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Cache

For Administrators

As an administrator, Joomla provides you with the ability to cache parts of your site. You can choose to cache whole web pages or just parts of those pages. This guide explains how.

On a Joomla site web page there are 3 things which may be cached:

  1. The whole page itself – the Page cache
  2. The output from the Joomla component for that web page – known as the View cache
  3. The output from the modules shown on that page – known as the Module cache

You have a number of cache settings which allow you to control what gets cached:

  1. The system plugin "System – Page Cache"
  2. The Global Configuration, System tab, Cache Settings. Here the System Cache option may be set to
    • OFF – Caching disabled
    • ON – Conservative caching
    • ON – Progressive caching
  3. Many modules within their options have an Advanced tab in which the Caching can be set to Use global or No caching

As described below, there are also rules for caching which are implemented within the Joomla code, and over which you have no control.

You can clear the cache through the Administrator → System → Clear Cache menu selection. In general, you can think of Joomla having 3 levels of cache, increasing in aggressiveness

  1. Conservative caching
  2. Progressive caching
  3. Page caching

In addition, Joomla developers can use caching facilities to store the result of database queries, for example, to increase the responsiveness of the site, but this is outside the scope of Administrator capabilities.

Page Caching

To switch this on, go to Administrator → Extensions → Plugins. Then find the System – Page Cache plugin, and enable it. This means that site pages will now be cached and whenever they're requested again, the cached page will be served, rather than it being generated by Joomla from the information in the database. The cached page will continue to be served until it's expired – as defined by the Cache Time parameter in the Administrator → Global Configuration → System tab → Cache Settings.

If you're reading this page as a tutorial and want to test the page caching, it's best to set the Global Configuration cache settings to:

  • Cache Handler – File
  • Path to Cache Folder – leave blank
  • Cache Time – 15 (the default of 15 minutes)
  • Platform Specific Caching - No
  • System Cache – OFF – Caching disabled

To verify that page caching is working, go to a website page that displays an article. After you display that page you should find in the file system a cache/page directory with a file in it which has a filename like xxx-cache-page-yyy.php where xxx and yyy are long hash strings. Joomla has to store separate cache pages for separate URLs so the second string of hex digits is a hash of the URL of the site web page, to make the filename unique to that page.

Then use the Administrator functionality to change the text of that article, and redisplay the site web page. You should find the cached version, not your modified text.

Changing an article (or other Joomla item) does not clear the page cache for the web page(s) where that article is displayed. To clear the page cache go to Administrator → System → Clear Cache. Click on the checkbox next to the Cache Group called "page", and press the Delete button. When you redisplay your web page it should now show your amended text.

If your site has a function like a shopping basket, applying page caching will cause problems, as pages have to show what the customer has already selected, rather than displaying a cached page which is common to everyone. However, you can configure the System - Page Cache plugin to exclude caching specified Menu Items or specified URLs and URL ranges (in the Advanced tab), so that only truly static pages are cached.

Conservative Caching

With Conservative Caching you can cache the View output from components and the output from those Modules which allow caching. But note that this will work only on pages which are not cached using the Page Cache. For those pages the whole web page is cached, and Conservative Caching isn't even considered.

To switch on Conservative Caching:

  1. Go to Administrator → System → Global Configuration → System tab and within Cache Settings, set System Cache to ON – Conservative caching
  2. Go to Administrator → Extensions → Modules and select the modules which you'd like to be cached. If that module permits caching then under the Advanced tab you should be able to set Caching to
  • Use Global – this module will be cached (with the Global option now having been set to Conservative caching)
  • No caching – this module will not be cached.

(Note that the Cache Time in the Global Configuration is in minutes but the Cache Time in the Module settings is in seconds.)

To check it's working, go to your site, ensure that you are logged out, and navigate to a web page which displays an article. Check your file system and you should find a folder cache/com_content containing a cache file.

You'll also find other directories such as cache/com_languages (as displaying the page involves loading the current language, and this will be cached as well) and also directories relating to module cache, e.g. cache/com_modules. These result from the use of cache which developers have coded within the Joomla application.

If you edit and save that article then refresh the site page, you will find that the site displays the updated text this time. This is because whenever the edit is saved, Joomla clears the cache for that article.

However, you can demonstrate that the cache is working if you edit the cache file in the cache/com_content directory using a basic text editor. Using the editor, change one letter within the article text in the cache file and save the file. Then when you refresh the web page you should see the change that you made to the cache file.

How can you select which component views get cached, and under what circumstances? Alas, you can't do this. This is determined by the Joomla core component developers and coded in the component PHP code. The criteria are different for each component. However, you can easily discover what criteria are used because for each of the site components they are coded in the site DisplayController.php file. For example, at the time of this revision (Joomla version 5) for the Contacts component we find in components/com_contact/src/Controller/DisplayController.php

    public function display($cachable = false, $urlparams = [])
    {
        if ($this->app->getUserState('com_contact.contact.data') === null) {
            $cachable = true;
        }

This means that the views associated with contacts will be cachable unless there is session data keyed by com_contact.contact.data – which will be the case if in the user session the user has displayed a contact form (e.g. on a page pointed to by a menu item of type Contacts → Single Contact).

The equivalent file for articles components/com_content/src/Controller/DisplayController.php contains:

    public function display($cachable = false, $urlparams = false)
    {
        $cachable = true;

        /**
         * Set the default view name and format from the Request.
         * Note we are using a_id to avoid collisions with the router and the return page.
         * Frontend is a bit messier than the backend.
         */
        $id    = $this->input->getInt('a_id');
        $vName = $this->input->getCmd('view', 'categories');
        $this->input->set('view', $vName);

        $user = $this->app->getIdentity();

        if (
            $user->get('id')
            || ($this->input->getMethod() === 'POST'
            && (($vName === 'category' && $this->input->get('layout') !== 'blog') || $vName === 'archive'))
        ) {
            $cachable = false;
        }

The expression $user->get('id') is true if this is a logged-in user. This means that articles are never cached for logged-in users. The subsequent expressions relate to other conditions when the caching is not performed, even if the user is not logged in.

So in this way you can discover the circumstances under which caching is performed, but changing these is not advisable. You can also demonstrate that modules are being cached by using the Joomla Breadcrumbs module, ensuring it's displayed in some module position on the web page, setting its Caching option and manually editing the cached file in cache/mod_breadcrumbs.

Progressive Caching

Like Conservative Caching, Progressive Caching also caches the output from component views and from modules. The functional difference between the two is that with Progressive Caching for logged-off users all modules are always cached. In this case, setting the No Caching option for a module has no effect. If the caching storage option is set to File, you can find the modules cache file (the output from all modules is stored within the same file) within the cache/com_modules directory.

To switch on Progressive Caching, go to Administrator → System → Global Configuration → System tab and within Cache Settings set System Cache to ON – Progressive caching.

As regards the conditions for caching of Joomla core component views, there is no difference between conservative and progressive caching. Despite what you may read on some websites and responses to Stack Overflow questions, it is not the case that Conservative Caching relates to when the user is not logged on and Progressive Caching to when the user is logged on.

Summary

A summary of the caching types is below.

Whole Page Caching

  • Configuration: Built-in Plugin (Administrator → Extensions → Plugin Manager → System - Page Cache)
  • Caches: each whole page of your site
  • Based on: URL
  • More info:
    • Optional browser caching: Also caches on your visitors' browser/computer
    • Only caches pages for guest visitors (not for logged in visitors). Be careful using this plugin if you have an interactive site where you want to server content based on session/cookie information rather than on the plain URL only. Features like a shopping cart will not work.

View Caching

  • Configuration: Global Config -> Cache
  • Caches: each view of a component
  • Based on: URL, view, parameters, ...
  • More info: Component developers have to include this in their code to work. Mostly this is not done. The Joomla main content component uses this, but only for guest visitors of your site though this is not obligated for every component.

Module Caching

  • Configuration: Global Config -> Cache
  • Caches: each module (individually customized via each module's Advanced Parameters)
  • Based on: the module id, the user's view levels and the Itemid parameter in the HTTP request
  • More info: You must disable it on some modules to avoid problems

Further Caching

If you want to check out other cache systems and possibilities, you might want to check out the third-party extensions around caching.

Caching engines or storages

  • Configuration: Global Config -> Cache

Here you can choose which system you want your site to use for all caching. Some options are: APC, Eaccelorator, File, Memcache, Redis, XCache.

APC, for example, also caches your PHP opcode.

For Developers

This section needs revision for Joomla! 4/5.

The class JCache allows a lot of different sorts and levels of caching. The following subclasses are made specifically, but you can add your own, or use the main one in many different ways.

Don't forget that the first level of cache encountered, will override any deeper caching. I suppose that too many levels is also counterproductive (to be verified though).

  • JCacheView caches and returns the output of a given view (in MVC). A cache id is automatically generated from the URI, specific view and its specific method, or you can give your own.

This can automatically be done via the base controller's display function. For example in the controller of your component:

class DeliciousController extends JController {
    function display() {
        parent::display(true); //true asks for caching.
    }
}

There are also some urlparams to consider. Check this Joomla StackExchange

Also, be aware that any updates (such as hits or visit counts) will NOT be updated (unless you add this outside this method and thus any deeper MVC-part.)

  • JCachePage caches and returns the body of the page.
  • JCacheCallback caches and returns the output and results of functions or methods.

If you want to cache queries, this is a good class for it, as illustrated here: Using caching to speed up your code

  • JCacheOutput caches and returns output.

This is rather meant for caching a specific part of PHP code. It acts like an output buffer, but cached.