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.
The "undef" keyword cannot be used with the slapd-mdb(5)
backend as it requires all schema elements be fully
defined.
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.
