Provided by: slapd_2.4.15-1ubuntu3_i386 bug


       slapo-pcache - proxycache 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  proxytemplate  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 proxycache overlay can
       be prefixed by proxycache-, 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.

       proxycache   <database>   <max_entries>   <numattrsets>   <entry_limit>
              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  proxyattrset  directives.  Queries are cached only if
              they correspond  to  a  cacheable  template  (specified  by  the
              proxytemplate  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:

              proxycache bdb 10000 1 50 100

       proxyattrset <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 proxytemplate
              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.

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

       proxycheckcacheability { 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.

       proxysavequeries { 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 proxycache 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 proxyattrset  and  proxytemplate
              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

       proxytemplate   <template_string>   <attrset_index>   <ttl>   [<negttl>
              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).

       response-callback { 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 proxyattrset;

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

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

              proxyattrset 0 mail postaladdress telephonenumber
              proxytemplate (&(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  queryid  attribute  should  be
       configured, to assist in the removal of expired query data.


       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 proxytemplate TTL.

       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.