Plugin Ordering


This document describes the design and implementation of a feature to allow Plug-in execution order to be defined.

Background

The current implementation of the SLAPI plug-in interface does not allow the execution order of plug-ins to be defined within the same plug-in type. It would be useful to plug-in developers to be able to define an order for plug-in execution. This would allow for more complex plug-in interaction, such as having a pre-operation plug-in that relies on another pre-operation plug-in completing it’s job first.

Design

To define plug-in execution order, it will be possible to set a plug-in precedence in a plug-in configuration entry. This precedence value will then be used to determine the order in which to call the plug-ins within a given plug-in type. The precedence will be defined as an integer value between 1-99 with 1 being the highest priority and 99 being the lowest priority. The precedence value will be stored in the nsslapd-pluginPrecedence attribute within a plug-in configuration entry. If the precedence attribute is not set for a particular plug-in, the value will default to 50 internally. Here is an example configuration entry:

dn: cn=ACL Plugin,cn=plugins,cn=config
objectClass: top
objectClass: nsSlapdPlugin
objectClass: extensibleObject
cn: ACL Plugin
nsslapd-pluginPath: libacl-plugin
nsslapd-pluginInitfunc: acl_init
nsslapd-pluginType: accesscontrol
nsslapd-pluginEnabled: on
nsslapd-plugin-depends-on-type: database
nsslapd-pluginId: acl
nsslapd-pluginVersion: 1.2.3
nsslapd-pluginVendor: 389 Project
nsslapd-pluginDescription: acl access check plugin
nsslapd-pluginPrecedence: 50

The precedence of a plug-in will only order plug-ins within the same plug-in type. The available plug-in types are:

It should be noted that the ability to set a precedence doesn’t make sense for all of the plug-in types. Some examples of where precedence do not make sense are the syntax, matchingrule, and extendedop plug-in types, since these are implementing callbacks to add support for specific datatypes or operations. The types where precedence is really desirable are for the six preoperation and postoperation plug-in types.

It is important to not confuse plug-in dependencies with plug-in precedence. Dependencies are used to determine plug-in startup and shutdown order, not execution order when they are called to process LDAP operations. The precedence setting will not affect the plug-in startup and shutdown order. Precedence will only be used to define execution order when processing LDAP operations.

Implementation

The plug-in structures are currently stored in a set of lists, with one list for each plug-in type. The execution order is the same as the list order, so the order can be set properly by adding a plug-in to the list in the proper place based off of the value of the precedence attribute.

Tasks

Last modified on 7 August 2014