Remove `DateSchedule` and `NoSchedule`

[jlowin]

Prefect currently supports two schedule classes that are probably not useful: DateSchedule (a list of datetimes) and NoSchedule (no schedule at all).

In Prefect's original design, schedules were created when the flow was created and was therefore impossible to change, so it was desirable to have

1. a uniform interface for accessing the schedule's information, even if there wasn't a schedule
2. a way to specify that the flow should run on a few (off-schedule) dates

Now, with @dylanbhughes's great observation that schedules should/can be modified after the flow is created, we may do schedule examination and validation at any time. NoSchedule can simply be replaced by None and DatesSchedule can be replaced by using Prefect's API to simply schedule flow runs by hand on the appropriate dates.

Therefore, the only built-in schedule classes should handle ongoing, regular schedules. Currently we have CronSchedule and IntervalSchedule and I can imagine more complex versions (like BusinessDaySchedule).

[jlowin]

cc @cicdw and @joshmeek for comment

[cicdw]

Overall +1, although I do still see a benefit to DateSchedule: it seems easier to provide a list of dates that I manually update whenever I feel like and let Prefect handle the scheduling / running than to have to know how to manually start jobs at precisely the right time myself.

[jlowin]

I think that you'll be better served by generating that list of dates and iterating over it with client.create_flow_run(flow_id, start_time=schedule_dt).

Putting it in the schedule object creates difficulty server side (we have to check every schedule to see if it should run, even if it turns out to have no dates in the future), as well as a UI difficulty (how to display arbitrarily long list of dates, and how to create an analogous input?)

I would prefer that Schedules are for legitimate regular schedules where exact dates don't have to be passed, and ad-hoc runs, even taking place years in the future, are added by users by hand (via GQL or Client)

[cicdw]

Fair - are we going to encourage the use of the Client to users?

[jlowin]

Yes, as an easier way of writing GQL

[jlowin]

I have a better way to explain my thinking:

The Schedule object is a stand-in for simple computations, like needing to create runs every hour, or week, or day. If you already know the dates you want a Flow to run -- or you don't need it to run at all -- then you can simply create the flow runs yourself on those dates. The Schedule object is only needed if the required dates are infinite or overwhelming -- something that requires a monitor process to create them. I think if we had designed Prefect from day 0 knowing schedules could be set via API, we'd never have considered Schedule objects that didn't require some computation.

Not directly related, but worth mentioning:

In the case of very complex schedules, I think best practice will be to create a simple flow that evaluates those complex schedules and creates the flow runs directly. For example: every other business day, but not a holiday; or monthends with an adjustment for weekends.

[joshmeek]

I'm not entirely sold on this. My first time using schedules in Prefect I immediately went towards the Date Schedule. I understand what you're saying in that schedules seem like they should be more infinite instead of a list of finite dates, however attaching a one-off date schedule seemed natural.

How easy / obvious is it going to be to send an API call with a new scheduled run? It comes across as second to just attaching a simple datetime to a flow.