المعاملات والكتابة المتزامنة

from django.conf import settings
settings.configure(
    SECRET_KEY="synthetic-course-key",
    USE_I18N=False, USE_TZ=True,
    INSTALLED_APPS=[],
    DATABASES={"default": {"ENGINE": "django.db.backends.sqlite3", "NAME": ":memory:"}},
    DEFAULT_AUTO_FIELD="django.db.models.AutoField",
    ALLOWED_HOSTS=["testserver"],
)
import django
django.setup()

from django.db import connection, models, transaction
from django.db.models import F

class Course(models.Model):
    seats = models.PositiveIntegerField()
    class Meta:
        app_label = "lab"

with connection.schema_editor() as editor:
    editor.create_model(Course)
course = Course.objects.create(seats=3)
def reserve(quantity):
    with transaction.atomic():
        return Course.objects.filter(pk=course.pk, seats__gte=quantity).update(seats=F("seats") - quantity)

print(reserve(2))
print(reserve(2))
print(Course.objects.get(pk=course.pk).seats)

1
0
1

يميز عدد الصفوف القبول من نقص السعة بقاعدة ذاكرية معزولة.

from django.conf import settings
settings.configure(
    SECRET_KEY="synthetic-course-key",
    USE_I18N=False, USE_TZ=True,
    INSTALLED_APPS=[],
    DATABASES={"default": {"ENGINE": "django.db.backends.sqlite3", "NAME": ":memory:"}},
    DEFAULT_AUTO_FIELD="django.db.models.AutoField",
    ALLOWED_HOSTS=["testserver"],
)
import django
django.setup()

from django.db import transaction
events = []
with transaction.atomic():
    transaction.on_commit(lambda: events.append("committed"))
    print(events)
print(events)
try:
    with transaction.atomic():
        transaction.on_commit(lambda: events.append("rolled-back"))
        raise ValueError("Synthetic rollback")
except ValueError:
    pass
print(events)

[]
['committed']
['committed']

ينفذ الالتزام الأول ويسقط التراجع الثاني دون بريد أو طابور أو شبكة.