I have deployed openstack 2024.2 with kolla-ansible in HA setup with 3 control nodes. Everything works without issues, but when I stop memcached node with IP address 192.168.56.12 (or when node goes down), it is not possible to login to Horizon with error message: Something went wrong! An unexpected error has occurred. Try refreshing the page. If that doesn't help, contact your local administrator. In horizon log I have an error message: 2025-05-12 18:05:37.406588 Internal Server Error: / 2025-05-12 18:05:37.406613 Traceback (most recent call last): 2025-05-12 18:05:37.406615 File "/var/lib/kolla/venv/lib64/python3.9/site-packages/django/core/handlers/exception.py", line 55, in inner 2025-05-12 18:05:37.406617 response = get_response(request) 2025-05-12 18:05:37.406619 File "/var/lib/kolla/venv/lib/python3.9/site-packages/horizon/middleware/simultaneous_sessions.py", line 30, in __call__ 2025-05-12 18:05:37.406621 self._process_request(request) 2025-05-12 18:05:37.406623 File "/var/lib/kolla/venv/lib/python3.9/site-packages/horizon/middleware/simultaneous_sessions.py", line 37, in _process_request 2025-05-12 18:05:37.406625 cache_value = cache.get(cache_key) 2025-05-12 18:05:37.406627 File "/var/lib/kolla/venv/lib64/python3.9/site-packages/django/core/cache/backends/memcached.py", line 75, in get 2025-05-12 18:05:37.406628 return self._cache.get(key, default) 2025-05-12 18:05:37.406630 File "/var/lib/kolla/venv/lib/python3.9/site-packages/pymemcache/client/hash.py", line 347, in get 2025-05-12 18:05:37.406632 return self._run_cmd("get", key, default, default=default, **kwargs) 2025-05-12 18:05:37.406634 File "/var/lib/kolla/venv/lib/python3.9/site-packages/pymemcache/client/hash.py", line 322, in _run_cmd 2025-05-12 18:05:37.406636 return self._safely_run_func(client, func, default_val, *args, **kwargs) 2025-05-12 18:05:37.406637 File "/var/lib/kolla/venv/lib/python3.9/site-packages/pymemcache/client/hash.py", line 199, in _safely_run_func 2025-05-12 18:05:37.406639 result = func(*args, **kwargs) 2025-05-12 18:05:37.406640 File "/var/lib/kolla/venv/lib/python3.9/site-packages/pymemcache/client/base.py", line 687, in get 2025-05-12 18:05:37.406642 return self._fetch_cmd(b"get", [key], False, key_prefix=self.key_prefix).get( 2025-05-12 18:05:37.406644 File "/var/lib/kolla/venv/lib/python3.9/site-packages/pymemcache/client/base.py", line 1133, in _fetch_cmd 2025-05-12 18:05:37.406645 self._connect() 2025-05-12 18:05:37.406647 File "/var/lib/kolla/venv/lib/python3.9/site-packages/pymemcache/client/base.py", line 424, in _connect 2025-05-12 18:05:37.406648 sock.connect(sockaddr) 2025-05-12 18:05:37.406650 ConnectionRefusedError: [Errno 111] Connection refused so horizon tries to connect to memcached node which is down. I have default kolla-ansible config with enabled memcached and horizon config is following: SESSION_ENGINE = 'django.contrib.sessions.backends.cache' CACHES['default']['LOCATION'] = ['192.168.56.11:11211','192.168.56.12:11211 ','192.168.56.21:11211'] When I stop any other memcached node, horizon is working without issues. Why is that exact node important for the horizon? Does anyone else have the same experience? Thanks for any advice. -- Kamil Madac