As dicussed in the previous chapter, WebDoubler offers a choice in caching mechanisms. In some circumstances, the relatively limited size of caches possible using the RAM Cache may not be large enough to attain reasonable cache hit rates in your organization. In this case, the disk-based Cache Manager will increase the hit rate, and therefore overall performance of the server, by maintaining a much larger cache store.
To switch from the default RAM Cache to the disk-based Cache Manager, first quit WebDoubler. Open the WebDoubler "Plug-Ins" folder, and move the "WDPI RAM Cache" file into the "Disabled Plug-Ins" folder. Then move the "WDPI Cache Manager" file from the disabled folder into the active "Plug-ins" folder and restart.
The Cache Manager is responsible for storing objects retrieved from the Web that may be needed again in the future. As each request is processed, the Cache Manager oversees the transfer of the data to the Web browser once the data is retrieved by the HTTP Client. During this processing time, the Cache Manager determines whether or not the response to the request can be used again to service future requests for the same object. Objects that can be reused are considered "cachable", and are stored locally in RAM, on disk, or both.
If the response can be cached, then the object is stored in one of the cache folders that the Cache Manager has been configured to use. Subsequent requests for the item will receive the data from the cache, bypassing the process of retrieving the response from the remote server.
The more RAM and disk space made available for caching, the better. The Cache Manager settings allow you to adjust the cache to scale to the number of clients that utilize WebDoubler and to fit the hardware you are running WebDoubler on. The WebDoubler Cache Plug-In window is shown below. The Web administration of the Cache Manager is very similar and provides access to the same options.
Figure 7: The Cache Manager Window
Enable WebDoubler Cache
- This checkbox simply turns the Cache Manager on and off. Disabling the Cache Manager prevents retrieved objects from being stored locally and causes all requests to be completed by connecting to remote Web servers. This option is configured in the Web administration interface by setting the "Cache is" field to either "Enabled" or "Disabled".
Maintain Cache Decision Log
- The Cache Manager uses a complex set of rules to determine if an object is cachable or not. These rules are also used when a request is received by WebDoubler to determine if a cached object can be served from the cache. For debugging purposes, or for the adventurous soul seeking to optimize cache performance at a very detailed level, a "decision log" can be maintained that can be used to review how these rules are applied. This option is configured in the Web administration interface by setting the "Decision Logging" field to either "Enabled" or "Disabled".
The decision log is technical and can be complicated. We recommend that decision logging be left off unless you are having a specific problem with WebDoubler.
Cache Aggressiveness
- The cache supports 4 "aggressiveness levels": Ultra Conservative, Conservative (HTTP 1.1), Aggressive, and Very Aggressive.
Ultra Conservative: In this mode, the Cache Manager strictly follows the HTTP 1.1 draft standard rules for caching content. In addition, requests containing search or path arguements will never be cached or served from the cache.
- Conservative: In Conservative mode, the Cache Manager strictly follows the HTTP 1.1 draft standard. In some cases, this allows caching and serving of URLs containing search or path arguments.
- Aggressive: In this mode, the Cache Manager will serve standard Netscape Navigator reloads from the cache. Super reload bypasses the cache. This will serve "Not Modified" responses if it can, or may send the whole file as appropriate. Note that according to the HTTP 1.1 draft, Netscape reloads should NOT be served from the cache, because they contain a "Pragma: no-cache" cache-control directive.
- Very Aggressive: This mode breaks the HTTP 1.1 rules in many ways, including:
- Entities are served from cache if they exist, even if they are stale. This overrides reload and super-reload.
- Entities with no expires or last-modified information are still cached, with a default "shelf-life" of 12 hours.
- Heuristic shelf-life computation (if last-modified, but no expires provided) results in much higher values than conservative/standard computation.
Cache Locations
- The Cache Manager stores retrieved cachable items in "Cache Locations", also known as "Cache Folders". A Cache Location is a folder on your hard disk that contains the cached items as well as associated record keeping information.
Note: You should never delete these folders directly. Instead, use the Cache Location settings to add, remove, or change the capacity of a Cache Location.
Each Cache Location that you specify has a capacity associated with it. This capacity setting specifies the maximum amount of data (in megabytes) that the Cache Location will use on disk. This nature of Cache Locations allows you to predictably use specific hard disks without worrying how much space the cache will grow to use.
To add a new Cache Location:
- Using WebDoubler's application interface:
- Create a new folder that will contain the cached data by choosing "New Folder" from the File menu in the Finder.
- In the WebDoubler "Cache Manager" window, click the "Add..." button.
- Navigate using the Standard File dialog to the folder that you created in step 1.
- Click "Select" to specify the folder.
- Using remote Web administration:
- On the Cache Administration page, enter the full path to the Cache Folder in the blank "Full Path" field provided. A Mac OS "Full Path" is made up of the volume name (usually a hard disk), followed by each folder name leading to the cache folder. Each folder name is seperated by a colon, as in:
- MacHD:WebDoubler:CacheFolders:Cache1
If the specified Cache Folder doesn't exist, WebDoubler will create it.
- Also enter the maximum size (in MB) of the Cache Location in the column labeled "Capacity".
- After you have carefully entered the Cache Folder path, click the "Save Changes" button.
To remove a Cache Location:
- Using WebDoubler's application interface:
- In the WebDoubler "Cache Manager" window, select the Cache Location you want to remove by clicking on its name.
- Click the "Remove" button.
- In the Finder, drag the folder that contained the Cache Location to the trash.
- Using remote Web administration:
- On the Cache Administration page, click the "Delete" link next to the Cache Location to be removed.
- A warning will be displayed asking you to confirm the action. Click "Confirm Delete".
- At any time later (for example, during routine maintenance), go to the WebDoubler Macintosh. In the Finder, drag the folder that contained the Cache Location to the trash.
To set the capacity of a Cache Location:
- Using WebDoubler's application interface:
- In the "Cache Manager" window in WebDoubler, select the Cache Location you want to change by clicking on its name.
- Type in the new capacity (in MB).
- Click the "Change" button.
- Using remote Web administration:
- On the Cache Administration page, find the line that defines the Cache Location in the Disk Cache table. Enter the maximum size (in MB) of the Cache Location in the column labeled "Capacity".
- Click the "Save Changes" button.
