.lf 1 stdin
.TH SLAPO-PCACHE 5 "2009/07/13" "OpenLDAP 2.4.17"
.\" Copyright 1998-2009 The OpenLDAP Foundation, All Rights Reserved.
.\" Copying restrictions apply.  See the COPYRIGHT file.
.\" Copyright 2001, Pierangelo Masarati, All rights reserved. <ando@sys-net.it>
.\" $OpenLDAP: pkg/ldap/doc/man/man5/slapo-pcache.5,v 1.14.2.10 2009/06/03 01:41:59 quanah Exp $
.SH NAME
slapo\-pcache \- proxycache overlay to slapd
.SH SYNOPSIS
/c/openldap/slapd.conf
.SH DESCRIPTION
The
.B pcache
overlay to
.BR slapd (8)
allows caching of LDAP search requests (queries) in a local database.
For an incoming query, the
proxy cache determines its corresponding \fBtemplate\fP. If the template
was specified as cacheable using the \fBproxytemplate\fP 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.
.LP

A template is defined by a filter string and an index identifying a set of
attributes. The \fBtemplate string\fP 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 \fB(mail=)\fP, \fB(|(sn=)(cn=))\fP,
\fB(&(sn=)(givenName=))\fP.

.LP 
The config directives that are specific to the
.B proxycache
overlay can be prefixed by
.BR 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: 
.TP
.B 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
.BR ldap ,
.BR meta ,
and
.BR sql
backends.
.TP
.B proxycache <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 \fBproxyattrset\fP
directives. Queries are cached only if they correspond to a cacheable template
(specified by the \fBproxytemplate\fP 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(\fBTTL\fP)" are removed. A sample cache configuration is: 
.LP
.RS
proxycache \fBbdb 10000 1 50 100\fP
.RE

.TP
.B 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 \fBproxytemplate\fP 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.

.TP
.B proxycachequeries <queries>
Specify the maximum number of queries to cache. The default is 10000.

.TP
.B 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.

.TP
.B 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 FALSE.

.BR 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
.B proxyattrset
and
.B 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 affected.

.TP
.B proxytemplate <template_string> <attrset_index> <ttl> [<negttl> [<limitttl>]]
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).

.TP
.B response\-callback { head | tail }
Specifies whether the response callback should be placed at the
.B tail
(the default) or at the 
.B 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 \fIafter\fP massaging the first
time, and \fIbefore\fP massaging when cached.

.TP
There are some constraints:

all values must be positive;

.B <entry_limit>
must be less than or equal to
.BR <max_entries> ;

.B <numattrsets>
attribute sets SHOULD be defined by using the directive
.BR proxyattrset ;

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

.LP
The following adds a template with filter string \fB(&(sn=)(givenName=))\fP 
and attributes mail, postaladdress, telephonenumber and a TTL of 1 hour. 
.LP
.RS
.nf
proxyattrset \fB0 mail postaladdress telephonenumber\fP
proxytemplate \fB(&(sn=)(givenName=)) 0 3600\fP
.fi
.RE

.LP
Directives for configuring the underlying database must also be given, as
shown here:
.LP
.RS
.nf
directory /var/tmp/cache
cachesize 100
.fi
.RE
.LP
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 \fBqueryid\fP attribute should be configured, to
assist in the removal of expired query data.
.SH 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
.B proxytemplate
.BR TTL .

The remote server should expose the
.B 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
.BR back-ldap ,
proxy caching be used in conjunction with the 
.I identity assertion
feature of
.BR slapd\-ldap (5)
(see the
.B idassert\-bind
and the
.B idassert\-authz
statements), so that remote server interrogation occurs with a vanilla identity 
that has some relatively high
.B search
and
.B 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.
.SH FILES

.TP
/c/openldap/slapd.conf
default slapd configuration file
.SH SEE ALSO
.BR slapd.conf (5),
.BR slapd\-config (5),
.BR slapd\-ldap (5),
.BR slapd\-meta (5),
.BR slapd\-sql (5),
.BR slapd (8).
.SH AUTHOR
Originally implemented by Apurva Kumar as an extension to back-meta;
turned into an overlay by Howard Chu.
