.

The general-purpose web cache is used to cache XUL files whether
FastLoad is at work or not. To turn that other
caching off as well, either disable caching generally as in the
previous example, or else set this preference:

nglayout.debug.disable_xul_cache /* default = false, set to true */

A further hack described by some pundits is to delete the
XUL.mfl file while Firefox is shut down and
replace it with a directory of the same name. That makes Firefox
think the FastLoad cache is corrupt or missing,
and XUL files in the chrome are thus always reloaded from cache or
directly. This kind of trickery is necessary only if preferences
can't be set for some obscure reason.