Comment 1 for bug 248901

James Henstridge had this advice on how to do this:

The guts of the caching code in ViewVC can be found in the
check_freshness() function found in viewvc.py:

    http://viewvc.tigris.org/source/browse/viewvc/trunk/lib/viewvc.py?view=markup

This function takes the etag value and last modified date for the
page, and does the following:
 * If an etag was passed in and an If-Not-Modified header is sent with
a matching value, consider the request as fresh.
 * If a last modified date was passed in an an If-Modified-Since
header is sent with a date equal or newer, consider the request fresh.
 * If the request is fresh, generate a "304 Not Modified" response.
 * Otherwise, add ETag and Last-Modified response headers.
 * Return whether the request is fresh.

If check_freshness() returns True, then the caller is expected to exit
with no further processing (so this should be done before as much
heavy work as possible).

As for picking etags, the main constraint is that they should change
if the content changes. For many ViewVC pages I used "weak etags"
since there were a few cases where the constraint could be broken
(software/template upgrades). If we ignore changes from templates,
then we can see that rendering of most pages are dependent on a single
revision:
 * changes pages: the first revision in the list.
 * annotate pages: the revision of the file being annotated.
 * files pages: the revision of the tree being displayed
 * revision pages: the revision being displayed

So for these cases, it should be possible to use the relevant revision
ID as an etag. And if you've got a revision ID, you should be able to
look this up for a date fairly cheaply.

Now if you have Squid sitting in front, it will use conditional GETs
to validate its cache, so the only points where Loggerhead would need
to fully render a page are:
 * the first time a particular page is rendered, or if the page falls
out of the cache.
 * the underlying branch has changed, so the given URL now points at a
different revision.