Refs #27683 -- Allowed setting isolation level in DATABASES ['OPTIONS'] on MySQL.

django/db/backends/mysql/base.py

@@ -219,6 +219,13 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         'iendswith': "LIKE CONCAT('%%', {})",
     }
 
+    isolation_levels = {
+        'read uncommitted',
+        'read committed',
+        'repeatable read',
+        'serializable',
+    }
+
     Database = Database
     SchemaEditorClass = DatabaseSchemaEditor
     # Classes instantiated in __init__().
@@ -252,7 +259,23 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         # We need the number of potentially affected rows after an
         # "UPDATE", not the number of changed rows.
         kwargs['client_flag'] = CLIENT.FOUND_ROWS
-        kwargs.update(settings_dict['OPTIONS'])
+        # Validate the transaction isolation level, if specified.
+        options = settings_dict['OPTIONS'].copy()
+        isolation_level = options.pop('isolation_level', None)
+        if isolation_level:
+            isolation_level = isolation_level.lower()
+            if isolation_level not in self.isolation_levels:
+                raise ImproperlyConfigured(
+                    "Invalid transaction isolation level '%s' specified.\n"
+                    "Use one of %s, or None." % (
+                        isolation_level,
+                        ', '.join("'%s'" % s for s in sorted(self.isolation_levels))
+                    ))
+            # The variable assignment form of setting transaction isolation
+            # levels will be used, e.g. "set tx_isolation='repeatable-read'".
+            isolation_level = isolation_level.replace(' ', '-')
+        self.isolation_level = isolation_level
+        kwargs.update(options)
         return kwargs
 
     def get_new_connection(self, conn_params):
@@ -262,13 +285,20 @@ class DatabaseWrapper(BaseDatabaseWrapper):
         return conn
 
     def init_connection_state(self):
+        assignments = []
         if self.features.is_sql_auto_is_null_enabled:
-            with self.cursor() as cursor:
             # SQL_AUTO_IS_NULL controls whether an AUTO_INCREMENT column on
             # a recently inserted row will return when the field is tested
             # for NULL. Disabling this brings this aspect of MySQL in line
             # with SQL standards.
-                cursor.execute('SET SQL_AUTO_IS_NULL = 0')
+            assignments.append('SQL_AUTO_IS_NULL = 0')
+
+        if self.isolation_level:
+            assignments.append("TX_ISOLATION = '%s'" % self.isolation_level)
+
+        if assignments:
+            with self.cursor() as cursor:
+                cursor.execute('SET ' + ', '.join(assignments))
 
     def create_cursor(self, name=None):
         cursor = self.connection.cursor()

docs/ref/databases.txt

@@ -492,6 +492,32 @@ like other MySQL options: either in a config file or with the entry
 ``'init_command': "SET sql_mode='STRICT_TRANS_TABLES'"`` in the
 :setting:`OPTIONS` part of your database configuration in :setting:`DATABASES`.
 
+.. _mysql-isolation-level:
+
+Isolation level
+~~~~~~~~~~~~~~~
+
+.. versionadded:: 1.11
+
+When running concurrent loads, database transactions from different sessions
+(say, separate threads handling different requests) may interact with each
+other. These interactions are affected by each session's `transaction isolation
+level`_. You can set a connection's isolation level with an
+``'isolation_level'`` entry in the :setting:`OPTIONS` part of your database
+configuration in :setting:`DATABASES`. Valid values for
+this entry are the four standard isolation levels:
+
+* ``'read uncommitted'``
+* ``'read committed'``
+* ``'repeatable read'``
+* ``'serializable'``
+
+or ``None`` to use the server's configured isolation level. However, Django
+works best with read committed rather than MySQL's default, repeatable read.
+Data loss is possible with repeatable read.
+
+.. _transaction isolation level: https://dev.mysql.com/doc/refman/en/innodb-transaction-isolation-levels.html
+
 Creating your tables
 --------------------
 

docs/releases/1.11.txt

@@ -255,6 +255,11 @@ Database backends
   the worker memory load (used to hold query results) to the database and might
   increase database memory usage.
 
+* Added MySQL support for the ``'isolation_level'`` option in
+  :setting:`OPTIONS` to allow specifying the :ref:`transaction isolation level
+  <mysql-isolation-level>`. To avoid possible data loss, it's recommended to
+  switch from MySQL's default level, repeatable read, to read committed.
+
 Email
 ~~~~~
 

tests/backends/test_mysql.py

@@ -2,6 +2,7 @@ from __future__ import unicode_literals
 
 import unittest
 
+from django.core.exceptions import ImproperlyConfigured
 from django.db import connection
 from django.test import TestCase, override_settings
 
@@ -10,6 +11,12 @@ from django.test import TestCase, override_settings
 @unittest.skipUnless(connection.vendor == 'mysql', 'MySQL specific test.')
 class MySQLTests(TestCase):
 
+    @staticmethod
+    def get_isolation_level(connection):
+        with connection.cursor() as cursor:
+            cursor.execute("SELECT @@session.tx_isolation")
+            return cursor.fetchone()[0]
+
     def test_auto_is_null_auto_config(self):
         query = 'set sql_auto_is_null = 0'
         connection.init_connection_state()
@@ -18,3 +25,42 @@ class MySQLTests(TestCase):
             self.assertIn(query, last_query)
         else:
             self.assertNotIn(query, last_query)
+
+    def test_connect_isolation_level(self):
+        read_committed = 'read committed'
+        repeatable_read = 'repeatable read'
+        isolation_values = {
+            level: level.replace(' ', '-').upper()
+            for level in (read_committed, repeatable_read)
+        }
+        configured_level = connection.isolation_level or isolation_values[repeatable_read]
+        configured_level = configured_level.upper()
+        other_level = read_committed if configured_level != isolation_values[read_committed] else repeatable_read
+
+        self.assertEqual(self.get_isolation_level(connection), configured_level)
+
+        new_connection = connection.copy()
+        new_connection.settings_dict['OPTIONS']['isolation_level'] = other_level
+        try:
+            self.assertEqual(self.get_isolation_level(new_connection), isolation_values[other_level])
+        finally:
+            new_connection.close()
+
+        # Upper case values are also accepted in 'isolation_level'.
+        new_connection = connection.copy()
+        new_connection.settings_dict['OPTIONS']['isolation_level'] = other_level.upper()
+        try:
+            self.assertEqual(self.get_isolation_level(new_connection), isolation_values[other_level])
+        finally:
+            new_connection.close()
+
+    def test_isolation_level_validation(self):
+        new_connection = connection.copy()
+        new_connection.settings_dict['OPTIONS']['isolation_level'] = 'xxx'
+        msg = (
+            "Invalid transaction isolation level 'xxx' specified.\n"
+            "Use one of 'read committed', 'read uncommitted', "
+            "'repeatable read', 'serializable', or None."
+        )
+        with self.assertRaisesMessage(ImproperlyConfigured, msg):
+            new_connection.cursor()