[doc] Document how to use "local" memcached
Having "local" memcached might be valuable for some usecases, like availability zones, where distance or connection between controllers is a concern, and memcached with lowest latency should be preffered whenever possible. Change-Id: Ie285e582fc9ada149c199a3bd4e6344035068bd3
This commit is contained in:
Dmitriy Rabotyagov
Amy Marrich
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====================================
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Alternative Memcached configurations
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====================================
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By default Memcached servers are deployed on each controller host as a part of
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`shared-infra_containers` group. Drivers, like `oslo_cache.memcache_pool <https://github.com/openstack/oslo.cache/blob/master/oslo_cache/backends/memcache_pool.py>`_
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support marking memcache backends as dead, however not all services allow you
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to select the driver which will be used for interaction with Memcached.
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In the meanwhile, you may face services API response delays or even unresponsive
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APIs while one of the memcached backends is down.
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This is why you may want to use HAProxy for handling access and to check for backend
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aliveness or use always "local" to the service memcached server.
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Configuring Memcached through HAProxy
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Setting haproxy in front of the Memcached servers and relying on it for checking
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aliveness of the backends gives more reliable failover and minimize delays
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in case of backend failure.
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We need to define the following in your ``user_variables.yml``:
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.. code-block:: yaml
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haproxy_memcached_allowlist_networks: "{{ haproxy_allowlist_networks }}"
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memcached_servers: "{{ internal_lb_vip_address ~ ':' ~ memcached_port }}"
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haproxy_extra_services:
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- service:
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haproxy_service_name: memcached
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haproxy_backend_nodes: "{{ groups['memcached'] | default([]) }}"
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haproxy_bind: "{{ [internal_lb_vip_address] }}"
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haproxy_port: 11211
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haproxy_balance_type: tcp
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haproxy_balance_alg: source
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haproxy_backend_ssl: False
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haproxy_backend_options:
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- tcp-check
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haproxy_allowlist_networks: "{{ haproxy_memcached_allowlist_networks }}"
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After setting this you will need to update haproxy and all services configuration
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to use new memcached backend:
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.. code-block:: shell-session
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# openstack-ansible playbooks/haproxy-install.yml
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# openstack-ansible playbooks/setup-openstack.yml
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Using only "local" Memcached
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The idea behind this, is to configure services to use Memcached, that will
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reside only on the local control plane. Here "local" means not only having memcahed
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inside the container with the service itslef, but also having memcached inside
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a separate container on the same controller as the service well.
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This will reduce latency and improve stability, since service and memcached
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instance will be running on the same control plane just on different containers
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that are connected through the same L2 bridge.
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Among the cons of this approach is that there won't be any failover available
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in case of the memcached container crush, so caching on this controller won't
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work. For pros it's only 1 controller that will be affected and not
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all of them in the case of remote memcached unavailability. Also, most the common
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scenario of API response delays is when whole controller goes down, since
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connection drop causes memcache_pool to wait for connection timeout rather
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then connection is rejected when memcached service goes down and memcache_pool
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instantly switches to another backend.
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.. note::
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In case some service won't have memcached server "locally"
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to the service that is running, behaviour will fallback to using
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all available memcached servers and memcached client will decide
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which one to use.
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In order to always use "local" memcached you need to define the following
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in the ``user_variables.yml`` file:
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.. code-block:: yaml
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memcached_servers: |-
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{% set service_controller_group = group_names | select('regex', '.*-host_containers') | first | default('memcached') %}
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{{
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groups['memcached'] | intersect(groups[service_controller_group])
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| map('extract', hostvars, 'management_address')
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| map('regex_replace', '(.+)', '\1:' ~ memcached_port)
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| list | join(',')
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}}
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After setting that you need to update all services configuration
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to use new memcached backend:
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.. code-block:: shell-session
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# openstack-ansible playbooks/setup-openstack.yml
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@ -1,45 +0,0 @@
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=================================
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Making Memcached highly-available
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=================================
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By default Memcached servers are deployed on each controller host as a part of
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`shared-infra_containers` group. Drivers, like `oslo_cache.memcache_pool <https://github.com/openstack/oslo.cache/blob/master/oslo_cache/backends/memcache_pool.py>`_
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support marking memcache backends as dead, however not all services allow you
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to select driver which will be used for interaction with Memcached.
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In the meanwhile you may face services API response delays or even unresponsive
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APIs while one of the memcached backends is down.
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That's why you may want to use HAProxy for handling access and check of backend
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aliveness.
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Configuring Memcached through HAProxy
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Setting haproxy in front of the Memcached servers and relying it in checking
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aliveness of the backends gives more reliable failover and minimize delays
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in case of backend failure.
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We need to define the following in your ``user_variables.yml``:
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.. code-block:: yaml
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haproxy_memcached_allowlist_networks: "{{ haproxy_allowlist_networks }}"
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memcached_servers: "{{ internal_lb_vip_address ~ ':' ~ memcached_port }}"
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haproxy_extra_services:
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- service:
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haproxy_service_name: memcached
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haproxy_backend_nodes: "{{ groups['memcached'] | default([]) }}"
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haproxy_bind: "{{ [internal_lb_vip_address] }}"
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haproxy_port: 11211
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haproxy_balance_type: tcp
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haproxy_balance_alg: source
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haproxy_backend_ssl: False
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haproxy_backend_options:
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- tcp-check
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haproxy_allowlist_networks: "{{ haproxy_memcached_allowlist_networks }}"
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After setting that you need to update haproxy and all services configuration
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to use new memcached backend:
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.. code-block:: shell-session
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# openstack-ansible playbooks/haproxy-install.yml
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# openstack-ansible playbooks/setup-openstack.yml
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@ -5,7 +5,7 @@ OpenStack-Ansible Memcached server
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.. toctree::
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:maxdepth: 2
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configure-ha.rst
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alternate-configurations.rst
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Ansible role to install and configure Memcached.
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