Offload more work to database replica

src/pretix/base/models/event.py

@@ -164,7 +164,7 @@ class EventMixin:
     def annotated(cls, qs, channel='web'):
         from pretix.base.models import Item, ItemVariation, Quota
 
-        sq_active_item = Item.objects.filter_available(channel=channel).filter(
+        sq_active_item = Item.objects.using(settings.DATABASE_REPLICA).filter_available(channel=channel).filter(
             Q(variations__isnull=True)
             & Q(quotas__pk=OuterRef('pk'))
         ).order_by().values_list('quotas__pk').annotate(
@@ -186,7 +186,7 @@ class EventMixin:
             Prefetch(
                 'quotas',
                 to_attr='active_quotas',
-                queryset=Quota.objects.annotate(
+                queryset=Quota.objects.using(settings.DATABASE_REPLICA).annotate(
                     active_items=Subquery(sq_active_item, output_field=models.TextField()),
                     active_variations=Subquery(sq_active_variation, output_field=models.TextField()),
                 ).exclude(

src/pretix/base/models/items.py

@@ -1290,7 +1290,8 @@ class Quota(LoggedModel):
                     'cached_availability_state', 'cached_availability_number', 'cached_availability_time',
                     'cached_availability_paid_orders'
                 ],
-                clear_cache=False
+                clear_cache=False,
+                using='default'
             )
 
         if _cache is not None:

src/pretix/helpers/database.py

@@ -76,3 +76,21 @@ class GroupConcat(Aggregate):
             function='string_agg',
             template="%(function)s(%(field)s::text, '%(separator)s')",
         )
+
+
+class ReplicaRouter:
+
+    def db_for_read(self, model, **hints):
+        return 'default'
+
+    def db_for_write(self, model, **hints):
+        return 'default'
+
+    def allow_relation(self, obj1, obj2, **hints):
+        db_list = ('default', 'replica')
+        if obj1._state.db in db_list and obj2._state.db in db_list:
+            return True
+        return None
+
+    def allow_migrate(self, db, app_label, model_name=None, **hintrs):
+        return True

src/pretix/presale/utils.py

@@ -21,6 +21,10 @@ def _detect_event(request, require_live=True, require_plugin=None):
     if hasattr(request, '_event_detected'):
         return
 
+    db = 'default'
+    if request.method == 'GET':
+        db = settings.DATABASE_REPLICA
+
     url = resolve(request.path_info)
     try:
         if hasattr(request, 'organizer_domain'):
@@ -31,24 +35,24 @@ def _detect_event(request, require_live=True, require_plugin=None):
                 path = "/" + request.get_full_path().split("/", 2)[-1]
                 return redirect(path)
 
-            request.event = request.organizer.events\
+            request.event = request.organizer.events.using(db).get(
-                .get(
+                slug=url.kwargs['event'],
-                    slug=url.kwargs['event'],
+                organizer=request.organizer,
-                    organizer=request.organizer,
+            )
-                )
             request.organizer = request.organizer
         else:
             # We are on our main domain
             if 'event' in url.kwargs and 'organizer' in url.kwargs:
                 request.event = Event.objects\
                     .select_related('organizer')\
+                    .using(db)\
                     .get(
                         slug=url.kwargs['event'],
                         organizer__slug=url.kwargs['organizer']
                     )
                 request.organizer = request.event.organizer
             elif 'organizer' in url.kwargs:
-                request.organizer = Organizer.objects.get(
+                request.organizer = Organizer.objects.using(db).get(
                     slug=url.kwargs['organizer']
                 )
             else:

src/pretix/presale/views/event.py

@@ -50,32 +50,34 @@ def item_group_by_category(items):
 
 
 def get_grouped_items(event, subevent=None, voucher=None, channel='web'):
-    items = event.items.filter_available(channel=channel, voucher=voucher).select_related(
+    items = event.items.using(settings.DATABASE_REPLICA).filter_available(channel=channel, voucher=voucher).select_related(
         'category', 'tax_rule',  # for re-grouping
     ).prefetch_related(
         Prefetch('quotas',
                  to_attr='_subevent_quotas',
-                 queryset=event.quotas.filter(subevent=subevent)),
+                 queryset=event.quotas.using(settings.DATABASE_REPLICA).filter(subevent=subevent)),
         Prefetch('bundles',
-                 queryset=ItemBundle.objects.prefetch_related(
+                 queryset=ItemBundle.objects.using(settings.DATABASE_REPLICA).prefetch_related(
                      Prefetch('bundled_item',
-                              queryset=event.items.select_related('tax_rule').prefetch_related(
+                              queryset=event.items.using(settings.DATABASE_REPLICA).select_related('tax_rule').prefetch_related(
                                   Prefetch('quotas',
                                            to_attr='_subevent_quotas',
-                                           queryset=event.quotas.filter(subevent=subevent)),
+                                           queryset=event.quotas.using(settings.DATABASE_REPLICA).filter(subevent=subevent)),
                               )),
                      Prefetch('bundled_variation',
-                              queryset=ItemVariation.objects.select_related('item', 'item__tax_rule').filter(item__event=event).prefetch_related(
+                              queryset=ItemVariation.objects.using(
+                                  settings.DATABASE_REPLICA
+                              ).select_related('item', 'item__tax_rule').filter(item__event=event).prefetch_related(
                                   Prefetch('quotas',
                                            to_attr='_subevent_quotas',
-                                           queryset=event.quotas.filter(subevent=subevent)),
+                                           queryset=event.quotas.using(settings.DATABASE_REPLICA).filter(subevent=subevent)),
                               )),
                  )),
         Prefetch('variations', to_attr='available_variations',
-                 queryset=ItemVariation.objects.filter(active=True, quotas__isnull=False).prefetch_related(
+                 queryset=ItemVariation.objects.using(settings.DATABASE_REPLICA).filter(active=True, quotas__isnull=False).prefetch_related(
                      Prefetch('quotas',
                               to_attr='_subevent_quotas',
-                              queryset=event.quotas.filter(subevent=subevent))
+                              queryset=event.quotas.using(settings.DATABASE_REPLICA).filter(subevent=subevent))
                  ).distinct()),
     ).annotate(
         quotac=Count('quotas'),
@@ -229,7 +231,7 @@ class EventIndex(EventViewMixin, EventListMixin, CartMixin, TemplateView):
 
         if request.event.has_subevents:
             if 'subevent' in kwargs:
-                self.subevent = request.event.subevents.filter(pk=kwargs['subevent'], active=True).first()
+                self.subevent = request.event.subevents.using(settings.DATABASE_REPLICA).filter(pk=kwargs['subevent'], active=True).first()
                 if not self.subevent:
                     raise Http404()
                 return super().get(request, *args, **kwargs)
@@ -287,7 +289,7 @@ class EventIndex(EventViewMixin, EventListMixin, CartMixin, TemplateView):
 
             ebd = defaultdict(list)
             add_subevents_for_days(
-                filter_qs_by_attr(self.request.event.subevents_annotated(self.request.sales_channel), self.request),
+                filter_qs_by_attr(self.request.event.subevents_annotated(self.request.sales_channel).using(settings.DATABASE_REPLICA), self.request),
                 before, after, ebd, set(), self.request.event,
                 kwargs.get('cart_namespace')
             )
@@ -297,7 +299,7 @@ class EventIndex(EventViewMixin, EventListMixin, CartMixin, TemplateView):
             context['years'] = range(now().year - 2, now().year + 3)
         else:
             context['subevent_list'] = self.request.event.subevents_sorted(
-                filter_qs_by_attr(self.request.event.subevents_annotated(self.request.sales_channel), self.request)
+                filter_qs_by_attr(self.request.event.subevents_annotated(self.request.sales_channel).using(settings.DATABASE_REPLICA), self.request)
             )
 
         context['show_cart'] = (

src/pretix/presale/views/organizer.py

@@ -93,7 +93,7 @@ class EventListMixin:
 
     def _get_event_queryset(self):
         query = Q(is_public=True) & Q(live=True)
-        qs = self.request.organizer.events.filter(query)
+        qs = self.request.organizer.events.using(settings.DATABASE_REPLICA).filter(query)
         qs = qs.annotate(
             min_from=Min('subevents__date_from'),
             min_to=Min('subevents__date_to'),
@@ -126,7 +126,7 @@ class EventListMixin:
 
     def _set_month_to_next_subevent(self):
         tz = pytz.timezone(self.request.event.settings.timezone)
-        next_sev = self.request.event.subevents.filter(
+        next_sev = self.request.event.subevents.using(settings.DATABASE_REPLICA).filter(
             active=True,
             is_public=True,
             date_from__gte=now()
@@ -141,14 +141,14 @@ class EventListMixin:
             self.month = now().month
 
     def _set_month_to_next_event(self):
-        next_ev = filter_qs_by_attr(Event.objects.filter(
+        next_ev = filter_qs_by_attr(Event.objects.using(settings.DATABASE_REPLICA).filter(
             organizer=self.request.organizer,
             live=True,
             is_public=True,
             date_from__gte=now(),
             has_subevents=False
         ), self.request).order_by('date_from').first()
-        next_sev = filter_qs_by_attr(SubEvent.objects.filter(
+        next_sev = filter_qs_by_attr(SubEvent.objects.using(settings.DATABASE_REPLICA).filter(
             event__organizer=self.request.organizer,
             event__is_public=True,
             event__live=True,
@@ -353,14 +353,14 @@ class CalendarView(OrganizerViewMixin, EventListMixin, TemplateView):
     def _events_by_day(self, before, after):
         ebd = defaultdict(list)
         timezones = set()
-        add_events_for_days(self.request, Event.annotated(self.request.organizer.events, 'web'), before, after, ebd, timezones)
+        add_events_for_days(self.request, Event.annotated(self.request.organizer.events, 'web').using(settings.DATABASE_REPLICA), before, after, ebd, timezones)
         add_subevents_for_days(filter_qs_by_attr(SubEvent.annotated(SubEvent.objects.filter(
             event__organizer=self.request.organizer,
             event__is_public=True,
             event__live=True,
         ).prefetch_related(
             'event___settings_objects', 'event__organizer___settings_objects'
-        )), self.request), before, after, ebd, timezones)
+        )), self.request).using(settings.DATABASE_REPLICA), before, after, ebd, timezones)
         self._multiple_timezones = len(timezones) > 1
         return ebd
 

src/pretix/settings.py

@@ -109,6 +109,7 @@ if config.has_section('replica'):
             'COLLATION': 'utf8mb4_unicode_ci',
         } if 'mysql' in db_backend else {}
     }
+    DATABASE_ROUTERS = ['pretix.helpers.database.ReplicaRouter']
 
 STATIC_URL = config.get('urls', 'static', fallback='/static/')