Tech Trivia : Solve the Deadlock !

A piece of code gave a DB deadlock error. Figure out the reason ! ( and possibly the solution )

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The code is in Python. But the trivia is less about the code and more about problem-solving.

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Write your answers in the comments section. The first few right answers get a shoutout.

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Good luck !

 

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Error :

deadlock detected
DETAIL: Process 10137 waits for ShareLock on transaction 1842665572; blocked by process 32398.
Process 32398 waits for ShareLock on transaction 1842665620; blocked by process 10137.
HINT: See server log for query details.
CONTEXT: while updating tuple (4368,11) in relation "person" while processing record

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Code :

@transaction.atomic
def update_person_parameters(self, person_id, **kwargs):

    Person = apps.get_model("params", "Person")
    PersonSteps = apps.get_model("params", "PersonSteps")
    PersonHeartRate = apps.get_model("params", "PersonHeartRate")
    PersonTemperature = apps.get_model("params", "PersonTemperature")
    TrainingSlot = apps.get_model("slot", "TrainingSlot")
    TrainerBooking = apps.get_model("params", "TrainerBooking")

    steps = kwargs.pop("steps")
    heart_rate = kwargs.pop("heart_rate")
    temperature = kwargs.pop("temperature", [])

    trainer_bookings = self.model.objects.filter(
        status__in=(self.model.STATUS_UPCOMING, self.model.COMPLETED,
                    self.model.MISSED),
        person__person_id=person_id,
    ).values_list("primary_key", flat=True)

    if not trainer_bookings.exists():
        return # Nothing to do

    person_queryset = Person.objects.filter(
        booking_id__in=trainer_bookings, person_id=person_id
    )

    person_queryset.update(**kwargs)

    centers = HealthCenter.objects\
        .filter(pk__in=person_queryset.values("trainer_bookings__centers__primary_key")[:1])

    mapping_queryset = TrainingSlot.objects\
        .filter(center__bookings=OuterRef("trainer_booking"))
        .values("training_slot")[:1]

    Person.objects.update(session=Subquery(mapping_queryset))

    person_subquery_qs = Person.objects
        .filter(trainer_booking=OuterRef("primary_key"))
        .values("training_slot")[:1]

    TrainerBooking.objects.filter(guests__in=person_queryset)\
        .update(session=Subquery(person_subquery_qs))

    #     A lot of other work
    #     ...
    #     ...
    #     ...

 

The solution is in the next blog …

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