From 7ed35e06ba6c4e1950ca747efa890751001c00ef Mon Sep 17 00:00:00 2001
From: Raphael Michel <mail@raphaelmichel.de>
Date: Thu, 14 Feb 2019 10:14:12 +0100
Subject: [PATCH] Allow to configure a database replica

---
 doc/admin/config.rst   | 17 +++++++++++++++++
 src/pretix/settings.py | 17 +++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/doc/admin/config.rst b/doc/admin/config.rst
index aecfda11c..f1223d929 100644
--- a/doc/admin/config.rst
+++ b/doc/admin/config.rst
@@ -125,6 +125,23 @@ Example::
     Indicates if the database backend is a MySQL/MariaDB Galera cluster and
     turns on some optimizations/special case handlers. Default: ``False``
 
+Database replica settings
+-------------------------
+
+If you use a replicated database setup, pretix expects that the default database connection always points to the primary database node.
+Routing read queries to a replica on databse layer is **strongly** discouraged since this can lead to inaccurate such as more tickets
+being sold than are actually available.
+
+However, pretix can still make use of a database replica to keep some expensive queries with that can tolerate some latency from your
+primary database, such as backend search queries. The ``replica`` configuration section can have the same settings as the ``database``
+section (except for the ``backend`` setting) and will default back to the ``database`` settings for all values that are not given. This
+way, you just need to specify the settings that are different for the replica.
+
+Example::
+
+    [replica]
+    host=192.168.0.2
+
 URLs
 ----
 
diff --git a/src/pretix/settings.py b/src/pretix/settings.py
index 5e831e137..8e04ccc96 100644
--- a/src/pretix/settings.py
+++ b/src/pretix/settings.py
@@ -92,6 +92,23 @@ DATABASES = {
         } if 'mysql' in db_backend else {}
     }
 }
+DATABASE_REPLICA = 'default'
+if config.has_section('replica'):
+    DATABASE_REPLICA = 'replica'
+    DATABASES['replica'] = {
+        'ENGINE': 'django.db.backends.' + db_backend,
+        'NAME': config.get('replica', 'name', fallback=DATABASES['default']['NAME']),
+        'USER': config.get('replica', 'user', fallback=DATABASES['default']['USER']),
+        'PASSWORD': config.get('replica', 'password', fallback=DATABASES['default']['PASSWORD']),
+        'HOST': config.get('replica', 'host', fallback=DATABASES['default']['HOST']),
+        'PORT': config.get('replica', 'port', fallback=DATABASES['default']['PORT']),
+        'CONN_MAX_AGE': 0 if db_backend == 'sqlite3' else 120,
+        'OPTIONS': db_options,
+        'TEST': {
+            'CHARSET': 'utf8mb4',
+            'COLLATION': 'utf8mb4_unicode_ci',
+        } if 'mysql' in db_backend else {}
+    }
 
 STATIC_URL = config.get('urls', 'static', fallback='/static/')