Locking backend with database (#185)

* Locking backend with database

* documentation of database lock

Author: nerjan

demo/demo/settings.py

@@ -127,3 +127,5 @@
 CRON_CLASSES = [
     "demo.cron.EmailUsercountCronJob",
 ]
+# If you want to test django locking with database
+# DJANGO_CRON_LOCK_BACKEND = "django_cron.backends.lock.database.DatabaseLock"

django_cron/admin.py

@@ -4,7 +4,7 @@
 from django.db.models import F
 from django.utils.translation import gettext_lazy as _
 
-from django_cron.models import CronJobLog
+from django_cron.models import CronJobLog, CronJobLock
 from django_cron.helpers import humanize_duration
 
 
@@ -54,3 +54,4 @@ def humanize_duration(self, obj):
 
 
 admin.site.register(CronJobLog, CronJobLogAdmin)
+admin.site.register(CronJobLock)

django_cron/backends/lock/database.py

@@ -0,0 +1,29 @@
+from django_cron.backends.lock.base import DjangoCronJobLock
+from django_cron.models import CronJobLock
+from django.db import transaction
+
+
+class DatabaseLock(DjangoCronJobLock):
+    """
+    Locking cron jobs with database. Its good when you have not parallel run and want to make sure 2 jobs won't be
+    fired at the same time - which may happened when job execution is longer that job interval.
+    """
+
+    @transaction.atomic
+    def lock(self):
+        lock, created = CronJobLock.objects.get_or_create(job_name=self.job_name)
+        if lock.locked:
+            return False
+        else:
+            lock.locked = True
+            lock.save()
+            return True
+
+    @transaction.atomic
+    def release(self):
+        lock = CronJobLock.objects.filter(
+            job_name=self.job_name,
+            locked=True
+        ).first()
+        lock.locked = False
+        lock.save()

django_cron/migrations/0003_cronjoblock.py

@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('django_cron', '0002_remove_max_length_from_CronJobLog_message'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='CronJobLock',
+            fields=[
+                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
+                ('job_name', models.CharField(max_length=200, unique=True)),
+                ('locked', models.BooleanField(default=False)),
+            ],
+        ),
+    ]

django_cron/models.py

@@ -29,3 +29,8 @@ class Meta:
             ('code', 'start_time')  # useful when finding latest run (order by start_time) of cron
         ]
         app_label = 'django_cron'
+
+
+class CronJobLock(models.Model):
+    job_name = models.CharField(max_length=200, unique=True)
+    locked = models.BooleanField(default=False)

django_cron/tests.py

@@ -14,7 +14,7 @@
 from django.contrib.auth.models import User
 
 from django_cron.helpers import humanize_duration
-from django_cron.models import CronJobLog
+from django_cron.models import CronJobLog, CronJobLock
 import test_crons
 
 
@@ -105,6 +105,17 @@ def test_file_locking_backend(self):
         self._call(self.success_cron, force=True)
         self.assertEqual(CronJobLog.objects.all().count(), logs_count + 1)
 
+    @override_settings(DJANGO_CRON_LOCK_BACKEND='django_cron.backends.lock.database.DatabaseLock')
+    def test_database_locking_backend(self):
+        # TODO: to test it properly we would need to run multiple jobs at the same time
+        logs_count = CronJobLog.objects.all().count()
+        cron_job_locks = CronJobLock.objects.all().count()
+        for _ in range(3):
+            call(self.success_cron, force=True)
+        self.assertEqual(CronJobLog.objects.all().count(), logs_count + 3)
+        self.assertEqual(CronJobLock.objects.all().count(), cron_job_locks + 1)
+        self.assertEqual(CronJobLock.objects.first().locked, False)
+
     @patch.object(test_crons.TestSuccessCronJob, 'do')
     def test_dry_run_does_not_perform_task(self, mock_do):
         response = self._call(self.success_cron, dry_run=True)

docs/locking_backend.rst

@@ -1,10 +1,11 @@
 Locking Backend
 ===============
 
-You can use one of two built-in locking backends by setting ``DJANGO_CRON_LOCK_BACKEND`` with one of:
+You can use one of three built-in locking backends by setting ``DJANGO_CRON_LOCK_BACKEND`` with one of:
 
     - ``django_cron.backends.lock.cache.CacheLock`` (default)
     - ``django_cron.backends.lock.file.FileLock``
+    - ``django_cron.backends.lock.database.DatabaseLock``
 
 
 Cache Lock
@@ -16,6 +17,10 @@ File Lock
 ---------
 This backend creates a file to mark current job as "already running", and delete it when lock is released.
 
+Database Lock
+---------
+This backend creates new model for jobs, saving their state as locked when they starts, and setting it to unlocked when
+job is finished. It may help preventing multiple instances of the same job running.
 
 Custom Lock
 -----------