start_time for frequency-based trips exact_times=0 is immutable #34
Description
Wednesday Apr 26, 2017 at 00:17 GMT
Originally opened as CUTR-at-USF#154
Summary:
From the spec (https://developers.google.com/transit/gtfs-realtime/reference/TripDescriptor):
If the frequency-based trip corresponds to exact_times=0, then its start_time may be arbitrary and it is expected to be the first departure of the trip.
Once established, the start_time of a frequency-based trip (exact_times=0) should be considered immutable, even if the first departure time changes; that time change may instead be reflected in a StopTimeUpdate.
If start_time changes for a trip instance that's exact_times=0 after it's created (for same start_date and vehicle_id), this is an error.
This might be tricky to detect, as you need to look at the current and past updates to detect changes. We currently have access to both the current and previous feed message in the validator classes, and I'd like to try to stick to examining only these two messages if possible.
We may not be able to detect this problem in feeds with 100% confidence, so in that case we need to decide if we're comfortable still calling this an error or if we should drop it down to a warning.