[Impact]

 * When using an LDAP backend for Keystone, the performance can be slow if 
there are
   a large number of users using the cloud. This is due in large part to 
querying the
   SQL database for the identity mapping information of each user in a separate 
transaction.
   For example, an environment with 12,000 users will result in 12,000 sql 
queries to the
   backend database in order to fulfill a user list request. This causes some 
admin
   functions in Horizon UI to take several minutes, which often exceeds the 
WSGI and any
   haproxy timeouts configured.

 * This is fixed by backporting a series of patches which caches previously 
fetched identity
   mapping information in a memcached instance and changes the logic to query 
all of the
   user id mapping by the domain the id mapping is in. Additionally, the 
keystone-manage
   command to sync the id mapping information with a backend database in an 
offline manner
   is included to allow offline syncing of the data.

[Test Case]

 * Install keystone using an ldap backend w/ large number of users.
 * List user information: openstack user list --domain <domain_id>
 * observe slow down

[Regression Potential]

 * For Mitaka, the caching backends such as memcached or mongodb will likely 
see more
   usage and an increased footprint due to additional data being cached. 
Caching the
   identity mapping information is now standard since Newton and no major 
issues have
   been seen coming from this.

 * This code affects the identity mapping between keystone user and the ldap 
user
   (essentially the bridge between the two). While it does not functionally 
alter the
   information that is mapped (e.g. no difference in how the identity mapping 
is calculated),
   it does alter a key code path for information regarding user identity 
mappings.

[Other Info]
 
 * These patches have been run and tested in a staging environment to 
production and
   have had exposure in the Mitaka path for approximately one month to show 
their stability.

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https://bugs.launchpad.net/bugs/1582585

Title:
  the speed of query user from ldap server is very slow

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