SLAPO-PCACHE(5)               File Formats Manual              SLAPO-PCACHE(5)

       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 mdb 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.  Attributes
              prefixed by "undef:" need not be present in the schema.

       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.

OpenLDAP 2.4.48                   2019/07/24                   SLAPO-PCACHE(5)