# $OpenLDAP$
-# Copyright 1999-2006 The OpenLDAP Foundation, All Rights Reserved.
+# Copyright 1999-2011 The OpenLDAP Foundation, All Rights Reserved.
# COPYING RESTRICTIONS APPLY, see COPYRIGHT.
H1: Introduction to OpenLDAP Directory Services
While some consider the Internet {{TERM[expand]DNS}} (DNS) is an
example of a globally distributed directory service, DNS is not
-browsable nor searchable. It is more properly described as a
-globaly distributed {{lookup}} service.
+browseable nor searchable. It is more properly described as a
+globally distributed {{lookup}} service.
H2: What is LDAP?
you can think of. Figure 1.1 shows an example LDAP directory tree
using traditional naming.
-!import "intro_tree.gif"; align="center"; \
+!import "intro_tree.png"; align="center"; \
title="LDAP directory tree (traditional naming)"
FT[align="Center"] Figure 1.1: LDAP directory tree (traditional naming)
Figure 1.2 shows an example LDAP directory tree using domain-based
naming.
-!import "intro_dctree.gif"; align="center"; \
+!import "intro_dctree.png"; align="center"; \
title="LDAP directory tree (Internet naming)"
FT[align="Center"] Figure 1.2: LDAP directory tree (Internet naming)
services.
+H2: When should I use LDAP?
+
+This is a very good question. In general, you should use a Directory
+server when you require data to be centrally managed, stored and accessible via
+standards based methods.
+
+Some common examples found throughout the industry are, but not limited to:
+
+* Machine Authentication
+* User Authentication
+* User/System Groups
+* Address book
+* Organization Representation
+* Asset Tracking
+* Telephony Information Store
+* User resource management
+* E-mail address lookups
+* Application Configuration store
+* PBX Configuration store
+* etc.....
+
+There are various {{SECT:Distributed Schema Files}} that are standards based, but
+you can always create your own {{SECT:Schema Specification}}.
+
+There are always new ways to use a Directory and apply LDAP principles to address
+certain problems, therefore there is no simple answer to this question.
+
+If in doubt, join the general LDAP forum for non-commercial discussions and
+information relating to LDAP at:
+{{URL:http://www.umich.edu/~dirsvcs/ldap/mailinglist.html}} and ask
+
+H2: When should I not use LDAP?
+
+When you start finding yourself bending the directory to do what you require,
+maybe a redesign is needed. Or if you only require one application to use and
+manipulate your data (for discussion of LDAP vs RDBMS, please read the
+{{SECT:LDAP vs RDBMS}} section).
+
+It will become obvious when LDAP is the right tool for the job.
+
+
H2: How does LDAP work?
LDAP utilizes a {{client-server model}}. One or more LDAP servers
LDAPv3 was developed in the late 1990's to replace LDAPv2.
LDAPv3 adds the following features to LDAP:
- - Strong authentication and data security services via {{TERM:SASL}}
- - Certificate authentication and data security services via {{TERM:TLS}} (SSL)
- - Internationalization through the use of Unicode
- - Referrals and Continuations
- - Schema Discovery
- - Extensibility (controls, extended operations, and more)
+ * Strong authentication and data security services via {{TERM:SASL}}
+ * Certificate authentication and data security services via {{TERM:TLS}} (SSL)
+ * Internationalization through the use of Unicode
+ * Referrals and Continuations
+ * Schema Discovery
+ * Extensibility (controls, extended operations, and more)
LDAPv2 is historic ({{REF:RFC3494}}). As most {{so-called}} LDAPv2
implementations (including {{slapd}}(8)) do not conform to the
-LDAPv2 technical specification, interoperatibility amongst
+LDAPv2 technical specification, interoperability amongst
implementations claiming LDAPv2 support is limited. As LDAPv2
differs significantly from LDAPv3, deploying both LDAPv2 and LDAPv3
simultaneously is quite problematic. LDAPv2 should be avoided.
LDAPv2 is disabled by default.
+H2: LDAP vs RDBMS
+
+This question is raised many times, in different forms. The most common,
+however, is: {{Why doesn't OpenLDAP drop Berkeley DB and use a relational
+database management system (RDBMS) instead?}} In general, expecting that the
+sophisticated algorithms implemented by commercial-grade RDBMS would make
+{{OpenLDAP}} be faster or somehow better and, at the same time, permitting
+sharing of data with other applications.
+
+The short answer is that use of an embedded database and custom indexing system
+allows OpenLDAP to provide greater performance and scalability without loss of
+reliability. OpenLDAP uses Berkeley DB concurrent / transactional
+database software. This is the same software used by leading commercial
+directory software.
+
+Now for the long answer. We are all confronted all the time with the choice
+RDBMSes vs. directories. It is a hard choice and no simple answer exists.
+
+It is tempting to think that having a RDBMS backend to the directory solves all
+problems. However, it is a pig. This is because the data models are very
+different. Representing directory data with a relational database is going to
+require splitting data into multiple tables.
+
+Think for a moment about the person objectclass. Its definition requires
+attribute types objectclass, sn and cn and allows attribute types userPassword,
+telephoneNumber, seeAlso and description. All of these attributes are multivalued,
+so a normalization requires putting each attribute type in a separate table.
+
+Now you have to decide on appropriate keys for those tables. The primary key
+might be a combination of the DN, but this becomes rather inefficient on most
+database implementations.
+
+The big problem now is that accessing data from one entry requires seeking on
+different disk areas. On some applications this may be OK but in many
+applications performance suffers.
+
+The only attribute types that can be put in the main table entry are those that
+are mandatory and single-value. You may add also the optional single-valued
+attributes and set them to NULL or something if not present.
+
+But wait, the entry can have multiple objectclasses and they are organized in
+an inheritance hierarchy. An entry of objectclass organizationalPerson now has
+the attributes from person plus a few others and some formerly optional attribute
+types are now mandatory.
+
+What to do? Should we have different tables for the different objectclasses?
+This way the person would have an entry on the person table, another on
+organizationalPerson, etc. Or should we get rid of person and put everything on
+the second table?
+
+But what do we do with a filter like (cn=*) where cn is an attribute type that
+appears in many, many objectclasses. Should we search all possible tables for
+matching entries? Not very attractive.
+
+Once this point is reached, three approaches come to mind. One is to do full
+normalization so that each attribute type, no matter what, has its own separate
+table. The simplistic approach where the DN is part of the primary key is
+extremely wasteful, and calls for an approach where the entry has a unique
+numeric id that is used instead for the keys and a main table that maps DNs to
+ids. The approach, anyway, is very inefficient when several attribute types from
+one or more entries are requested. Such a database, though cumbersomely,
+can be managed from SQL applications.
+
+The second approach is to put the whole entry as a blob in a table shared by all
+entries regardless of the objectclass and have additional tables that act as
+indices for the first table. Index tables are not database indices, but are
+fully managed by the LDAP server-side implementation. However, the database
+becomes unusable from SQL. And, thus, a fully fledged database system provides
+little or no advantage. The full generality of the database is unneeded.
+Much better to use something light and fast, like Berkeley DB.
+
+A completely different way to see this is to give up any hopes of implementing
+the directory data model. In this case, LDAP is used as an access protocol to
+data that provides only superficially the directory data model. For instance,
+it may be read only or, where updates are allowed, restrictions are applied,
+such as making single-value attribute types that would allow for multiple values.
+Or the impossibility to add new objectclasses to an existing entry or remove
+one of those present. The restrictions span the range from allowed restrictions
+(that might be elsewhere the result of access control) to outright violations of
+the data model. It can be, however, a method to provide LDAP access to preexisting
+data that is used by other applications. But in the understanding that we don't
+really have a "directory".
+
+Existing commercial LDAP server implementations that use a relational database
+are either from the first kind or the third. I don't know of any implementation
+that uses a relational database to do inefficiently what BDB does efficiently.
+For those who are interested in "third way" (exposing EXISTING data from RDBMS
+as LDAP tree, having some limitations compared to classic LDAP model, but making
+it possible to interoperate between LDAP and SQL applications):
+
+OpenLDAP includes back-sql - the backend that makes it possible. It uses ODBC +
+additional metainformation about translating LDAP queries to SQL queries in your
+RDBMS schema, providing different levels of access - from read-only to full
+access depending on RDBMS you use, and your schema.
+
+For more information on concept and limitations, see {{slapd-sql}}(5) man page,
+or the {{SECT: Backends}} section. There are also several examples for several
+RDBMSes in {{F:back-sql/rdbms_depend/*}} subdirectories.
+
+
H2: What is slapd and what can it do?
{{slapd}}(8) is an LDAP directory server that runs on many different
{{B:{{TERM[expand]TLS}}}}: {{slapd}} supports certificate-based
authentication and data security (integrity and confidentiality)
services through the use of TLS (or SSL). {{slapd}}'s TLS
-implementation utilizes {{PRD:OpenSSL}} software.
+implementation can utilize {{PRD:OpenSSL}}, {{PRD:GnuTLS}},
+or {{PRD:MozNSS}} software.
{{B:Topology control}}: {{slapd}} can be configured to restrict
access at the socket layer based upon network topology information.
customized modules which extend {{slapd}} in numerous ways. Also,
a number of {{programmable database}} modules are provided. These
allow you to expose external data sources to {{slapd}} using popular
-programming languages ({{PRD:Perl}}, {{shell}}, {{TERM:SQL}}, and
-{{PRD:TCL}}).
+programming languages ({{PRD:Perl}}, {{shell}}, and {{TERM:SQL}}).
{{B:Threads}}: {{slapd}} is threaded for high performance. A single
multi-threaded {{slapd}} process handles all incoming requests using
{{B:Replication}}: {{slapd}} can be configured to maintain shadow
copies of directory information. This {{single-master/multiple-slave}}
replication scheme is vital in high-volume environments where a
-single {{slapd}} just doesn't provide the necessary availability
-or reliability. {{slapd}} includes support for {{LDAP Sync}}-based
+single {{slapd}} installation just doesn't provide the necessary availability
+or reliability. For extremely demanding environments where a
+single point of failure is not acceptable, {{multi-master}} replication
+is also available. {{slapd}} includes support for {{LDAP Sync}}-based
replication.
{{B:Proxy Cache}}: {{slapd}} can be configured as a caching
{{B:Configuration}}: {{slapd}} is highly configurable through a
single configuration file which allows you to change just about
everything you'd ever want to change. Configuration options have
-reasonable defaults, making your job much easier.
+reasonable defaults, making your job much easier. Configuration can
+also be performed dynamically using LDAP itself, which greatly
+improves manageability.
projects / openldap / blobdiff