.TH SLAPO-PCACHE 5 "RELEASEDATE" "OpenLDAP LDVERSION"
.\" Copyright 1998-2007 The OpenLDAP Foundation, All Rights Reserved.
.\" Copying restrictions apply.  See the COPYRIGHT file.
.\" Copyright 2001, Pierangelo Masarati, All rights reserved. <[email protected]>
.\" $OpenLDAP: pkg/ldap/doc/man/man5/slapo-pcache.5,v 1.3.2.10 2007/01/02 21:43:45 kurt Exp $
.SH NAME
slapo-pcache \- proxycache overlay
.SH SYNOPSIS
ETCDIR/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 2254 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 proxycachequeries <queries>
Specify the maximum number of queries to cache. The default is 10000.

.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. 

.TP
.B proxytemplate <template_string> <attrset_index> <ttl> [<negttl>]
Specifies a cacheable template and "time to live" (in sec) <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 number of seconds. Negative
results are not cached by default.

.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.

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
ETCDIR/slapd.conf
default slapd configuration file
.SH SEE ALSO
.BR slapd.conf (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.