Fetch Diagnosis Keys for current day (contributes to corona-warn-app/cwa-documentation#236)

[kbobrowski]

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Description

Currently only daily bundles of Diagnosis Keys are fetched, which introduces in worst-case additional 24 hours delay in exposure notification. Fetching hourly Diagnosis Keys decreases this delay and aligns Android implementation with iOS implementation.

Unit test is extended but I have not tested it with live API.

Contributes to corona-warn-app/cwa-documentation#236

[jakobmoellerdev]

Hi! Where does iOS actually have the fetchHours method used? From what I see, the behaviour aligns right now.

[kbobrowski]

Hey :) fetchHours is used here on iOS (within fetchDays method). But I'm not iOS dev, so difficult for me to say for certain if in the end hour endpoint is queried.

But I think this is not the main point (alignment with iOS), I wonder why we cannot fetch also hour endpoint? it seems we only fetch days right now, and functionality to fetch hours used to be there. This would greatly decrease delay in providing user with exposure notification. This is a subject of this issue: corona-warn-app/cwa-documentation#236 , if you could clear our doubts over there (especially regarding limits imposed on provideDiagnosisKeys) - it would be great, thanks!

(this issue is quite lengthy, but in the end everything seems to be connected to limits imposed on provideDiagnosisKeys - does it limit number of files passed, or number of calls to the method?, there seems to be discrepancy right now in interpretation of these limits, please check my last comment)

[jakobmoellerdev]

We are preparing a common statement that is applicable for both Apps and should give some clarity on the behaviour

[kbobrowski]

@jakobmoellersap thanks for the update, statement applicable to both apps will be of course useful and I'll wait for it then, but in a meantime I think it would still be important to clarify the specific issue of provideDiagnosisKeys - what are the limits here. This is something that community cannot easily test (perhaps using rev-eng tools). I'm curious if you experienced limits on number of files passed or on number of calls to this method - this would help a lot in supporting you with dev work here

[kbobrowski]

I've experimented with exposure notification client captured from Java heap and fully understand now limitations of current provideDiagnosisKeys, it will not accept multiple files which have batch_size set to 1. Google should allow soon multiple batches per request (https://github.com/corona-warn-app/cwa-documentation/issues/236#issuecomment-647618727), perhaps then it would be possible to include hourly batches without making deep changes to current architecture

[jakobmoellerdev]

As discussed, this will most likely be revisited with the next major Google API Update and or a our architecture alignment regarding background jobs. Thus I will close the PR as it would not make sense to introduce changes to the logic that is subject to change right now.