473,287 Members | 1,655 Online
Bytes | Software Development & Data Engineering Community
Post Job

Home Posts Topics Members FAQ

Join Bytes and contribute your articles to a community of 473,287 developers and data experts.

Advanced caching with mod_expires

docdiesel
297 Expert 100+
The fastest (and cheapest) transfer of data is that which doesn't have to be done - because the data is already there. Or still there - the caching of files is widely known and practised. Each web browser is caching the files he formerly requested from the web server. But if the file is to be loaded a second time, still a request is sent to the web server which then usually is answering with a "304 Not modified" [1] as can be seen in his access log:

Expand|Select|Wrap|Line Numbers
  1. 215.82.12.78 - - [02/Nov/2007:10:20:33 +0100] "GET /images/logo.gif HTTP/1.1" 304 -
Receiving this, finally the browser is loading the file from his cache. Instead of transfering the file just a couple of bytes went through the cables. But if a page uses lots of cached files like gif images and javascript files, even those 304 requests sum up - especially on connections with high latency.

To get rid of this unnecessary load the Apache web server is providing the module mod_expires [2] which enables you to stamp each delivered file with a kind of "valid until" mark. Therefore and according to the HTTP/1.1 specifications ([3], subsection 14.21), one line like the following is added to the http response header:

Expand|Select|Wrap|Line Numbers
  1. Expires: Thu, 01 Mar 2007 09:30:00 GMT
This would cause the browser not to ask again for this document or file unless he looses it from his cache. Now lets have a closer look to the directives of mod_expires and the resulting possibilities. Here's an example:

Expand|Select|Wrap|Line Numbers
  1. ExpiresActive  On
  2. ExpiresDefault "access plus 5 minutes"
  3. ExpiresByType  image/gif  "access plus 2 days"
  4. ExpiresByType  text/html  "modification plus 5 minutes"
ExpiresActive is used to enable or disable (on/off) the modification of the http response header. ExpiresDefault gives a default value for all documents, which are not captured by a rule on their own. Here the expiration date will be five minutes since time of access. This ensures that the document isn't reloaded by the client every few seconds while on the other hand he'll get the newer version within reasonable time, should the document be changed on the server.

ExpiresByType is giving you the possibility to control the expiration based on mime types. In the example gif images won't be reloaded from the server for 24 hours from of the point of downloading - based on the assumption that the gif images aren't changing but rather would be replaced by new files with new names. If your site is using gif buttons with roll over effects this takes lots of requests off your apache. On the other hand: If the layout of the web page and the gif images are changed while the names of the images remain the same, this would lead to rather strange looking pages in browsers which firstly accessed the page less than 24 hours ago. So this directive, though it offers a lot of reduction of traffic, is also to be handled with care.

In the example html pages won't be reloaded if their content is younger than five minutes. Imagine a highly frequented front page of a web portal which is generated every five minutes from dynamic content but saved as static html file for reasons of performance. If this was managed by the "access" alternative, a client that just loaded the 4:59 minutes old page would miss the newer version for five minutes. Based on "modification" it's ensured that nobody would miss the latest news.

In short, use the access rule for content that doesn't change (or at least not frequently). If modifications are necessary, try to use a new name. Use the modification rule for content that often is modified or for what reasons ever has a short lifetime.

Last but not least, this way you don't only reduce the load on your apache but also speed up your web application on client side: images and javascripts that don't have to be asked for every time they are to be used maybe displayed just some fractions of a secong faster. But in the end (and at least subjectively) it all sums up.

----

[1] http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
[2] http://httpd.apache.org/docs/2.2/mod/mod_expires.html
[3] http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html
Nov 25 '07 #1
0 14691

Sign in to post your reply or Sign up for a free account.

Similar topics

0
by: Subtext Whore | last post by:
My new web host doesn't have mod_expires compiled into Apache and this is problematic because the server isn't encouraging proxies and browsers to cache images by default. The images are even...
6
by: ZagrebMike | last post by:
On our (intranet) web site we have an ASP page that runs a report which serves up the results in a dynamic Excel file (i.e it specifies Response.ContentType = "application/vnd.ms-excel"). This...
15
by: olle | last post by:
Hi folks. I learning asp.net and compare it with traditional asp and Access-developing. The issue is this one: 1/I have this Ms Acceess adp-project application that works fine on my Ms Sql...
0
by: Troy Simpson | last post by:
Hi, I have a website which is made up of dynamic pages. Each page that's loaded has some code which looks at which template to load amongst other things, which causes the page to take a little...
3
by: DC | last post by:
Hi, (ASP.Net 1.1) is it possible to (programmatically and globally) deactivate page fragment caching? We have only two scenarios, development stage where we want caching off and testing where we...
5
by: Raj | last post by:
What is the purpose of file system caching while creating a tablespace? Memory on the test server gets used up pretty quickly after a user executes a complex query(database is already activated),...
2
by: George1776 | last post by:
All, I've recently upgraded our production ASP.NET/C# application from framework 1.1 to 2.0. Since then I've been plagued by out-of-memory errors and problems with the cache object (which may...
0
by: jason | last post by:
hi experts, support.microsoft.com/kb/917072 and http://msdn.microsoft.com/msdnmag/issues/06/07/WebAppFollies/ As pointed out in these articles, users might get session variables belong to...
4
by: Hermann | last post by:
My site is a bit slow showing the main page so I thought caching query result in PHP will improve performace. Then I read MySQL documentation and saw that MySQL does have a caching feature. So......
2
isladogs
by: isladogs | last post by:
The next Access Europe meeting will be on Wednesday 7 Feb 2024 starting at 18:00 UK time (6PM UTC) and finishing at about 19:30 (7.30PM). In this month's session, the creator of the excellent VBE...
0
by: MeoLessi9 | last post by:
I have VirtualBox installed on Windows 11 and now I would like to install Kali on a virtual machine. However, on the official website, I see two options: "Installer images" and "Virtual machines"....
0
by: DolphinDB | last post by:
The formulas of 101 quantitative trading alphas used by WorldQuant were presented in the paper 101 Formulaic Alphas. However, some formulas are complex, leading to challenges in calculation. Take...
0
by: ryjfgjl | last post by:
ExcelToDatabase: batch import excel into database automatically...
0
isladogs
by: isladogs | last post by:
The next Access Europe meeting will be on Wednesday 6 Mar 2024 starting at 18:00 UK time (6PM UTC) and finishing at about 19:15 (7.15PM). In this month's session, we are pleased to welcome back...
0
by: marcoviolo | last post by:
Dear all, I would like to implement on my worksheet an vlookup dynamic , that consider a change of pivot excel via win32com, from an external excel (without open it) and save the new file into a...
0
by: jfyes | last post by:
As a hardware engineer, after seeing that CEIWEI recently released a new tool for Modbus RTU Over TCP/UDP filtering and monitoring, I actively went to its official website to take a look. It turned...
0
by: ArrayDB | last post by:
The error message I've encountered is; ERROR:root:Error generating model response: exception: access violation writing 0x0000000000005140, which seems to be indicative of an access violation...
1
by: PapaRatzi | last post by:
Hello, I am teaching myself MS Access forms design and Visual Basic. I've created a table to capture a list of Top 30 singles and forms to capture new entries. The final step is a form (unbound)...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.