Provided by: slapd_2.4.49+dfsg-2ubuntu1.10_amd64 
NAME
slapo-pcache - proxy cache overlay to slapd
SYNOPSIS
/etc/ldap/slapd.conf
DESCRIPTION
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=)),
(&(sn=)(givenName=)).
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 rootdn.
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 10000.
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 FALSE.
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 hour.
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.
BACKWARD COMPATIBILITY
The configuration keywords have been renamed and the older form is deprecated. These older keywords are
still recognized but may disappear in future releases.
proxycache
use pcache
proxyattrset
use pcacheAttrset
proxycachequeries
use pcacheMaxQueries
proxycheckcacheability
use pcacheValidate
proxysavequeries
use pcachePersist
proxytemplate
use pcacheTemplate
response-callback
use pcachePosition
CAVEATS
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
/etc/ldap/slapd.conf
default slapd configuration file
SEE ALSO
slapd.conf(5), slapd-config(5), slapd-ldap(5), slapd-meta(5), slapd-sql(5), slapd(8).
AUTHOR
Originally implemented by Apurva Kumar as an extension to back-meta; turned into an overlay by Howard
Chu.
OpenLDAP 2020/01/30 SLAPO-PCACHE(5)