perf(SF2.0): Stream predictions to UpcomingDepartures.Server instead of polling

[joshlarson]

Scope

Asana Ticket: 📅🔎🎭 Extract prediction fetching out of Upcoming Departures GenServer

Implementation

(Don't bother looking at this commit-by-commit - the commits are kind of a mess - sorry!)

Dotcom.Predictions

This introduces a new namespace with the following modules:

• Dotcom.Predictions.Manager
  • The entrypoint to the outside world - consumers can call subscribe or unsubscribe in order to receive messages when predictions data is added or modified. When a process subscribes to a predictions stream, it creates an EventSupervisor that (see below) gathers up predictions events and sends them back to its subscribers.
• Dotcom.Predictions.EventSupervisor
  • A supervisor that supervises two children - a ServerSentEventStage that hooks into the V3 API to get 🥟steamed🥟 messages, and an EventBroadcaster.
• Dotcom.Predictions.EventBroadcaster
  • A simple worker set up to receive events from the server sent events stream and broadcast them back to the Manager.

Other Changes

There are other oddball changes in this PR:

• Dotcom.UpcomingDepartures.Server
  • Re-architect this module a decent amount in order to handle messages from the predictions stream, rather than polling @predictions_repo.
• DotcomWeb.Live.UpcomingDeparturesLive
  • Update to handle a :loading? state from UpcomingDepartures.Server.
• PredictedSchedule.Collection
  • Add some more convenience functions to make it respond nicely to the types of prediction update events that we get from the API.
• Predictions.StreamParser
  • Its original implementation dropped timezone information; this PR updates it to work the same as Predictions.Parser.

Screenshots

Enjoy this side-by-side of local and prod for Green Line at Fenway Westbound, on either side of a photo of a Green Line train at Fenway going Westbound.

You may notice that local has one timestamp different from prod - that's because by virtue of streaming, this branch responds more quickly to updates than prod does, hence local showing a lower number of minutes remaining than prod (prod updated to also show 17 about 3 seconds after I took ☝️ screenshot).

How to test

Visit any departures page and verify that it loads upcoming departures as expected.

You won't notice much in the way of visual changes, but you might notice that this branch responds to changes in a snappier way.

[thecristen]

I'm feeling good about this and all the different bugs we've tackled on it today. I'd like to give this a whirl.