Improve way of cache backend selection

At the moment we don't provide any option rather then use memcached
backend. With that we also hardocde list of packages that should be
installed inside virtualenv for selected backend.

Adding bmemcached requirement to oslo_cache.memcache_pool [1] gives us
opportunity to refactor this bit of deployment and allow to be more
flexible in backend selection and requirements installation for it.

[1] https://review.opendev.org/c/openstack/oslo.cache/+/854628

Depends-On: https://review.opendev.org/c/openstack/openstack-ansible/+/858981/4
Change-Id: I2810208301fb31eeeabf31e4b38add7f8aa3e00b

defaults/main.yml

@@ -357,6 +357,8 @@ keystone_memcached_servers: "{{ memcached_servers }}"
 # The cache_servers default backend is memcached, so this variable
 # should point to a list of memcached servers.
 # If empty, caching is disabled.
+keystone_cache_backend: "{{ openstack_cache_backend | default('oslo_cache.memcache_pool') }}"
+keystone_cache_backend_map: "{{ openstack_cache_backend_map | default(_keystone_cache_backend_map) }}"
 keystone_cache_servers: "{{ keystone_memcached_servers.split(',') }}"
 
 ## LDAP Section
@@ -572,8 +574,7 @@ keystone_pip_packages:
   - ldappool
   - osprofiler
   - PyMySQL
-  - pymemcache
-  - python-memcached
+  - "{{ _keystone_cache_backend_package }}"
   - python-openstackclient
   - systemd-python
   - uWSGI

releasenotes/notes/cache_backend-3ac67f78fa111445.yaml

@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    New variables have been added to manage used cache backends:
+
+      * ``openstack_cache_backend``: defines driver, that will be used for
+        caching.
+        Default: oslo_cache.memcache_pool
+      * ``openstack_cache_backend_map``: maps selected backend to
+        the oslo driver that should be installed and configured for it.

templates/keystone.conf.j2

@@ -43,13 +43,15 @@ transport_url = {{ keystone_oslomsg_notify_transport }}://{% for host in keyston
 
 {% if keystone_cache_servers | length > 0 %}
 [cache]
-backend = oslo_cache.memcache_pool
+backend = {{ keystone_cache_backend }}
+{% if keystone_cache_backend in keystone_cache_backend_map['dogpile'] %}
 # FIXME(lbragstad): Some strange behaviors have been reported when using
 # multiple memcached instances with backend_argument. This has been documented
 # in https://bugs.launchpad.net/oslo.cache/+bug/1743036
 # For the time being, memcache_servers works with a single memcached instance
 # and multiple instances.
 memcache_servers = {{ keystone_cache_servers | join(',') }}
+{% endif %}
 config_prefix = cache.keystone
 enabled = true
 {% endif %}

vars/main.yml

@@ -37,3 +37,22 @@ uwsgi_keystone_services: |-
 
 _keystone_is_first_play_host: "{{ (keystone_services['keystone-wsgi-public']['group'] in group_names and inventory_hostname == ((groups[keystone_services['keystone-wsgi-public']['group']] | intersect(ansible_play_hosts)) | list)[0]) | bool }}"
 _keystone_is_last_play_host: "{{ (keystone_services['keystone-wsgi-public']['group'] in group_names and inventory_hostname == ((groups[keystone_services['keystone-wsgi-public']['group']] | intersect(ansible_play_hosts)) | list)[-1]) | bool }}"
+_keystone_cache_backend_map:
+  dogpile:
+    - oslo_cache.memcache_pool
+    - dogpile.cache.pymemcache
+    - dogpile.cache.memcached
+    - dogpile.cache.bmemcached
+  mongo:
+    - oslo_cache.mongo
+  etcd3gw:
+    - oslo_cache.etcd3gw
+
+_keystone_cache_backend_package: |-
+    {% set oslo = namespace(backend='dogpile') %}
+    {% for key, value in _keystone_cache_backend_map.items() %}
+    {%   if keystone_cache_backend in value %}
+    {%     set oslo.backend = key %}
+    {%-   endif %}
+    {%- endfor %}
+    oslo.cache[{{ oslo.backend }}]