Read operations only from slaves

docs/advanced_configuration.rst

@@ -135,7 +135,52 @@ read/write server and secondary servers as read-only.
         }
     }
 
+Additionally if you set ``MASTER_CACHE_ONLY_FOR_WRITE`` to True the primary
+server will be write-only.
 
+**For example:**
+
+Two data centers - DC1 and DC2. Redis master is in DC1, slave in DC2. With
+MASTER_CACHE_ONLY_FOR_WRITE option it's possible to configure application
+instances to write to the master and read from the closer data center.
+
+DC1 configuration:
+
+.. code:: python
+
+    CACHES = {
+        'default': {
+            'LOCATION': [
+                '10.10.0.1:6379',  # Master, DC1
+            ],
+            'OPTIONS': {
+                'PASSWORD': 'yadayada',
+                'MASTER_CACHE': '10.10.0.1:6379',
+                ...
+            },
+            ...
+        }
+    }
+
+DC2 configuration:
+
+.. code:: python
+
+    CACHES = {
+        'default': {
+            'LOCATION': [
+                '10.10.0.1:6379',  # Master, DC1
+                '10.20.0.1:6379',  # Slave, DC2
+            ],
+            'OPTIONS': {
+                'PASSWORD': 'yadayada',
+                'MASTER_CACHE': '10.10.0.1:6379',
+                'MASTER_CACHE_ONLY_FOR_WRITE': True,
+                ...
+            },
+            ...
+        }
+    }
 
 
 Pluggable Parser Classes

install_redis.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-: ${REDIS_VERSION:="2.6"}
+: ${REDIS_VERSION:="2.8"}
 
 test -d redis || git clone https://github.com/antirez/redis
 git -C redis checkout $REDIS_VERSION

redis_cache/backends/single.py

@@ -23,10 +23,17 @@ def __init__(self, server, params):
 
         self.client_list = self.clients.values()
         self.master_client = self.get_master_client()
+        self.ro_slaves_list = []
+        if self.options.get('MASTER_CACHE_ONLY_FOR_WRITE', False):
+            for identifier, client in self.clients.items():
+                if identifier != self.master_client.connection_pool.connection_identifier:  # noqa: E501
+                    self.ro_slaves_list.append(client)
 
     def get_client(self, key, write=False):
         if write and self.master_client is not None:
             return self.master_client
+        if self.ro_slaves_list:
+            return random.choice(self.ro_slaves_list)
         return random.choice(list(self.client_list))
 
     ####################

tests/testapp/tests/master_slave_tests.py

@@ -16,24 +16,7 @@
 ]
 
 
-@override_settings(CACHES={
-    'default': {
-        'BACKEND': 'redis_cache.RedisCache',
-        'LOCATION': LOCATIONS,
-        'OPTIONS': {
-            'DB': 1,
-            'PASSWORD': 'yadayada',
-            'PARSER_CLASS': 'redis.connection.HiredisParser',
-            'PICKLE_VERSION': -1,
-            'MASTER_CACHE': MASTER_LOCATION,
-        },
-    },
-})
-class MasterSlaveTestCase(SetupMixin, TestCase):
-
-    def setUp(self):
-        super(MasterSlaveTestCase, self).setUp()
-        pool.reset()
+class BaseMasterSlaveTestMixin(SetupMixin):
 
     def test_master_client(self):
         # Reset the caches at the beginning of the test.
@@ -93,3 +76,59 @@ def test_clear(self):
         time.sleep(.2)
         for client in self.cache.clients.values():
             self.assertEqual(len(client.keys('*')), 0)
+
+
+@override_settings(CACHES={
+    'default': {
+        'BACKEND': 'redis_cache.RedisCache',
+        'LOCATION': LOCATIONS,
+        'OPTIONS': {
+            'DB': 1,
+            'PASSWORD': 'yadayada',
+            'PARSER_CLASS': 'redis.connection.HiredisParser',
+            'PICKLE_VERSION': -1,
+            'MASTER_CACHE': MASTER_LOCATION,
+        },
+    },
+})
+class MasterSlaveTestCase(BaseMasterSlaveTestMixin, TestCase):
+
+    def setUp(self):
+        super(MasterSlaveTestCase, self).setUp()
+        pool.reset()
+
+
+@override_settings(CACHES={
+    'default': {
+        'BACKEND': 'redis_cache.RedisCache',
+        'LOCATION': LOCATIONS,
+        'OPTIONS': {
+            'DB': 1,
+            'PASSWORD': 'yadayada',
+            'PARSER_CLASS': 'redis.connection.HiredisParser',
+            'PICKLE_VERSION': -1,
+            'MASTER_CACHE': MASTER_LOCATION,
+            'MASTER_CACHE_ONLY_FOR_WRITE': True,
+        },
+    },
+})
+class MasterCacheOnlyForWriteTestCase(BaseMasterSlaveTestMixin, TestCase):
+
+    def setUp(self):
+        super(MasterCacheOnlyForWriteTestCase, self).setUp()
+        pool.reset()
+
+    def test_master_client_only_for_write(self):
+        caches._caches.caches = {}
+
+        cache = self.get_cache()
+        master_client = cache.master_client
+        slaves = []
+        for slave in cache.ro_slaves_list:
+            slaves.append(
+                slave.connection_pool.connection_identifier
+            )
+        self.assertTrue(
+            master_client.connection_pool.connection_identifier not in slaves
+        )
+        self.assertEqual(len(slaves), 2)