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

Change-Id: I48e193ef29e56aa8639511c5b5dcddc70f5e1198

defaults/main.yml

@@ -25,7 +25,10 @@ nova_ceilometer_enabled: "{{ (groups['ceilometer_all'] is defined) and (groups['
 # Enable/Disable nova versioned notification
 nova_versioned_notification_enabled: False
 
-nova_memcached_servers: "{{ memcached_servers }}"
+# Caching
+nova_cache_servers: "{{ nova_memcached_servers | default(memcached_servers) }}"
+nova_cache_backend: "{{ openstack_cache_backend | default('oslo_cache.memcache_pool') }}"
+nova_cache_backend_map: "{{ openstack_cache_backend_map | default(_nova_cache_backend_map) }}"
 
 ## Verbosity Options
 debug: False
@@ -496,8 +499,7 @@ nova_pip_packages:
   - "git+{{ nova_git_repo }}@{{ nova_git_install_branch }}#egg=nova"
   - osprofiler
   - PyMySQL
-  - pymemcache
-  - python-memcached
+  - "{{ _nova_cache_backend_package }}"
   - systemd-python
 
 # Specific pip packages provided by the user

releasenotes/notes/nova_memcached_servers-6c40d3b1714786bf.yaml

@@ -0,0 +1,7 @@
+---
+deprecations:
+  - |
+    Variable ``nova_memcached_servers`` has been deprecated and replaced with
+    ``nova_cache_servers`` that defaults to ``memcached_servers``. For
+    backpwards compatability ``nova_memcached_servers`` is still respected
+    but will be removed in future releases.

templates/nova.conf.j2

@@ -68,8 +68,10 @@ manager_interval = {{ nova_image_cache_manager_interval }}
 # Cache
 [cache]
 enabled = true
-backend = oslo_cache.memcache_pool
-memcache_servers = {{ nova_memcached_servers }}
+backend = {{ nova_cache_backend }}
+{% if nova_cache_backend in nova_cache_backend_map['dogpile'] %}
+memcache_servers = {{ nova_cache_servers }}
+{% endif %}
 
 
 # Cinder
@@ -194,13 +196,15 @@ service_token_roles_required = {{ nova_service_token_roles_required | bool }}
 service_token_roles = {{ nova_service_token_roles | join(',') }}
 service_type = {{ nova_service_type }}
 
-memcached_servers = {{ memcached_servers }}
+{% if nova_cache_backend in nova_cache_backend_map['dogpile'] %}
+memcached_servers = {{ nova_cache_servers }}
 
 token_cache_time = 300
 
 # if your memcached server is shared, use these settings to avoid cache poisoning
 memcache_security_strategy = ENCRYPT
 memcache_secret_key = {{ memcached_encryption_key }}
+{% endif %}
 
 {% if group_names | intersect(nova_services.keys() | difference('nova-compute') | map('extract', nova_services, 'group') | list) | count > 0 %}
 [database]

vars/main.yml

@@ -116,3 +116,23 @@ _qemu_default_conf_dict: |-
 _nova_qemu_conf: "{{ _qemu_default_conf_dict | combine(qemu_conf_dict) }}"
 
 _nova_oslomsg_heartbeat_in_pthread: "{{ (nova_services['nova-compute']['group'] in group_names) | ternary(False, True) }}"
+
+_nova_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
+
+_nova_cache_backend_package: |-
+    {% set oslo = namespace(backend='dogpile') %}
+    {% for key, value in _nova_cache_backend_map.items() %}
+    {%   if nova_cache_backend in value %}
+    {%     set oslo.backend = key %}
+    {%-   endif %}
+    {%- endfor %}
+    oslo.cache[{{ oslo.backend }}]