[Catalyst] modules for conditional GET ?

There's a good example using the 'Cache-Control' header in the new
Catalyst book, Chapter 11, section 'Deploy with a Cache'.

From: "Dami Laurent (PJ)" <[email protected]>

Hi Catalysters,

For some actions of a Catalyst app, I would like to implement
conditional GET (using If-Modified-Since HTTP header), where the
timestamp of one config file decides whether the page should be
refreshed or not --- this is because that page is quite expensive to
compute.

This scenario sounds like a common thing to do, so I expected to find
some Catalyst plugins/extensions in CPAN to do that, but didn't find
any. Did I just miss some CPAN modules, or should I really start from
scratch ?

Thanks in advance for any hints,

Laurent Dami




Try Catalyst::Plugin::Cache::HTTP.



Octavian

I agree with the others who have responded, but they didn?t
explain why theirs was the right answer, so:

You?re not checking whether any state has changed since the
last GET to decide whether to recompute. That is the case in
which conditional GET would be appropriate. That would allow
you to avoid recomputing the page indefinitely as long as no
state changes necessitate it, but it requires that the clients
keep asking.

Instead you merely want to avoid doing any recomputation for
some predefined period of time, regardless of your state. This
is a case for caching: since you aren?t going to recompute the
page until said time has passed, you may as well tell the client
that it?s superfluous for them to try asking again before that
period is up.

Regards,

Indeed, this is exactly what I want to do. The app has a config file (not a Catalyst
config file, but another file having to do with business logic), and some super-users
have a mechanism for hot uploading of a new config to the server, at any time.
A few app pages are expensive to compute, and they depend on the client and on that config file. So clients should keep asking for those pages at each request, and depending on the If-Modified-Since header and on the timestamp for the config file, the server can decide if it's worth recomputing the page for that client, or rather send a cheap 304 Not Modified.

Cheers, L. Dami

I suggest you send an ETag and check `If-None-Match` (possibly
just a hash of the timestamp for the config file) instead of
(or if you have HTTP/1.0 clients, in addition to) relying on the
timestamp and `If-Modified-Since`.

Beyond that:

It?s easy to write a module that will conserve bandwidth using
these headers, by hashing the body after all computation is done
and checking whether to send it or just a 304.

But conserving server CPU requires intimate knowledge of both the
model and the structure of the controllers. It seems hard to find
a generically useful abstraction beyond a utility method or two
for setting the headers.

Regards,

Be careful about using timestamps if you are running multiple web servers
behind a load balancer (or may expand to where you will be behind a
balancer). Here's a read on Etags:

http://developer.yahoo.net/blog/archives/2007/07/high_performanc_11.html

For resources such as css, js, images I tend to create URLs that include an
md5. Those include cache headers that don't expire and thus when the
content changes the URL changes.

I have also done that with text/html pages, but it's less common. For a
config file you can send the config through Object::Signature to get an
md5. You could recalculate and cache that whenever a new config is
uploaded.

For "static" pages (for non-logged in users) the pages tend to get cached
for some number of minutes as it's not critical that a change is seen
exactly the same time by all users. Dynamic content is not cached, of
course, but elements of the page may be cached in memcached.