[tor-bugs] #25587 [Applications/Tor Browser]: Investigate optimizing APK for Smart App Updating

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#25587: Investigate optimizing APK for Smart App Updating
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     Reporter:  sysrqb                    |      Owner:  tbb-team
         Type:  project                   |     Status:  new
     Priority:  Medium                    |  Milestone:
    Component:  Applications/Tor Browser  |    Version:
     Severity:  Normal                    |   Keywords:  tbb-mobile
Actual Points:                            |  Parent ID:
       Points:                            |   Reviewer:
      Sponsor:                            |
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 In mid-2012, beginning with Jelly Bean (4.1), Google Play began using
 Smart App Updates [0]
 {{{
 Smart App Updates

 Smart app updates is a new feature of Google Play that introduces a better
 way
 of delivering app updates to devices. When developers publish an update,
 Google
 Play now delivers only the bits that have changed to devices, rather than
 the
 entire APK. This makes the updates much lighter-weight in most cases, so
 they
 are faster to download, save the device’s battery, and conserve bandwidth
 usage
 on users’ mobile data plan. On average, a smart app update is about 1/3
 the
 size of a full APK update.
 }}}

 Considering Orfox is now ~30MB, and this may increase with additional
 locales, we should track the current diff patch size Google Play
 distributes for incremental updates, and we should investigate if we can
 tweak our APK so it provides minimal diffs.

 Can we (nearly) reproduce their results?

 StackOverflow suggests they are using GDIFF [1].

 Google was nice enough to publish some more details (4 years later) about
 how they're now using a newer diff method in some situations [2]:

 {{{
 For approximately 98% of app updates from the Play Store, only changes
 (deltas)
 to APK files are downloaded and merged with the existing files, reducing
 the
 size of updates. Google Play has used delta algorithms since 2012, and we
 recently rolled out an additional delta algorithm, bsdiff (created by
 Colin
 Percival1), that our experimentation shows can reduce delta size by up to
 50%
 or more compared to the previous algorithm for some APKs. Bsdiff is
 specifically targeted to produce more efficient deltas of native libraries
 by
 taking advantage of the specific ways in which compiled native code
 changes
 between versions. To be most effective, native libraries should be stored
 uncompressed (compression interferes with delta algorithms).
 }}}

 Unfortunately, we are restricted by our user base and by which platforms
 Mozilla is targeting. At this point, Mozilla are still targeting SDK 16,
 which is good for us, but we don't get new optimizations:

 {{{
 However, please note, native libraries should only be uncompressed when
 the
 minimum SDK version for an APK is 23 (Marshmallow) or later
 }}}

 So we should keep bsdiff optimizations in mind for when Mozilla eventually
 move to min-sdk 23+.

 [0] https://developer.android.com/about/versions/jelly-bean.html
 [1] https://stackoverflow.com/questions/12860938/smart-app-updates-on-
 google-play-store-how-does-it-work/12877791#12877791
 [2] https://android-developers.googleblog.com/2016/07/improvements-for-
 smaller-app-downloads.html

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25587>
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