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.
+are only allowed to be configured on individual databases.
-Essentially, they represent a means to:
+Some can be stacked on the {{EX:frontend}} as well, for global use. 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.
+
+Essentially, overlays represent a means to:
* customize the behavior of existing backends without changing the backend
code and without requiring one to write a new custom backend with
* write functionality of general usefulness that can be applied to
different backend types
+When using {{slapd.conf}}(5), overlays that are configured before any other
+databases are considered global, as mentioned above. In fact they are implicitly
+stacked on top of the {{EX:frontend}} database. They can also be explicitly
+configured as such:
+
+> database frontend
+> overlay <overlay name>
+
Overlays are usually documented by separate specific man pages in section 5;
the naming convention is
> 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
+rationale of the overlay implementation, and may serve as a guideline for the
development of custom overlays.
Contribware overlays are located in
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
+All the current overlays in OpenLDAP are listed and described in detail in the
following sections.
In order to demonstrate how this overlay works, we shall discuss a typical
scenario which might be one master server and three Syncrepl slaves.
-On each replica, add this near the top of the file (global), before any database
-definitions:
+On each replica, add this near the top of the {{slapd.conf}}(5) file
+(global), before any database definitions:
> overlay chain
> chain-uri "ldap://ldapmaster.example.com"
bound to the slave will also exist on the master. If that DN does not have
update privileges on the master, nothing will happen.
-You will need to restart the slave after these changes. Then, if you are using
-{{loglevel stats}} (256), you can monitor an {{ldapmodify}} on the slave and the master.
+You will need to restart the slave after these {{slapd.conf}} changes.
+Then, if you are using {{loglevel stats}} (256), you can monitor an
+{{ldapmodify}} on the slave and the master. (If you're using {{cn=config}}
+no restart is required.)
Now start an {{ldapmodify}} on the slave and watch the logs. You should expect
something like:
H3: Overview
Overlays can be stacked, which means that more than one overlay
-can be instantiated for each database, or for the frontend.
-As a consequence, each overlays function is called, if defined,
+can be instantiated for each database, or for the {{EX:frontend}}.
+As a consequence, each overlay's function is called, if defined,
when overlay execution is invoked.
-Multiple overlays are executed in reverse order (it's a stack, all in all)
+Multiple overlays are executed in reverse order (as a stack)
with respect to their definition in slapd.conf (5), or with respect
to their ordering in the config database, as documented in slapd-config (5).
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