Alternative Memcached configurations

By default Memcached servers are deployed on each controller host as a part of shared-infra_containers group. Drivers, like oslo_cache.memcache_pool support marking memcache backends as dead, however not all services allow you to select the driver which will be used for interaction with Memcached. In the meanwhile, you may face services API response delays or even unresponsive APIs while one of the memcached backends is down.

This is why you may want to use HAProxy for handling access and to check for backend aliveness or use always “local” to the service memcached server.

Configuring Memcached through HAProxy

Setting haproxy in front of the Memcached servers and relying on it for checking aliveness of the backends gives more reliable failover and minimize delays in case of backend failure. We need to define the following in your user_variables.yml:

haproxy_memcached_allowlist_networks: "{{ haproxy_allowlist_networks }}"
memcached_servers: "{{ internal_lb_vip_address ~ ':' ~ memcached_port }}"
  - service:
      haproxy_service_name: memcached
      haproxy_backend_nodes: "{{ groups['memcached'] | default([]) }}"
      haproxy_bind: "{{ [internal_lb_vip_address] }}"
      haproxy_port: 11211
      haproxy_balance_type: tcp
      haproxy_balance_alg: source
      haproxy_backend_ssl: False
        - tcp-check
      haproxy_allowlist_networks: "{{ haproxy_memcached_allowlist_networks }}"

After setting this you will need to update haproxy and all services configuration to use new memcached backend:

# openstack-ansible playbooks/haproxy-install.yml
# openstack-ansible playbooks/setup-openstack.yml

Using only “local” Memcached

The idea behind this, is to configure services to use Memcached, that will reside only on the local control plane. Here “local” means not only having memcahed inside the container with the service itslef, but also having memcached inside a separate container on the same controller as the service well.

This will reduce latency and improve stability, since service and memcached instance will be running on the same control plane just on different containers that are connected through the same L2 bridge.

Among the cons of this approach is that there won’t be any failover available in case of the memcached container crush, so caching on this controller won’t work. For pros it’s only 1 controller that will be affected and not all of them in the case of remote memcached unavailability. Also, most the common scenario of API response delays is when whole controller goes down, since connection drop causes memcache_pool to wait for connection timeout rather then connection is rejected when memcached service goes down and memcache_pool instantly switches to another backend.


In case some service won’t have memcached server “locally” to the service that is running, behaviour will fallback to using all available memcached servers and memcached client will decide which one to use.

In order to always use “local” memcached you need to define the following in the user_variables.yml file:

memcached_servers: |-
    {% set service_controller_group = group_names | select('regex', '.*-host_containers') | first | default('memcached') %}
      groups['memcached'] | intersect(groups[service_controller_group])
        | map('extract', hostvars, 'management_address')
        | map('regex_replace', '(.+)', '\1:' ~ memcached_port)
        | list | join(',')

After setting that you need to update all services configuration to use new memcached backend:

# openstack-ansible playbooks/setup-openstack.yml