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RSLs: The Diet Pills for Flex-based Apps

Posted by Sravan | April 8, 2009 .

Less than a week after using the first release of Tour de Flex I made some charges against beloved Adobe AIR primarily regarding the installer size and the memory footprint. Last week, I made another charge this time regarding the hyperactivity of a couple of DLLs.

Thankfully, the latter post got a visit from the Adobe AIR PM Rob Christensen who subsequently made very useful clarifications. The following part especially caught my attention:

In general, AIR files tend to consist of files that are already compressed like (.swf, .jpg, .mp3, etc.). As such, you are not likely to see an .air file compress down much further.

Fair enough but disappointing. Large file sizes bother me because I live in India where my 256 KBps broadband connection in reality gives me barely 25 KBps download speed.

Nevertheless Tour de Flex once again gave a new hope. I remember the installer for the earlier version to be about 52 MB. The installer for the latest version on the other hand, which comes with a lot more component samples and a more useful UI has been reduced to 36 MB. Nearly 30% weight loss.

TourDeFlex

Rich Tretola hinted the answer in his post about the updated Tour de Flex:

The download size is now 35% smaller thanks to RSLs.

I am not exactly sure how AIR RSLs work but the introduction to the Flex 3 Runtime Shared Libraries (RSLs) throws some light on this. These RSLs are SWF files which can be shared by different application SWF files. In addition, the signed kind of RSLs (signed by Adobe) can be even shared by entirely different apps. So if you download any signed RSLs by visiting one Flex website and then visit a completely unrelated Flex website which uses the same signed RSLs, they of course won’t need to be downloaded again. The unsigned RSLs obviously can’t be shared by all Flex apps because of security issues.

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2 Comments so far
  1. ariel sommeria  April 9, 2009 5:24 am

    Hi Sravan,
    downloading RSLs uses the browser cache, so if you get the same RSL at 2 different locations, you will download them at both places. However if you publish a new version of your main SWF but not the RSL, your user won’t have to download the RSL because it hasn’t changed.
    Another caveat: When you use a static library, the compiler eliminates the unused classes from the final SWF. However if you use a RSL, you have to download the whole RSL.
    So RSLs are still worth looking at, but only for really big projects, I think. Our project at pearltrees is just getting into that area, IHMO

  2. Sravan  April 9, 2009 8:12 am

    Hi Ariel,

    If the RSL only lies in the browser cache, then it means that I’ll have to download it every time I free my cache, isn’t it?

    And please check your mail.

    Thanks,
    Sravan

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