Rescheduling commands

[gornik]

It is currently not possible to schedule a command using schedule_uuid that was previously cancelled:

iex> Scheduler.schedule_once(seat_id, %TimeoutSeatReservation{...}, ~N[2020-01-01 12:00:00])
:ok
iex> Scheduler.cancel_schedule(seat_id)
:ok
iex> Scheduler.schedule_once(seat_id, %TimeoutSeatReservation{...}, ~N[2020-01-01 14:00:00])
{:error, :already_scheduled}

This is because command handlers for scheduling are allowed only when the schedule aggregate instance does not exist yet:

  def execute(%Schedule{schedule_uuid: nil} = schedule, %ScheduleOnce{} = once) do
    ...
  end

  def execute(%Schedule{}, %ScheduleOnce{}), do: {:error, :already_scheduled}

https://github.com/commanded/commanded-scheduler/blob/master/lib/commanded/scheduler/schedule/schedule.ex#L27

This means that if we need rescheduling, we need to generate a new random schedule_uuid for each new schedule and track it client side. This makes it a bit more difficult and doesn't guarantee schedule uniqueness (in the example above that we have a single active schedule per single seat), e.g.:

iex> Scheduler.schedule_once(seat_id, %TimeoutSeatReservation{...}, ~N[2020-01-01 12:00:00])
:ok
iex> Scheduler.cancel_schedule(seat_id)
:ok
iex> Scheduler.schedule_once(seat_id, %TimeoutSeatReservation{...}, ~N[2020-01-01 14:00:00])
:ok
iex> Scheduler.schedule_once(seat_id, %TimeoutSeatReservation{...}, ~N[2020-01-01 16:00:00])
{:error, :already_scheduled}

I believe it might also be useful to have some kind of rescheduling of a command, something like:

Scheduler.reschedule(seat_id, ~N[2020-01-01 14:00:00])

[slashdotdash]

As you've discovered the current behaviour is by design (a command or batch may only be scheduled once), but it could be changed. I'm happy to accept a PR to add a Scheduler.reschedule function.

[gornik]

Thank you! I'll try to prepare a PR with support for rescheduling.

Regarding allowing a schedule to be scheduled again after cancellation: I think if we have rescheduling implemented, this would no longer be necessary, but still worth adding. Let me know what you prefer.

[slashdotdash]

My preference would be to only allow rescheduling when the schedule is active.