Redis: Initial support for Redis Sentinel

This adds basic support for using Redis Sentinel to mediate connections
to the primary Redis server used by the API functionality.

Setting `REDIS_SENTINEL_HOSTS` to a dict of "'host': port" key/values
will override any settings for `REDIS_DB_HOST` and `REDIS_DB_PORT` with
values provided by Sentinel.

Note that for the purposes of running tests, this will circumvent the
patching of `api_keys.utils.redis.StrictRedis` by mockredis as calls to
`redis_connection()` will use `sentinel.master_for` rather than
`redis.StrictRedis`, so you'll need functioning Redis and Sentinel
services in this case. Set `REDIS_SENTINEL_HOSTS` to `null` to fall-back
to the existing mocked connection.

Fixes #132

api_keys/utils.py

@@ -1,4 +1,5 @@
 import redis
+from redis.sentinel import Sentinel
 
 from django.conf import settings
 
@@ -9,12 +10,27 @@
 def redis_connection():
     global _connection
     if _connection is None:
-        _connection = redis.StrictRedis(
-            host=settings.REDIS_DB_HOST,
-            port=settings.REDIS_DB_PORT,
-            db=settings.REDIS_DB_NUMBER,
-            password=settings.REDIS_DB_PASSWORD
-        )
+        if settings.REDIS_SENTINEL_HOSTS is not None:
+            # If we have listed any sentinels, use those to manage the connection.
+            # REDIS_SENTINEL_HOSTS will be a dict, but this needs an array of tuples [('host', port)]
+            sentinel = Sentinel(
+                [(host, port) for host, port in settings.REDIS_SENTINEL_HOSTS.items()],
+                socket_timeout=0.1
+            )
+            _connection = sentinel.master_for(
+                settings.REDIS_SENTINEL_SET,
+                socket_timeout=0.1,
+                db=settings.REDIS_DB_NUMBER,
+                password=settings.REDIS_DB_PASSWORD
+            )
+        else:
+            # Otherwise fall back to a regular connection
+            _connection = redis.StrictRedis(
+                host=settings.REDIS_DB_HOST,
+                port=settings.REDIS_DB_PORT,
+                db=settings.REDIS_DB_NUMBER,
+                password=settings.REDIS_DB_PASSWORD
+            )
     return _connection
 
 

conf/general.yml-example

@@ -14,10 +14,24 @@ MAPIT_DB_PORT: '5432'
 MAPIT_DB_RO_HOST: 'replica'
 MAPIT_DB_RO_PORT: '5433'
 
+# Connection details for Redis.
+# Note that REDIS_DB_HOST and REDIS_DB_PORT will be ignored
+# if REDIS_SENTINEL_HOSTS is set as the connection to the
+# Redis primary will be mediated by Sentinel.
+# REDIS_DB_NUMBER and REDIS_DB_PASSWORD will be used in either
+# case when making the connection to the primary.
 REDIS_DB_HOST: 'localhost'
 REDIS_DB_PORT: 6379
 REDIS_DB_NUMBER: 0
 REDIS_DB_PASSWORD: null
+# REDIS_SENTINEL_HOSTS should be a dict of "'host': port" pairs.
+# If you don't want to use Sentinel, set it to null.
+REDIS_SENTINEL_HOSTS:
+  'localhost': 26379
+# Note that this will not be used unless REDIS_SENTINEL_HOSTS is set.
+# It refers to the set of Redis hosts Sentinel should return connection
+# details for.
+REDIS_SENTINEL_SET: data
 
 # Country is currently one of GB, NO, or KE. Optional; country specific things won't happen if not set.
 COUNTRY: 'GB'

mapit_mysociety_org/settings.py

@@ -117,6 +117,8 @@ def allow_migrate(self, db, app_label, model_name=None, **hints):
 REDIS_DB_PORT = config.get('REDIS_DB_PORT')
 REDIS_DB_NUMBER = config.get('REDIS_DB_NUMBER')
 REDIS_DB_PASSWORD = config.get('REDIS_DB_PASSWORD')
+REDIS_SENTINEL_HOSTS = config.get('REDIS_SENTINEL_HOSTS', None)
+REDIS_SENTINEL_SET = config.get('REDIS_SENTINEL_SET')
 
 EMAIL_BACKEND = "mailer.backend.DbBackend"
 # Configurable email port, to make it easier to develop email sending