db

type DB interface {
	// Kind identifies this particular database implementation.
	//
	// Among other things it is used in Cloud Tasks messages involved in the
	// implementation of the distributed sweeping.
	Kind() string

	// Defer defers the execution of the callback until the transaction lands.
	//
	// Panics if called outside of a transaction.
	//
	// The callback will receive the original non-transactional context.
	Defer(context.Context, func(context.Context))

	// SaveReminder persists reminder in a transaction context.
	//
	// Tags retriable errors as transient.
	SaveReminder(context.Context, *reminder.Reminder) error

	// DeleteReminder deletes reminder in a non-transaction context.
	DeleteReminder(context.Context, *reminder.Reminder) error

	// FetchRemindersMeta fetches Reminders with Ids in [low..high) range.
	//
	// RawPayload of Reminders should not be fetched.
	// Both fresh & stale reminders should be fetched.
	// The reminders should be returned in order of ascending Id.
	//
	// In case of error, partial result of fetched Reminders so far should be
	// returned alongside the error. The caller will later call this method again
	// to fetch the remaining of Reminders in range of [<lastReturned.Id+1> .. high).
	FetchRemindersMeta(ctx context.Context, low, high string, limit int) ([]*reminder.Reminder, error)

	// FetchReminderRawPayloads fetches raw payloads of a batch of Reminders.
	//
	// The Reminder objects are re-used in the returned batch.
	// If any Reminder is no longer found, it is silently omitted in the returned
	// batch.
	// In case of any other error, partial result of fetched Reminders so far
	// should be returned alongside the error.
	FetchReminderRawPayloads(context.Context, []*reminder.Reminder) ([]*reminder.Reminder, error)
}

type Impl struct {
	// Kind identifies this particular DB implementation.
	//
	// Must match Kind() of the produced DB instance.
	Kind string

	// ProbeForTxn "probes" a context for an active transaction, returning a DB
	// that can be used to transactionally submit reminders or nil if this is not
	// a transactional context.
	ProbeForTxn func(context.Context) DB

	// NonTxn returns an instance of DB that can be used outside of
	// transactions.
	//
	// This is used by the sweeper to enumerate reminders.
	NonTxn func(context.Context) DB
}