Understanding Plug-In Types and Dependencies

The plug-in type tells Directory Server which type of plug-in functions can be registered by a plug-in. A plug-in type corresponds to the principal type of operation that the plug-in performs. Refer to How Plug-Ins Interact With the Server and Example Uses for explanations of plug-in types. Part II, Directory Server Plug-In API Reference describes plug-in types and lists type property settings that correspond to the type identifiers specified in slapi-plugin.h.

Determining the plug-in type is clear-cut when you know what the plug-in must do. For example, if you write a plug-in to handle authentication of a request before Directory Server processes the bind for that request, the type is preoperation. In other words, the plug-in does something to the request prior to the bind operation. Such a plug-in therefore registers at least a pre-bind plug-in function to process the request prior to the bind. If, on the other hand, you write a plug-in to encode passwords, and also compare incoming passwords with encoded passwords, the type is pwdstoragescheme. Such a plug-in registers at least password encoding, and also compares functions. In both cases, plug-in type follows plug-in function.

Plug-in dependencies tell Directory Server that a plug-in requires one or more other plug-ins in order to function properly. Directory Server resolves dependencies as the server loads the plug-ins. Therefore, Directory Server fails to register a plug-in if a plug-in that the plug-in depends on cannot be registered.

Specify plug-in dependencies by name, using depends-on-named, or by type, using depends-on-type. Here, plug-in name refers to the plug-in relative distinguished name (RDN) from the configuration entry. Plug-in type is a type identifier from the list of types in Chapter 18, Parameter Block Reference. Both depends-on-named and nsslapddepends-on-type can take multiple values.

A plug-in configuration entry can specify either kind or both kinds of dependencies. Use the configuration entry to identify dependencies on specific plug-ins by name. Identify dependencies on a type of plug-in by type. If a dependency identifies a plug-in by name, Directory Server only registers the plug-in after the server registers the named plug-in. If a dependency identifies a plug-in by type, Directory Server only registers the plug-in after the server registers all plug-ins of the specified type.

Do not confuse plug-in registration dependencies with a mechanism to specify the order in which Directory Server calls plug-ins.