# $OpenLDAP$
# Copyright 2007 The OpenLDAP Foundation, All Rights Reserved.
# COPYING RESTRICTIONS APPLY, see COPYRIGHT.

H1: Overlays

Overlays are software components that provide hooks to functions analogous to 
those provided by backends, which can be stacked on top of the backend calls 
and as callbacks on top of backend responses to alter their behavior. 

Overlays may be compiled statically into slapd, or when module support
is enabled, they may be dynamically loaded. Most of the overlays
are only allowed to be configured on individual databases, but some
may also be configured globally.

Essentially they represent a means to:

    * customise the behavior of existing backends without changing the backend 
      code and without requiring one to write a new custom backend with 
      complete functionality
    * write functionality of general usefulness that can be applied to 
      different backend types

Overlays are usually documented by separate specific man pages in section 5; 
the naming convention is

        slapo-<overlay name>

Not all distributed overlays have a man page yet. Feel free to contribute one, 
if you think you well understood the behavior of the component and the 
implications of all the related configuration directives.

Official overlays are located in

        servers/slapd/overlays/

That directory also contains the file slapover.txt, which describes the 
rationale of the overlay implementation, and may serve as guideline for the 
development of custom overlays.

Contribware overlays are located in

        contrib/slapd-modules/<overlay name>/

along with other types of run-time loadable components; they are officially 
distributed, but not maintained by the project.

They can be stacked on the frontend as well; this means that they can be 
executed after a request is parsed and validated, but right before the 
appropriate database is selected. The main purpose is to affect operations 
regardless of the database they will be handled by, and, in some cases, 
to influence the selection of the database by massaging the request DN. 

All the current overlays in 2.4 are listed and described in detail in the 
following sections.


H2: Access Logging


H3: Overview

This overlay can record accesses to a given backend database on another
database.


H3: Access Logging Configuration


H2: Audit Logging

This overlay records changes on a given backend database to an LDIF log
file.
   
   
H3: Overview


H3: Audit Logging Configuration


H2: Chaining


H3: Overview

The chain overlay provides basic chaining capability to the underlying 
database.

What is chaining? It indicates the capability of a DSA to follow referrals on 
behalf of the client, so that distributed systems are viewed as a single 
virtual DSA by clients that are otherwise unable to "chase" (i.e. follow) 
referrals by themselves.

The chain overlay is built on top of the ldap backend; it is compiled by 
default when --enable-ldap.


H3: Chaining Configuration


H2: Constraints


H3: Overview

This overlay enforces a regular expression constraint on all values
of specified attributes. It is used to enforce a more rigorous
syntax when the underlying attribute syntax is too general.


H3: Constraint Configuration
   
   
H2: Dynamic Directory Services


H3: Overview

This overlay supports dynamic objects, which have a limited life after
which they expire and are automatically deleted.
   
   
H3: Dynamic Directory Service Configuration


H2: Dynamic Groups


H3: Overview

This overlay extends the Compare operation to detect
members of a dynamic group.


H3: Dynamic Group Configuration


H2: Dynamic Lists
   
   
H3: Overview

This overlay allows expansion of dynamic groups and more.


H3: Dynamic List Configuration


H2: The Proxy Cache Engine

{{TERM:LDAP}} servers typically hold one or more subtrees of a
{{TERM:DIT}}. Replica (or shadow) servers hold shadow copies of
entries held by one or more master servers.  Changes are propagated
from the master server to replica (slave) servers using LDAP Sync
replication.  An LDAP cache is a special type of replica which holds
entries corresponding to search filters instead of subtrees.

H3: Overview

The proxy cache extension of slapd is designed to improve the
responseiveness of the ldap and meta backends. It handles a search
request (query)
by first determining whether it is contained in any cached search
filter. Contained requests are answered from the proxy cache's local
database. Other requests are passed on to the underlying ldap or
meta backend and processed as usual.

E.g. {{EX:(shoesize>=9)}} is contained in {{EX:(shoesize>=8)}} and
{{EX:(sn=Richardson)}} is contained in {{EX:(sn=Richards*)}}

Correct matching rules and syntaxes are used while comparing
assertions for query containment. To simplify the query containment
problem, a list of cacheable "templates" (defined below) is specified
at configuration time. A query is cached or answered only if it
belongs to one of these templates. The entries corresponding to
cached queries are stored in the proxy cache local database while
its associated meta information (filter, scope, base, attributes)
is stored in main memory. 

A template is a prototype for generating LDAP search requests.
Templates are described by a prototype search filter and a list of
attributes which are required in queries generated from the template.
The representation for prototype filter is similar to {{REF:RFC4515}},
except that the assertion values are missing. Examples of prototype
filters are: (sn=),(&(sn=)(givenname=)) which are instantiated by
search filters (sn=Doe) and (&(sn=Doe)(givenname=John)) respectively.

The cache replacement policy removes the least recently used (LRU)
query and entries belonging to only that query. Queries are allowed
a maximum time to live (TTL) in the cache thus providing weak
consistency. A background task periodically checks the cache for
expired queries and removes them.

The Proxy Cache paper
({{URL:http://www.openldap.org/pub/kapurva/proxycaching.pdf}}) provides
design and implementation details.


H3: Proxy Cache Configuration

The cache configuration specific directives described below must
appear after a {{EX:overlay proxycache}} directive within a
{{EX:"database meta"}} or {{EX:database ldap}} section of
the server's {{slapd.conf}}(5) file.

H4: Setting cache parameters

> proxyCache <DB> <maxentries> <nattrsets> <entrylimit> <period>

This directive enables proxy caching and sets general cache
parameters.  The <DB> parameter specifies which underlying database
is to be used to hold cached entries.  It should be set to
{{EX:bdb}} or {{EX:hdb}}.  The <maxentries> parameter specifies the
total number of entries which may be held in the cache.  The
<nattrsets> parameter specifies the total number of attribute sets
(as specified by the {{EX:proxyAttrSet}} directive) that may be
defined.  The <entrylimit> parameter specifies the maximum number of
entries in a cachable query.  The <period> specifies the consistency
check period (in seconds).  In each period, queries with expired
TTLs are removed.

H4: Defining attribute sets

> proxyAttrset <index> <attrs...>

Used to associate a set of attributes to an index. Each attribute
set is associated with an index number from 0 to <numattrsets>-1.
These indices are used by the proxyTemplate directive to define
cacheable templates.

H4: Specifying cacheable templates 

> proxyTemplate <prototype_string> <attrset_index> <TTL>

Specifies a cacheable template and the "time to live" (in sec) <TTL>
for queries belonging to the template. A template is described by
its prototype filter string and set of required attributes identified
by <attrset_index>.


H4: Example

An example {{slapd.conf}}(5) database section for a caching server
which proxies for the {{EX:"dc=example,dc=com"}} subtree held
at server {{EX:ldap.example.com}}.
 
>	database 	ldap
>	suffix 		"dc=example,dc=com" 
>	rootdn 		"dc=example,dc=com" 
>	uri    		ldap://ldap.example.com/dc=example%2cdc=com
>	overlay proxycache
>	proxycache    bdb 100000 1 1000 100
>	proxyAttrset  0 mail postaladdress telephonenumber 
>	proxyTemplate (sn=) 0 3600
>	proxyTemplate (&(sn=)(givenName=)) 0 3600
>	proxyTemplate (&(departmentNumber=)(secretary=*)) 0 3600
>
>	cachesize 20
>	directory ./testrun/db.2.a
>	index       objectClass eq
>	index       cn,sn,uid,mail  pres,eq,sub


H5: Cacheable Queries

A LDAP search query is cacheable when its filter matches one of the
templates as defined in the "proxyTemplate" statements and when it references
only the attributes specified in the corresponding attribute set. 
In the example above the attribute set number 0 defines that only the
attributes: {{EX:mail postaladdress telephonenumber}} are cached for the following
proxyTemplates.

H5: Examples:

>	Filter: (&(sn=Richard*)(givenName=jack)) 
>	Attrs: mail telephoneNumber

    is cacheable, because it matches the template {{EX:(&(sn=)(givenName=))}} and its
    attributes are contained in proxyAttrset 0.

>	Filter: (&(sn=Richard*)(telephoneNumber))
>	Attrs: givenName 

    is not cacheable, because the filter does not match the template,
    nor is the attribute givenName stored in the cache

>	Filter: (|(sn=Richard*)(givenName=jack))
>	Attrs: mail telephoneNumber

    is not cacheable, because the filter does not match the template ( logical
    OR "|" condition instead of logical AND "&" )
                           
                           
H2: Password Policies


H3: Overview

This overlay provides a variety of password control mechanisms,
e.g. password aging, password reuse and duplication control, mandatory
password resets, etc.


H3: Password Policy Configuration


H2: Referential Integrity


H3: Overview

This overlay can be used with a backend database such as slapd-bdb (5)
to maintain the cohesiveness of a schema which utilises reference
attributes.


H3: Referential Integrity Configuration


H2: Return Code


H3: Overview

This overlay is useful to test the behavior of clients when
server-generated erroneous and/or unusual responses occur.


H3: Return Code Configuration


H2: Rewrite/Remap
            
            
H3: Overview

It performs basic DN/data rewrite and
objectClass/attributeType mapping.


H3: Rewrite/Remap Configuration


H2: Sync Provider


H3: Overview

This overlay implements the provider-side support for syncrepl
replication, including persistent search functionality


H3: Sync Provider Configuration


H2: Translucent Proxy


H3: Overview

This overlay can be used with a backend database such as slapd-bdb (5)
to create a "translucent proxy".

Content of entries retrieved from a remote LDAP server can be partially
overridden by the database.


H3: Translucent Proxy Configuration


H2: Attribute Uniqueness


H3: Overview

This overlay can be used with a backend database such as slapd-bdb (5)
to enforce the uniqueness of some or all attributes within a subtree.


H3: Attribute Uniqueness Configuration


H2: Value Sorting


H3: Overview

This overlay can be used to enforce a specific order for the values
of an attribute when it is returned in a search.


H3: Value Sorting Configuration


H2: Overlay Stacking


H3: Overview


H3: Example Senarios


H4: Samba
