Google explained how the system works in a blog post this week. Currently, app updates require downloading and installing a new package that contains most of the app’s files. This means its file size is relatively high, leading to longer download times.
The new method uses file-by-file patching to only send your device the actual changes made to the app. Google likened it to an edit that needs to be made to a sentence before a book is published. An editor could send you an entirely new book with the relevant sentence updated. It’s far easier if they just tell you what sentence needs changing and what to change it to though.
File-by-File patching enables Google to do this for app updates. It looks for changes in the uncompressed files of the old and new versions of the app. When your phone needs to download an update, the old app package on your phone and the new one on the server are decompressed. Changes in the files can then be detected and a patch generated, based on only the edits made over your existing app version.
The result of this process is significantly smaller app updates. On average, Google has observed a 65 percent reduction in app file size when using file-by-file patching. In some cases, the updates were as much as 90 percent smaller. This has allowed Google to save the equivalent of six petabytes of customer data every day.
File-by-File patching isn’t a perfect solution to app updates. It has one significant limitation that’s restricting its use for now. Because the process involves decompressing and then recompressing the app package stored on your phone, additional computing power is required over a regular app update.
On modern phones, it takes around one second per megabyte to recompress the app. On older devices, it can be considerably longer. This makes app installation times noticeably slower than when file-by-file patching is disabled, slowing down devices and increasing the time until an app is usable after being updated.
As a consequence of this, Google is currently enabling file-by-file patching for automatic updates only. These occur in the background when the phone is idling, usually at night or while charging. This should cut down on waiting times caused by the recompression procedure while allowing your phone to save data the rest of the time. Google said it will continue to develop the technique though, potentially enabling an expansion to manual updates in the future.
“Android users are downloading tens of billions of apps and games on Google Play. We’re also seeing developers update their apps frequently in order to provide users with great content, improve security, and enhance the overall user experience,” said Google. “It takes a lot of data to download these updates and we know users care about how much data their devices are using.”
