slapo-pcache(5) — Linux manual page


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

NAME         top

       slapo-pcache - proxy cache overlay to slapd

SYNOPSIS         top


DESCRIPTION         top

       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

       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>
              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 affected.

       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

CAVEATS         top

       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.

FILES         top

              default slapd configuration file

SEE ALSO         top

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

AUTHOR         top

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

COLOPHON         top

       This page is part of the OpenLDAP (an open source implementation of
       the Lightweight Directory Access Protocol) project.  Information
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       have a bug report for this manual page, see
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       ⟨⟩ on 2020-11-01.  (At
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OpenLDAP LDVERSION               RELEASEDATE                 SLAPO-PCACHE(5)

Pages that refer to this page: slapd-asyncmeta(5)slapd-ldap(5)slapd-meta(5)slapd.overlays(5)slapd-sql(5)