Flex Article

Improving Flex application performance using the Flash Player cache

Working with the Flash Player cache

The Flash Player cache is a new feature available in Flash Player 9 Update 3 (9,0,115,0). The cache allows files signed by Adobe to be cached by Flash Player. These files end in the file extension .swz.

The Flash Player cache differs from the browser cache in some useful ways. Because the cached SWZ files are signed by Adobe, they can be reused in more than one domain and are not limited to reuse within the domain in which they originated. For example, imagine that framework.swz is loaded from http://www.a.com and is placed in the cache of Flash Player. If the same framework.swz file needs to be downloaded from http://www.b.com, Flash Player looks in its cache to find a.com's framework.swz file. When it finds a match, it uses the cached file rather than loading a new file from b.com.

The SWZ file persists in the cache even after the browser is closed. The file remains in the cache indefinitely, provided that the end user does not change settings in the Flash Player Settings Manager to reduce their caching ability. However, future SWZ files may eventually cause older, unused SWZ files to cycle out of the cache when the cache size limit is reached.

Fortunately, you need only one copy of a framework, since Flash Player uses a common cache for all browsers on a system. This means that if a user downloads a SWZ file using Microsoft Internet Explorer, that same SWZ file will also be available in Mozilla Firefox.

Flex 3 is the first technology to take advantage of the Flash Player cache. In Flex 3, the framework library code has been rolled into Runtime Shared Libraries (RSLs). An RSL can be either a SWZ file or a SWF file that is loaded at runtime instead of being statically linked into the application. Because of this, the application's file size becomes smaller.

A developer can still choose to use unsigned RSLs, which are normal SWF files that are not loaded into the Flash Player cache. Instead, these RSLs rely on the browser's cache to keep them from being downloaded multiple times.

Once the Flex-signed RSLs are cached on a user's machine from running one application, they are available for use by all other applications, regardless of the original domain used to load the application. As Flex applications start using signed RSLs, they will all be able to benefit from the RSLs loaded in the Flash Player cache with lowered bandwidth costs and faster Flex application startup times.

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