Push notification to update files

[tobiasKaminsky]

@AndyScherzinger and me brainstormed a bit how to enhance sync on Android or iOS.
He had the idea to use push notification to inform app about file changes.

On Android we store state of a folder in DB:
before

• go into a folder with 1k images
• downloads metadata of 1k images
• stores into DB
• refresh
• no eTag change -> nothing to do
• add one file on server
• refresh
• etag change
• syncs 1k + 1 image metadata

after:

• initial sync of 1k metadata
• receive push "added 1 image to folder xyz" + new etag
• app checks that folder is in DB
• app adds one missing file to DB
• checks etag, to verify that no change was missed
• user enters folder next time
• only quick etag check

[nickvergessen]

Just for the record, the text length we are able to transmit is rather short ~150 chars (200 total - some meta overhead).
Also they are not always reliable, but the last etag check would still catch that and proceed as of now, right?

The other question that arises for me is scaling. If a folder is changed on the company instance, hundreds of pushes will have to be prepared and performed, just to have an eventually improvement later on.