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[Lance Bragstad <lbragstad@xxxxxxxxx>]

I'm able to recreate this with 3 memcached instances, but only with the
following configuration:

backend_argument =
url:192.168.122.243,192.168.122.241,192.168.122.144:11211

If I rework that configuration option into using memcache_servers I
don't see the issue:

memcache_servers =
192.168.122.243:11211,192.168.122.241:11211,192.168.122.144:11211

I can confirm memcache_servers works as expected by looping a token
validation call and seeing traffic get routed to each instance in the
ring of memcached servers.

The backend_argument doesn't seem to be working as expected. Looks like
you can work around it with memcache_servers for the time being.

** Also affects: oslo.cache
   Importance: Undecided
       Status: New

** Changed in: oslo.cache
       Status: New => Confirmed

** Changed in: keystone
       Status: New => Invalid

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https://bugs.launchpad.net/bugs/1743036

Title:
  Multiple memcached back-end instances breaks caching

Status in OpenStack Identity (keystone):
  Invalid
Status in oslo.cache:
  Confirmed

Bug description:
  Environment
  ~~~~~~~~~~~
  * Keystone deployed in containers, running on uWSGI (per openstack-ansible defaults)
  * Keystone baseline (as provided by OSA 16.0.2): 6a67918f9d5f39564af8eacc57b80cba98242683 # HEAD of "stable/pike" as of 28.09.2017
  * openstack-ansible version: 16.0.2
  * Target OS: Ubuntu 16.04 Xenial
  * keystone.conf: http://paste.openstack.org/show/643865/ ([cache] backend = dogpile.cache.memcached)

  Symptom
  ~~~~~~~
  Running keystone against multiple memcached backends (as per OSA standard deployment pattern) results in caching being completely defeated - meaning Keystone's performance is as if caching was disabled.

  Switching `backend_argument =
  url:<cache1ip>,<cache2ip>,<cache3ip>:11211` to `url:<cache1ip>:11211`
  restores caching (and performance).

  
  Analysis
  ~~~~~~~~
  Having turned on oslo.cache debugging with the following settings:

  `
  [DEFAULT]
  debug = True
  default_log_levels = amqp=WARN,amqplib=WARN,boto=WARN,qpid=WARN,sqlalchemy=WARN,suds=INFO,oslo.messaging=INFO,oslo_messaging=INFO,iso8601=WARN,requests.packages.urllib3.connectionpool=WARN,urllib3.connectionpool=WARN,websocket=WARN,requests.packages.urllib3.util.retry=WARN,urllib3.util.retry=WARN,keystonemiddleware=WARN,routes.middleware=WARN,stevedore=WARN,taskflow=WARN,keystoneauth=WARN,oslo.cache=DEBUG,dogpile.core.dogpile=INFO

  [cache]
  debug_cache_backend = True
  `
  It becomes obvious that every attempt to retrieve a cached value fails.  This appears to be because a different key is generated (through hashing?) despite the paylod (eg: token) being identical.

  Evidence
  ~~~~~~~~
  The following log excerpts demonstrate the issue:
  * http://paste.openstack.org/show/643886/
  * http://paste.openstack.org/show/643887/

  This is generated through two subsequent attempts to validate the same
  token (payload shown on the last line of both logs is the same), 3
  seconds apart, within the same keystone uWSGI worker process, through
  the same client invokation:

  curl -X GET -H "X-Auth-Token: $ADMIN_TOKEN" -H "X-Subject-Token:
  $SUBJECT_TOKEN" http://${HOST}:35357/v3/auth/tokens  -w
  "\n\n%{time_total}\n\n"

  Yet the cache keys for both requests are different
  ('5bd08aa07bf8b4bcafeac88469769a4554297df6' vs
  '27eb98ba5a77f3701547be6ed06f3301e124e853').  Because the keys are
  different, the 2nd request (for the same token) doesn't hit the cache
  and the (same) token is stored a 2nd time in the cache with the 2nd
  key.

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