Provided by: slapd_2.4.25-1.1ubuntu4_i386 bug


       slapo-pcache - proxy cache overlay to slapd




       The  pcache  overlay to slapd(8) allows caching of LDAP search requests
       (queries) in a local database.  For an incoming query, the proxy  cache
       determines its corresponding template. If the template was specified as
       cacheable  using  the  pcacheTemplate  directive  and  the  request  is
       contained  in  a  cached  request, it is answered from the proxy cache.
       Otherwise, the search  is  performed  as  usual  and  cacheable  search
       results are saved in the cache for use in future queries.

       A template is defined by a filter string and an index identifying a set
       of attributes. The template string for  a  query  can  be  obtained  by
       removing  assertion  values  from  the  RFC  4515 representation of its
       search filter. A query belongs to a template if its template string and
       set  of  projected  attributes  correspond  to  a  cacheable  template.
       Examples   of   template   strings    are    (mail=),    (|(sn=)(cn=)),

       The  config  directives  that are specific to the pcache overlay can be
       prefixed by pcache-, to avoid conflicts with directives specific to the
       underlying  database  or  to  other  stacked  overlays.   This  may  be
       particularly useful for those directives that refer to the backend used
       for local storage.  The following cache specific directives can be used
       to configure the proxy cache:

       overlay pcache
              This directive adds the  proxy  cache  overlay  to  the  current
              backend.  The  proxy  cache overlay may be used with any backend
              but is intended for use with the ldap, meta, and  sql  backends.
              Please  note  that the underlying backend must have a configured

       pcache <database> <max_entries> <numattrsets> <entry_limit> <cc_period>
              The directive enables proxy caching in the current  backend  and
              sets general cache parameters. A <database> backend will be used
              internally to maintain the cached entries. The  chosen  database
              will  need  to  be  configured  as  well,  as shown below. Cache
              replacement  is  invoked  when   the   cache   size   grows   to
              <max_entries>  entries  and  continues till the cache size drops
              below this size.  <numattrsets> should be equal to the number of
              following  pcacheAttrset  directives. Queries are cached only if
              they correspond  to  a  cacheable  template  (specified  by  the
              pcacheTemplate  directive) and the number of entries returned is
              less than <entry_limit>. Consistency check  is  performed  every
              <cc_period>  duration (specified in secs). In each cycle queries
              with expired "time to live(TTL)" are  removed.  A  sample  cache
              configuration is:

              pcache bdb 10000 1 50 100

       pcacheAttrset <index> <attrs...>
              Used to associate a set of attributes <attrs..> with an <index>.
              Each attribute set is associated  with  an  integer  from  0  to
              <numattrsets>-1.  These  indices  are used by the pcacheTemplate
              directive to define cacheable templates.  A  set  of  attributes
              cannot  be  empty.   A set of attributes can contain the special
              attributes "*"  (all  user  attributes),  "+"  (all  operational
              attributes)  or both; in the latter case, any other attribute is
              redundant  and  should  be  avoided  for  clarity.   A  set   of
              attributes  can  contain  "1.1"  as  the only attribute; in this
              case, only the presence of the entries is cached.

       pcacheMaxQueries <queries>
              Specify the maximum number of queries to cache. The  default  is

       pcacheValidate { TRUE | FALSE }
              Check  whether  the results of a query being cached can actually
              be returned from the cache by the proxy DSA.  When enabled,  the
              entries  being returned while caching the results of a query are
              checked to ensure consistency with the schema known to the proxy
              DSA.   In case of failure, the query is not cached.  By default,
              the check is off.

       pcacheOffline { TRUE | FALSE }
              Set the cache to offline mode. While  offline,  the  consistency
              checker  will  be  stopped  and  no expirations will occur. This
              allows the cache contents to  be  used  indefinitely  while  the
              proxy  is  cut  off  from network access to the remote DSA.  The
              default is FALSE, i.e. consistency checks and  expirations  will
              be performed.

       pcachePersist { TRUE | FALSE }
              Specify  whether  the  cached  queries  should  be  saved across
              restarts of the caching proxy, to provide  hot  startup  of  the
              cache.   Only  non-expired queries are reloaded.  The default is

              CAVEAT: of course, the configuration of the proxy cache must not
              change  across restarts; the pcache overlay does not perform any
              consistency checks in this sense.  In detail, this option should
              be disabled unless the existing pcacheAttrset and pcacheTemplate
              directives are not changed neither in order nor in contents.  If
              new  sets  and  templates  are added, or if other details of the
              pcache overlay configuration changed, this feature should not be

       pcacheTemplate   <template_string>   <attrset_index>   <ttl>  [<negttl>
       [<limitttl> [<ttr>]]]
              Specifies a cacheable template  and  "time  to  live"  <ttl>  of
              queries  belonging  to the template. An optional <negttl> can be
              used to  specify  that  negative  results  (i.e.,  queries  that
              returned  zero  entries) should also be cached for the specified
              amount of time. Negative  results  are  not  cached  by  default
              (<negttl>  set  to  0).   An  optional <limitttl> can be used to
              specify that results hitting a sizelimit should also  be  cached
              for  the  specified amount of time.  Results hitting a sizelimit
              are not cached by default (<limitttl> set to  0).   An  optional
              <ttr>  "time  to  refresh"  can  be  used to specify that cached
              entries should be automatically refreshed after a certain  time.
              Entries  will  only be refreshed while they have not expired, so
              the <ttl> should be larger than the <ttr> for this option to  be
              useful. Entries are not refreshed by default (<ttr> set to 0).

       pcacheBind <filter_template> <attrset_index> <ttr> <scope> <base>
              Specifies  a  template for caching Simple Bind credentials based
              on an already defined pcacheTemplate. The  <filter_template>  is
              similar  to  a  <template_string>  except  that it may have some
              values present. Its purpose is to allow the overlay to  generate
              filters  similar  to  what  other applications do when they do a
              Search immediately  before  a  Bind.  E.g.,  if  a  client  like
              nss_ldap  is  configured  to  search  for a user with the filter
              "(&(objectClass=posixAccount)(uid=<username>))"     then     the
              corresponding   template   "(&(objectClass=posixAccount)(uid=))"
              should be used here. When converted to a regular  template  e.g.
              "(&(objectClass=)(uid=))"  this template and the <attrset_index>
              must match an already defined pcacheTemplate clause.  The  "time
              to  refresh"  <ttr> determines the time interval after which the
              cached credentials may be refreshed. The first Bind request that
              occurs  after  that  time  will  trigger  the  refresh  attempt.
              Refreshes are not performed when the overlay is  Offline.  There
              is  no  "time  to  live" parameter for the Bind credentials; the
              credentials will expire according to the pcacheTemplate ttl. The
              <scope>  and  <base> should match the search scope and base used
              by the authentication clients. The cached  credentials  are  not
              stored  in cleartext, they are hashed using the default password
              hash.  By default Bind caching is not enabled.

       pcachePosition { head | tail }
              Specifies whether the response callback should be placed at  the
              tail  (the  default)  or  at  the  head  (actually, wherever the
              stacking sequence would make it appear) of  the  callback  list.
              This  affects  how  the  overlay  interacts with other overlays,
              since the proxycache overlay should  be  executed  as  early  as
              possible  (and  thus  configured  as late as possible), to get a
              chance to return the cached results; however, if executed  early
              at response, it would cache entries that may be later "massaged"
              by other databases and thus returned after massaging  the  first
              time, and before massaging when cached.

       There are some constraints:

              all values must be positive;

              <entry_limit> must be less than or equal to <max_entries>;

              <numattrsets>  attribute  sets  SHOULD  be  defined by using the
              directive pcacheAttrset;

              all attribute sets  SHOULD  be  referenced  by  (at  least)  one
              pcacheTemplate directive;

       The  following  adds a template with filter string (&(sn=)(givenName=))
       and attributes mail, postaladdress, telephonenumber  and  a  TTL  of  1

              pcacheAttrset 0 mail postaladdress telephonenumber
              pcacheTemplate (&(sn=)(givenName=)) 0 3600

       Directives  for configuring the underlying database must also be given,
       as shown here:

              directory /var/tmp/cache
              cachesize 100

       Any valid directives for the chosen database type may be used. Indexing
       should  be  used  as  appropriate  for  the  queries  being handled. In
       addition, an equality index on the pcacheQueryid  attribute  should  be
       configured, to assist in the removal of expired query data.


       The  configuration  keywords  have  been  renamed and the older form is
       deprecated. These older keywords are still recognized but may disappear
       in future releases.

              use pcache

              use pcacheAttrset

              use pcacheMaxQueries

              use pcacheValidate

              use pcachePersist

              use pcacheTemplate

              use pcachePosition


       Caching  data is prone to inconsistencies because updates on the remote
       server will not be reflected in the response of the cache at least (and
       at   most)   for   the  duration  of  the  pcacheTemplate  TTL.   These
       inconsistencies can be minimized by careful use of the TTR.

       The remote server should expose the objectClass attribute  because  the
       underlying  database  that  actually caches the entries may need it for
       optimal local processing of the queries.

       The proxy server should contain all the schema information required for
       caching.   Significantly, it needs the schema of attributes used in the
       query templates.  If the objectClass  attribute  is  used  in  a  query
       template,  it  needs the definition of the objectClasses of the entries
       it is supposed to  cache.   It  is  the  responsibility  of  the  proxy
       administrator  to  keep  the  proxy  schema  lined  up with that of the
       proxied server.

       Another potential (and subtle) inconsistency may  occur  when  data  is
       retrieved  with  different  identities and specific per-identity access
       control is enforced by the remote server.  If data was  retrieved  with
       an identity that collected only partial results because of access rules
       enforcement on the remote server, other  users  with  different  access
       privileges  on  the  remote  server will get different results from the
       remote server and from the cache.  If those users  have  higher  access
       privileges  on  the  remote server, they will get from the cache only a
       subset of the results they would get directly from the  remote  server;
       but  if they have lower access privileges, they will get from the cache
       a superset of the results they  would  get  directly  from  the  remote
       server.   Either  occurrence may or may not be acceptable, based on the
       security policy of the cache and of the remote server.  It is important
       to  note  that  in this case the proxy is violating the security of the
       remote server by disclosing to an identity data that was  collected  by
       another  identity.   For  this reason, it is suggested that, when using
       back-ldap, proxy caching be  used  in  conjunction  with  the  identity
       assertion  feature  of  slapd-ldap(5)  (see  the  idassert-bind and the
       idassert-authz statements), so that remote server interrogation  occurs
       with  a  vanilla identity that has some relatively high search and read
       access privileges, and the "real" access control is  delegated  to  the
       proxy's  ACLs.   Beware that since only the cached fraction of the real
       datum is available to the cache, it may not be possible to enforce  the
       same access rules that are defined on the remote server.  When security
       is a concern, cached proxy access must be carefully tailored.


              default slapd configuration file


       slapd.conf(5),    slapd-config(5),    slapd-ldap(5),     slapd-meta(5),
       slapd-sql(5), slapd(8).


       Originally  implemented  by  Apurva Kumar as an extension to back-meta;
       turned into an overlay by Howard Chu.