NAME
slapd-sql - SQL backend to slapd
SYNOPSIS
/etc/ldap/slapd.conf
DESCRIPTION
The primary purpose of this slapd(8) backend 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
;).
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...
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
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.
The idea (detailed below) is to use some meta-information 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.
The SQL backend is designed to be tunable to virtually any relational
schema without having to change source (through that meta-information
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.
This backend is experimental.
CONFIGURATION
These slapd.conf options apply to the SQL backend database, which means
that they must follow a "database sql" line and come before any
subsequent "backend" or "database" lines. Other database options not
specific to this backend are described in the slapd.conf(5) manual
page.
DATA SOURCE CONFIGURATION
dbname <datasource name>
The name of the ODBC datasource to use.
dbhost <hostname>
dbpasswd <password>
dbuser <username>
The three above options are generally unneeded, because this
information is taken from the datasource specified by the dbname
directive. They allow to override datasource settings. Also,
several RDBMS' drivers tend to require explicit passing of
user/password, even if those are given in datasource (Note:
dbhost is currently ignored).
SCOPING CONFIGURATION
These options specify SQL query templates for scoping searches.
subtree_cond <SQL expression>
Specifies a where-clause template used to form a subtree search
condition (dn="(.+,)?<dn>$"). It may differ from one SQL
dialect to another (see samples). By default, it is constructed
based on the knowledge about how to normalize DN values (e.g.
"<upper_func>(ldap_entries.dn) LIKE CONCAT('%',?)"); see
upper_func, upper_needs_cast, concat_pattern and strcast_func in
"HELPER CONFIGURATION" for details.
children_cond <SQL expression>
Specifies a where-clause template used to form a children search
condition (dn=".+,<dn>$"). It may differ from one SQL dialect
to another (see samples). By default, it is constructed based
on the knowledge about how to normalize DN values (e.g.
"<upper_func>(ldap_entries.dn) LIKE CONCAT('%,',?)"); see
upper_func, upper_needs_cast, concat_pattern and strcast_func in
"HELPER CONFIGURATION" for details.
use_subtree_shortcut { YES | no }
Do not use the subtree condition when the searchBase is the
database suffix, and the scope is subtree; rather collect all
entries.
STATEMENT CONFIGURATION
These options specify SQL query templates for loading schema mapping
meta-information, 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. Note that the
parameter number and order must not be changed.
oc_query <SQL expression>
The query that is used to collect the objectClass mapping data
from table ldap_oc_mappings; see "METAINFORMATION USED" for
details. The default is "SELECT id, name, keytbl, keycol,
create_proc, delete_proc, expect_return FROM ldap_oc_mappings".
at_query <SQL expression>
The query that is used to collect the attributeType mapping data
from table ldap_attr_mappings; see "METAINFORMATION USED" for
details. The default is "SELECT name, sel_expr, from_tbls,
join_where, add_proc, delete_proc, param_order, expect_return
FROM ldap_attr_mappings WHERE oc_map_id=?".
id_query <SQL expression>
The query that is used to map a DN to an entry in table
ldap_entries; see "METAINFORMATION USED" for details. The
default is "SELECT id,keyval,oc_map_id,dn FROM ldap_entries
WHERE <DN match expr>", where <DN match expr> is constructed
based on the knowledge about how to normalize DN values (e.g.
"dn=?" if no means to uppercase strings are available;
typically, "<upper_func>(dn)=?" is used); see upper_func,
upper_needs_cast, concat_pattern and strcast_func in "HELPER
CONFIGURATION" for details.
insentry_stmt <SQL expression>
The statement that is used to insert a new entry in table
ldap_entries; see "METAINFORMATION USED" for details. The
default is "INSERT INTO ldap_entries (dn, oc_map_id, parent,
keyval) VALUES (?, ?, ?, ?)".
delentry_stmt <SQL expression>
The statement that is used to delete an existing entry from
table ldap_entries; see "METAINFORMATION USED" for details. The
default is "DELETE FROM ldap_entries WHERE id=?".
delobjclasses_stmt <SQL expression>
The statement that is used to delete an existing entry's ID from
table ldap_objclasses; see "METAINFORMATION USED" for details.
The default is "DELETE FROM ldap_entry_objclasses WHERE
entry_id=?".
HELPER CONFIGURATION
These statements are used to modify the default behavior of the backend
according to issues of the dialect of the RDBMS. The first options
essentially refer to string and DN normalization when building filters.
LDAP normalization is more than upper- (or lower-)casing everything;
however, as a reasonable trade-off, for case-sensitive RDBMSes the
backend can be instructed to uppercase strings and DNs by providing the
upper_func directive. Some RDBMSes, to use functions on arbitrary data
types, e.g. string constants, requires a cast, which is triggered by
the upper_needs_cast directive. If required, a string cast function
can be provided as well, by using the strcast_func directive. Finally,
a custom string concatenation pattern may be required; it is provided
by the concat_pattern directive.
upper_func <SQL function name>
Specifies the name of a function that converts a given value to
uppercase. This is used for case insensitive matching when the
RDBMS is case sensitive. It may differ from one SQL dialect to
another (e.g. UCASE, UPPER or whatever; see samples). By
default, none is used, i.e. strings are not uppercased, so
matches may be case sensitive.
upper_needs_cast { NO | yes }
Set this directive to yes if upper_func needs an explicit cast
when applied to literal strings. A cast in the form CAST (<arg>
AS VARCHAR(<max DN length>)) is used, where <max DN length> is
builtin in back-sql; see macro BACKSQL_MAX_DN_LEN (currently
255; note that slapd's builtin limit, in macro
SLAP_LDAPDN_MAXLEN, is set to 8192). This is experimental and
may change in future releases.
strcast_func <SQL function name>
Specifies the name of a function that converts a given value to
a string for appropriate ordering. This is used in "SELECT
DISTINCT" statements for strongly typed RDBMSes with little
implicit casting (like PostgreSQL), when a literal string is
specified. This is experimental and may change in future
releases.
concat_pattern <pattern>
This statement defines the pattern that is used to concatenate
strings. The pattern MUST contain two question marks, '?', that
will be replaced by the two strings that must be concatenated.
The default value is CONCAT(?,?); a form that is known to be
highly portable (IBM db2, PostgreSQL) is ?||?, but an explicit
cast may be required when operating on literal strings:
CAST(?||? AS VARCHAR(<length>)). On some RDBMSes (IBM db2,
MSSQL) the form ?+? is known to work as well. Carefully check
the documentation of your RDBMS or stay with the examples for
supported ones. This is experimental and may change in future
releases.
aliasing_keyword <string>
Define the aliasing keyword. Some RDBMSes use the word "AS"
(the default), others don't use any.
aliasing_quote <string>
Define the quoting char of the aliasing keyword. Some RDBMSes
don't require any (the default), others may require single or
double quotes.
has_ldapinfo_dn_ru { NO | yes }
Explicitly inform the backend whether the dn_ru column (DN in
reverse uppercased form) is present in table ldap_entries.
Overrides automatic check (this is required, for instance, by
PostgreSQL/unixODBC). This is experimental and may change in
future releases.
fail_if_no_mapping { NO | yes }
When set to yes it forces attribute write operations to fail if
no appropriate mapping between LDAP attributes and SQL data is
available. The default behavior is to ignore those changes that
cannot be mapped. It has no impact on objectClass mapping, i.e.
if the structuralObjectClass of an entry cannot be mapped to SQL
by looking up its name in ldap_oc_mappings, an add operation
will fail regardless of the fail_if_no_mapping switch; see
section "METAINFORMATION USED" for details. This is
experimental and may change in future releases.
allow_orphans { NO | yes }
When set to yes orphaned entries (i.e. without the parent entry
in the database) can be added. This option should be used with
care, possibly in conjunction with some special rule on the
RDBMS side that dynamically creates the missing parent.
baseObject [ <filename> ]
Instructs the database to create and manage an in-memory
baseObject entry instead of looking for one in the RDBMS. If
the (optional) <filename> argument is given, the entry is read
from that file in LDIF(5) format; otherwise, an entry with
objectClass extensibleObject is created based on the contents of
the RDN of the baseObject. This is particularly useful when
ldap_entries information is stored in a view rather than in a
table, and union is not supported for views, so that the view
can only specify one rule to compute the entry structure for one
objectClass. This topic is discussed further in section
"METAINFORMATION USED". This is experimental and may change in
future releases.
create_needs_select { NO | yes }
Instructs the database whether or not entry creation in table
ldap_entries needs a subsequent select to collect the
automatically assigned ID, instead of being returned by a stored
procedure.
fetch_attrs <attrlist>
fetch_all_attrs { NO | yes }
The first statement allows to provide a list of attributes that
must always be fetched in addition to those requested by any
specific operation, because they are required for the proper
usage of the backend. For instance, all attributes used in ACLs
should be listed here. The second statement is a shortcut to
require all attributes to be always loaded. Note that the
dynamically generated attributes, e.g. hasSubordinates, entryDN
and other implementation dependent attributes are NOT generated
at this point, for consistency with the rest of slapd. This may
change in the future.
check_schema { YES | no }
Instructs the database to check schema adherence of entries
after modifications, and structural objectClass chain when
entries are built. By default it is set to yes.
sqllayer <name> [...]
Loads the layer <name> onto a stack of helpers that are used to
map DNs from LDAP to SQL representation and vice-versa.
Subsequent args are passed to the layer configuration routine.
This is highly experimental and should be used with extreme
care. The API of the layers is not frozen yet, so it is
unpublished.
METAINFORMATION USED
Almost everything mentioned later is illustrated in examples located in
the servers/slapd/back-sql/rdbms_depend/ directory in the OpenLDAP
source tree, and contains scripts for generating sample database for
Oracle, MS SQL Server, mySQL and more (including PostgreSQL and IBM
db2).
The first thing that one must arrange is what set of LDAP object
classes can present your RDBMS information.
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.
Also you might want your object classes to conform to some of the
standard schemas like inetOrgPerson etc.
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.
Example: Let's suppose that we store information about persons working
in our organization in two tables:
PERSONS PHONES
---------- -------------
id integer id integer
first_name varchar pers_id integer references persons(id)
last_name varchar phone
middle_name varchar
...
(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:
person
-------
MUST cn
MAY telephoneNumber $ firstName $ lastName
...
To fetch all values for cn attribute given person ID, we construct the
query:
SELECT CONCAT(persons.first_name,' ',persons.last_name)
AS cn FROM persons WHERE persons.id=?
for telephoneNumber we can use:
SELECT phones.phone AS telephoneNumber FROM persons,phones
WHERE persons.id=phones.pers_id AND persons.id=?
If we wanted to service LDAP requests with filters like
(telephoneNumber=123*), we would construct something like:
SELECT ... FROM persons,phones
WHERE persons.id=phones.pers_id
AND persons.id=?
AND phones.phone like '%1%2%3%'
(note how the telephoneNumber match is expanded in multiple wildcards
to account for interspersed ininfluential chars like spaces, dashes and
so; this occurs by design because telephoneNumber is defined after a
specially recognized syntax). 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.
To store such information, we add three more tables to our schema and
fill it with data (see samples):
ldap_oc_mappings (some columns are not listed for clarity)
---------------
id=1
name="person"
keytbl="persons"
keycol="id"
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).
ldap_attr_mappings (some columns are not listed for clarity)
-----------
id=1
oc_map_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"
This table defines mappings between LDAP attributes and SQL queries
that load their values. Note that, unlike LDAP schema, these are not
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.
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).
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.
ldap_entries
------------
id=1
dn=<dn you choose>
oc_map_id=...
parent=<parent record id>
keyval=<value of primary key>
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.
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.
Also, several users 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):
CREATE VIEW ldap_entries (id, dn, oc_map_id, parent, keyval)
AS
SELECT 0, UPPER('o=MyCompany,c=NL'),
3, 0, 'baseObject' FROM unixusers WHERE userid='root'
UNION
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;
If your RDBMS does not support unions in views, only one objectClass
can be mapped in ldap_entries, and the baseObject cannot be created; in
this case, see the baseObject directive for a possible workaround.
TYPICAL SQL BACKEND OPERATION
Having meta-information 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.
Example: for our query with filter (telephoneNumber=123*) we would get
the following query generated (which loads candidate IDs)
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 '%1%2%3%')
(for ONELEVEL search) or "... AND dn=?" (for BASE search) or "... AND
dn LIKE '%?'" (for SUBTREE)
Then, for each candidate, we load the requested attributes using per-
attribute queries like
SELECT phones.phone AS telephoneNumber
FROM persons,phones
WHERE persons.id=? AND phones.pers_id=persons.id
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.
ADD, DELETE, MODIFY and MODRDN operations are also performed on per-
attribute meta-information (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 PostgreSQL, ORACLE and MSSQL - since as of this
writing there are no stored procs in MySQL).
We just add more columns to ldap_oc_mappings and ldap_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 the concepts expressed above.
COMMON TECHNIQUES
First of all, let's recall that among other major differences to the
complete LDAP data model, the above illustrated concept does not
directly support such features as multiple objectclasses per entry, and
referrals. Fortunately, they are easy to adopt in this scheme. The
SQL backend requires that one more table is added to the schema:
ldap_entry_objectclasses(entry_id,oc_name).
That table contains any number of objectclass names that corresponding
entries will possess, 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...
Referrals used to be implemented in a loose manner by adding an extra
table that allowed any entry to host a "ref" attribute, along with a
"referral" extra objectClass in table ldap_entry_objclasses. In the
current implementation, referrals are treated like any other user-
defined schema, since "referral" is a structural objectclass. The
suggested practice is to define a "referral" entry in ldap_oc_mappings,
holding a naming attribute, e.g. "ou" or "cn", a "ref" attribute,
containing the url; in case multiple referrals per entry are needed, a
separate table for urls can be created, where urls are mapped to the
respective entries. The use of the naming attribute usually requires
to add an "extensibleObject" value to ldap_entry_objclasses.
CAVEATS
As previously stated, this backend should not be considered a
replacement of other data storage backends, but rather a gateway to
existing RDBMS storages that need to be published in LDAP form.
The hasSubordintes operational attribute is honored by back-sql in
search results and in compare operations; it is partially honored also
in filtering. Owing to design limitations, a (brain-dead?) filter of
the form (!(hasSubordinates=TRUE)) will give no results instead of
returning all the leaf entries, because it actually expands into ...
AND NOT (1=1). If you need to find all the leaf entries, please use
(hasSubordinates=FALSE) instead.
A directoryString value of the form "__First___Last_" (where
underscores mean spaces, ASCII 0x20 char) corresponds to its prettified
counterpart "First_Last"; this is not currently honored by back-sql if
non-prettified data is written via RDBMS; when non-prettified data is
written through back-sql, the prettified values are actually used
instead.
BUGS
When the ldap_entry_objclasses table is empty, filters on the
objectClass attribute erroneously result in no candidates. A
workaround consists in adding at least one row to that table, no matter
if valid or not.
PROXY CACHE OVERLAY
The proxy cache overlay allows caching of LDAP search requests
(queries) in a local database. See slapo-pcache(5) for details.
EXAMPLES
There are example SQL modules in the slapd/back-sql/rdbms_depend/
directory in the OpenLDAP source tree.
ACCESS CONTROL
The sql backend honors access control semantics as indicated in
slapd.access(5) (including the disclose access privilege when enabled
at compile time).
FILES
/etc/ldap/slapd.conf
default slapd configuration file
SEE ALSO
slapd.conf(5), slapd(8).