openldap/doc/man/man5/slapd-sql.5

.TH SLAPD-SQL 5 "01 May 2002" "OpenLDAP LDVERSION"
.\" $OpenLDAP$
.SH NAME
slapd-sql \- SQL backend to slapd
.SH SYNOPSIS
ETCDIR/slapd.conf
.SH DESCRIPTION
The primary purpose of this backend (8) to
.BR slapd (8)
is to PRESENT information stored in some RDBMS as an LDAP subtree
without any programming (some SQL and maybe stored procedures can't be
considered programming, anyway ;).
.LP
That is, for example, when you (some ISP) have account information you
use in an RDBMS, and want to use modern solutions that expect such
information in LDAP (to authenticate users, make email lookups etc.).
Or you want to synchronize or distribute information between different
sites/applications that use RDBMSes and/or LDAP.
Or whatever else...
.LP
It is NOT designed as a general-purpose backend that uses RDBMS instead
of BerkeleyDB (as the standard BDB backend does), though it can be
used as such with several limitations.
You can take a look at
.B http://www.openldap.org/faq/index.cgi?file=378 
(OpenLDAP FAQ-O-Matic/General LDAP FAQ/Directories vs. conventional
databases) to find out more on this point.
.LP
The idea (detailed below) is to use some metainformation to translate
LDAP queries to SQL queries, leaving relational schema untouched, so
that old applications can continue using it without any
modifications.
This allows SQL and LDAP applications to inter-operate without
replication, and exchange data as needed.
.LP
The SQL backend is designed to be tunable to virtually any relational
schema without having to change source (through that metainformation
mentioned).
Also, it uses ODBC to connect to RDBMSes, and is highly configurable
for SQL dialects RDBMSes may use, so it may be used for integration
and distribution of data on different RDBMSes, OSes, hosts etc., in
other words, in highly heterogeneous environment.
.SH CONFIGURATION
These
.B slapd.conf
options apply to the SQL backend database.
That is, they must follow a "database sql" line and come before any
subsequent "backend" or "database" lines.
Other database options are described in the
.BR slapd.conf (5)
manual page.
.TP
.B dbname <datasource name>
The name of the ODBC datasource to use.
.TP
.B dbhost <hostname>
.TP
.B dbuser <username>
.TP
.B dbpasswd <password>
These three options are generally unneeded, because this information is already
taken from the datasource.
Use them if you need to override datasource settings.
Also, several RDBMS' drivers tend to require explicit passing of user/password,
even if those are given in datasource.
.TP
.B subtree_cond <SQL expression>
Specifies a where-clause template used to form a subtree search condition.
It may differ from one SQL dialect to another (see samples).
.TP
.B oc_query <SQL expression>
The default is
.B "SELECT id, name, keytbl, keycol, create_proc, delete_proc, expect_return FROM ldap_oc_mappings"
.TP
.B at_query <SQL expression>
The default is
.B "SELECT name, sel_expr, from_tbls, join_where, add_proc, delete_proc, param_order, expect_return FROM ldap_attr_mappings WHERE oc_map_id=?"
.TP
.B insentry_query <SQL expression>
The default is
.B "INSERT INTO ldap_entries (dn, oc_map_id, parent, keyval) VALUES (?, ?, ?, ?)"
.TP
.B delentry_query <SQL expression>
The default is
.B "DELETE FROM ldap_entries WHERE id=?"

These four options specify SQL query templates for loading schema mapping
metainformation,
adding and deleting entries to ldap_entries, etc.
All these and subtree_cond should have the given default values.
For the current value it is recommended to look at the sources,
or in the log output when slapd starts with "-d 5" or greater.
.TP
.B upper_func <SQL function name>
Specifies the name of a function that converts a given value to uppercase.
This is used for CIS matching when the RDBMS is case sensitive.

.SH METAINFORMATION USED
.LP
Almost everything mentioned later is illustrated in examples located
in the
.B slapd/back-sql/rdbms_depend/
directory in the OpenLDAP source tree, and contains scripts for
generating sample database for Oracle, MS SQL Server and mySQL.
.LP
The first thing that one must arrange is what set of LDAP
object classes can present your RDBMS information.
.LP
The easiest way is to create an objectclass for each entity you had in
ER-diagram when designing your relational schema.
Any relational schema, no matter how normalized it is, was designed
after some model of your application's domain (for instance, accounts,
services etc. in ISP), and is used in terms of its entities, not just
tables of normalized schema.
It means that for every attribute of every such instance there is an
effective SQL query that loads its values.
.LP
Also you might want your object classes to conform to some of the standard
schemas like inetOrgPerson etc.
.LP
Nevertheless, when you think it out, we must define a way to translate
LDAP operation requests to (a series of) SQL queries.
Let us deal with the SEARCH operation.
.LP
Example:
Let's suppose that we store information about persons working in our 
organization in two tables:
.LP
.nf
  PERSONS                 PHONES
  ----------              -------------
  id integer              id integer
  first_name varchar      pers_id integer references persons(id)
  last_name varchar       phone
  middle_name varchar
  ...
.fi
.LP
(PHONES contains telephone numbers associated with persons).
A person can have several numbers, then PHONES contains several
records with corresponding pers_id, or no numbers (and no records in
PHONES with such pers_id).
An LDAP objectclass to present such information could look like this:
.LP
.nf
  person
  -------
  MUST cn
  MAY telephoneNumber $ firstName $ lastName
  ...
.fi
.LP
To fetch all values for cn attribute given person ID, we construct the
query:
.LP
.nf
  SELECT CONCAT(persons.first_name,' ',persons.last_name)
      AS cn FROM persons WHERE persons.id=?
.fi
.LP
for telephoneNumber we can use:
.LP
.nf
  SELECT phones.phone AS telephoneNumber FROM persons,phones
   WHERE persons.id=phones.pers.id AND persons.id=?
.fi
.LP
If we wanted to service LDAP requests with filters like
(telephoneNumber=123*), we would construct something like:
.LP
.nf
  SELECT ... FROM persons,phones
   WHERE persons.id=phones.pers.id
     AND persons.id=?
     AND phones.phone like '123%'
.fi
.LP
So, if we had information about what tables contain values for each
attribute, how to join these tables and arrange these values, we could
try to automatically generate such statements, and translate search
filters to SQL WHERE clauses.
.LP
To store such information, we add three more tables to our schema
and fill it with data (see samples):
.LP
.nf
  ldap_oc_mappings (some columns are not listed for clarity)
  ---------------
  id=1
  name="person"
  keytbl="persons"
  keycol="id"
.fi
.LP
This table defines a mapping between objectclass (its name held in the
"name" column), and a table that holds the primary key for corresponding
entities.
For instance, in our example, the person entity, which we are trying
to present as "person" objectclass, resides in two tables (persons and
phones), and is identified by the persons.id column (that we will call
the primary key for this entity).
Keytbl and keycol thus contain "persons" (name of the table), and "id"
(name of the column).
.LP
.nf
  ldap_attr_mappings (some columns are not listed for clarity)
  -----------
  id=1
  oc_id=1
  name="cn"
  sel_expr="CONCAT(persons.first_name,' ',persons.last_name)"
  from_tbls="persons"
  join_where=NULL
  ************
  id=<n>
  oc_map_id=1
  name="telephoneNumber"
  sel_expr="phones.phone"
  from_tbls="persons,phones"
  join_where="phones.pers_id=persons.id"
.fi
.LP
This table defines mappings between LDAP attributes and SQL queries
that load their values.
Note that, unlike LDAP schema, these are not
.B attribute types
- the attribute "cn" for "person" objectclass can
have its values in different tables than "cn" for some other objectclass,
so attribute mappings depend on objectclass mappings (unlike attribute
types in LDAP schema, which are indifferent to objectclasses).
Thus, we have oc_map_id column with link to oc_mappings table.
.LP
Now we cut the SQL query that loads values for a given attribute into 3 parts.
First goes into sel_expr column - this is the expression we had
between SELECT and FROM keywords, which defines WHAT to load.
Next is table list - text between FROM and WHERE keywords.
It may contain aliases for convenience (see examples).
The last is part of the where clause, which (if it exists at all) expresses the
condition for joining the table containing values with the table
containing the primary key (foreign key equality and such).
If values are in the same table as the primary key, then this column is
left NULL (as for cn attribute above).
.LP
Having this information in parts, we are able to not only construct
queries that load attribute values by id of entry (for this we could
store SQL query as a whole), but to construct queries that load id's
of objects that correspond to a given search filter (or at least part of
it).
See below for examples.
.LP
.nf
  ldap_entries
  ------------
  id=1
  dn=<dn you choose>
  oc_map_id=...
  parent=<parent record id>
  keyval=<value of primary key>
.fi
.LP
This table defines mappings between DNs of entries in your LDAP tree,
and values of primary keys for corresponding relational data.
It has recursive structure (parent column references id column of the
same table), which allows you to add any tree structure(s) to your
flat relational data.
Having id of objectclass mapping, we can determine table and column
for primary key, and keyval stores value of it, thus defining the exact
tuple corresponding to the LDAP entry with this DN.
.LP
Note that such design (see exact SQL table creation query) implies one
important constraint - the key must be an integer.
But all that I know about well-designed schemas makes me think that it's
not very narrow ;) If anyone needs support for different types for
keys - he may want to write a patch, and submit it to OpenLDAP ITS,
then I'll include it.
.LP
Also, several people complained that they don't really need very
structured trees, and they don't want to update one more table every
time they add or delete an instance in the relational schema.
Those people can use a view instead of a real table for ldap_entries, something
like this (by Robin Elfrink):
.LP
.nf
  CREATE VIEW ldap_entries (id, dn, oc_map_id, parent, keyval)
      AS SELECT (1000000000+userid),
  UPPER(CONCAT(CONCAT('cn=',gecos),',o=MyCompany,c=NL')),
  1, 0, userid FROM unixusers UNION
          SELECT (2000000000+groupnummer),
  UPPER(CONCAT(CONCAT('cn=',groupnaam),',o=MyCompany,c=NL')),
  2, 0, groupnummer FROM groups;
.fi
.LP
.SH Typical SQL backend operation
Having metainformation loaded, the SQL backend uses these tables to
determine a set of primary keys of candidates (depending on search
scope and filter).
It tries to do it for each objectclass registered in ldap_objclasses.
.LP
Example:
for our query with filter (telephoneNumber=123*) we would get the following 
query generated (which loads candidate IDs)
.LP
.nf
  SELECT ldap_entries.id,persons.id, 'person' AS objectClass,
         ldap_entries.dn AS dn
    FROM ldap_entries,persons,phones
   WHERE persons.id=ldap_entries.keyval
     AND ldap_entries.objclass=?
     AND ldap_entries.parent=?
     AND phones.pers_id=persons.id
     AND (phones.phone LIKE '123%')
.fi
.LP
(for ONELEVEL search)
or "... AND dn=?" (for BASE search)
or "... AND dn LIKE '%?'" (for SUBTREE)
.LP
Then, for each candidate, we load the requested attributes using
per-attribute queries like
.LP
.nf
  SELECT phones.phone AS telephoneNumber
    FROM persons,phones
   WHERE persons.id=? AND phones.pers_id=persons.id
.fi
.LP
Then, we use test_filter() from the frontend API to test the entry for a full
LDAP search filter match (since we cannot effectively make sense of
SYNTAX of corresponding LDAP schema attribute, we translate the filter
into the most relaxed SQL condition to filter candidates), and send it to
the user.
.LP
ADD, DELETE, MODIFY operations are also performed on per-attribute
metainformation (add_proc etc.).
In those fields one can specify an SQL statement or stored procedure
call which can add, or delete given values of a given attribute, using
the given entry keyval (see examples -- mostly ORACLE and MSSQL - since
there're no stored procs in mySQL).
.LP
We just add more columns to oc_mappings and attr_mappings, holding
statements to execute (like create_proc, add_proc, del_proc etc.), and
flags governing the order of parameters passed to those statements.
Please see samples to find out what are the parameters passed, and other
information on this matter - they are self-explanatory for those familiar
with concept expressed above.
.LP
.SH common techniques (referrals, multiclassing etc.)
First of all, lets remember that among other major differences to the
complete LDAP data model, the concept above does not directly support
such things as multiple objectclasses per entry, and referrals.
Fortunately, they are easy to adopt in this scheme.
The SQL backend suggests two more tables being added to the schema -
ldap_entry_objectclasses(entry_id,oc_name), and
ldap_referrals(entry_id,url).
.LP
The first contains any number of objectclass names that corresponding
entries will be found by, in addition to that mentioned in
mapping.
The SQL backend automatically adds attribute mapping for the "objectclass"
attribute to each objectclass mapping that loads values from this table.
So, you may, for instance, have a mapping for inetOrgPerson, and use it
for queries for "person" objectclass...
.LP
The second table contains any number of referrals associated with a given entry.
The SQL backend automatically adds attribute mapping for "ref" attribute
to each objectclass mapping that loads values from this table.
So, if you add objectclass "referral" to this entry, and make one or
more tuples in ldap_referrals for this entry (they will be seen as
values of "ref" attribute), you will have slapd return a referral, as
described in the Administrators Guide.
.LP
.SH EXAMPLES
There are example SQL modules in the slapd/back-sql/rdbms_depend/
directory in the OpenLDAP source tree.
.SH FILES
ETCDIR/slapd.conf
.SH SEE ALSO
.BR slapd.conf (5),
.BR slapd (8).