What is a cacheable item?

First, let's cover the kind of web items that can and should be cached.

• static content (images and JavaScript)
• content for anonymous users. Items (such as Plone-generated HTML) can be cached with little danger.
• selected content for logged in users.

Warning Incorrect configuration of caching has the potential for authenticated requests being cached (which is bad). Generally, the two Plone products discussed below have sensible policies about not caching authenticated requests. Nonetheless, if you are creating a customized CacheFu policy or using another caching product for Plone, it's a good idea to confirm that authenticated requests are not being cached.

First Step: Install and Activate a Caching Product

Before you can cache, you must install and activate a product in Plone specifically for caching. With this product you can configure and customize cache settings.

Available Caching Products

• Important: you need to enable a caching policy for CacheFu to work in EP. On the first tab, you need to choose Zope Behind Squid as the Active Cache Policy.

 

• Chasseur is a Plone product included with Enfold Server (a commercial product). It was designed for ease-of-use and simplicity.  (Read more about Chasseur). The main two settings are Normal (cache images, js, css for one hour) and Aggressive (cache js, css, images for one hour and also cache content for anonymous users for 1 hour).

  • CacheFu will use cache headers different from chasseur. But EP is able to process both of them.
  • Normal is generally risk-free and unlikely to cause problems, especially if image files are unlikely to change.
  • Aggressive is generally not recommended because it interferes with EP's ability to purge content. When that happens, an individual browser might use private cache instead of checking with EP/ES to make sure it has fresh content.

To verify that caching is taking place, look for this line in your HTTP response header: X-Cache: HIT from www.originalfunsite.com

• If you see X-Cache: HIT, then yes, caching is occurring.
• If you see X-Cache: MISS, then this particular item was NOT cached.

Generally HIT's indicate successful caching.

Keep in mind that X-Cache: MISS is not always a bad thing. For example, if logged in as an administrator, many resources will not be cached on purpose. It's best to test as an anonymous web surfer (i.e., someone who is not logged in). If an item contains a modification, then a MISS reply is mandatory.

Careful: When testing, it's a good idea to use two different browsers. In Browser A (such as Internet Explorer), log in as Admin. In Browser B (i.e., Firefox), log in as anonymous or as a logged in user. It's also important to clear cache the right way and even to close the browser entirely (if you don't, the http headers you see won't be accurate). Consult the troubleshooting checklist for caching.

To see the HTTP headers directly, see Tools for Viewing Headers . You should probably pick two or three sample items to check for headers. Anything will do, but at least one should be a graphic and one should be a Zope page (such as http://www.originalfunsite.com/events ). In Chasseur, the maximum age is

Cache-Control: max-age=3600 X-Cache: HIT from www.originalfunsite.com

Optimize Cache Settings and Measure Caching Performance

Many of your cache settings can be tweaked in whatever Plone product you are using to cache web items. The CacheFu Plone product, for example, offers a lot of fine-grained controls and the ability to create caching rules.

But EP also offers some cache settings to tweak. (By the way,  there are actually separate screens for the Caching and the (Adv) link at the top. The screenshot below only shows what you see when you press the (Adv) screen.   

Here are some default settings for each proxy definition as set by Enfold Proxy:

• Maximum Size of the Cache: 10 gigabytes for each proxy definition
• Maximum number of items in the cache for each proxy definition: 50,000
• Maximum size of an item which can be cached: 100K
• Default-Age: 0

The main thing you can adjust here is storage amount and item size. The larger the storage amount you have, the more cacheable items EP can keep at once. Increasing maximum item size is recommended if your Plone server returns a lot of web items (which is not the same thing as "web pages") over 100K. If the maximum is increased to a very large size, this will also consume RAM on the machine with EP. Also, when you cache larger files, that will cause the maximum size of the cache to fill up more quickly. Theoretically, if your cache directory for your proxy definition is always at maximum and the hit percentage is declining, that could indicate you need to increase your maximum here.

Default-age=0 will cause EP to store cacheable items in the cache and always check to make sure if it is current before serving cached content to the browser. (This is the default and generally recommended). Even so, EP will not cache an item which is forbidden to be cached (according to Plone or whatever Plone Product is setting these commands).

If you check Enfold Proxy's cache.log messages for your proxy definition (and set the log level to Information), you will see every minute or so some statistics about caching:

2008-02-18 10:31:45,250|cache.host originalfunsite|STATS|3500|2856|Cache statistics:
        gets: 88, hits: 65 (33 validated), misses: 23 (0 uncachable)
        hitrate: 73% (58% excluding validations)
        size: 1943668 bytes, 324 items

The most important number here is the 58% in parenthesis.  The higher, the better.  This number refers to requests that EP returned without forwarding it to Enfold Server or Plone (thus resulting in the most time-saving). The 73% number includes those occasions when the item was stale and EP needed to make a conditional request to Enfold Server/Plone. So 15% (i.e., 73-58) is the percentage of times when EP had to revalidate stale cache by checking with Enfold Server or Plone. These revalidation requests generally do not result in significant time savings, so for all practical purposes the only number you need to worry about is the number in parenthesis.

The cache directory

The proxy allows you to configure the directory where cached items are stored for a particular proxy definition.

1. Open EP configuration tool.
2. Select your proxy definition. Select the Caching tab.
3. Leave this blank if you want your cache directory to exist under the application directory. The directory will be created if it does not exist. If you wish to put your cache files elsewhere, you should copy the complete path to the directory here (e.g., C:\My Documents\proxy definition 1\cache).

IIS must be restarted before a change to this option will take effect.  

When you change the cache_directory, the old cache is not migrated to the new location (i.e., the old cache remains and a new empty cache is created at the new location.)

Migrating your cache to a different place:

1. Stop IIS.
2. Move the cache directory into a new location.
3. Copy the absolute path into the Cache Directory field of your proxy definition.
4. restart IIS.