-----------------
External Routines
-----------------

Author:
	Adriano dos Santos Fernandes <adrianosf@uol.com.br>

Syntax:

{ CREATE [ OR ALTER ] | RECREATE | ALTER } PROCEDURE <name>
    [ ( <parameter list> ) ]
    [ RETURNS ( <parameter list> ) ]
    EXTERNAL NAME '<external name>' ENGINE <engine>

{ CREATE [ OR ALTER ] | RECREATE | ALTER } FUNCTION <name>
    [ <parameter list> ]
    RETURNS <data type>
    EXTERNAL NAME '<external name>' ENGINE <engine>

{ CREATE [ OR ALTER ] | RECREATE | ALTER } TRIGGER <name>
    ...
    EXTERNAL NAME '<external name>' ENGINE <engine>

Examples:

create procedure gen_rows (
  start_n integer not null,
  end_n integer not null
) returns (
  n integer not null
) external name 'udrcpp_example!gen_rows'
  engine udr;

create function wait_event (
  event_name varchar(31) character set ascii
) returns integer
  external name 'udrcpp_example!wait_event'
  engine udr;

create trigger persons_replicate
  after insert on persons
  external name 'udrcpp_example!replicate!ds1'
  engine udr;

How it works:

External names are opaque strings to Firebird. They are recognized by specific external engines.
External engines are declared in config files (possibly in the same file as a plugin, like in the
config example present below).

    <external_engine UDR>
        plugin_module UDR_engine
    </external_engine>

    <plugin_module UDR_engine>
        filename $(this)/udr_engine
        plugin_config UDR_config
    </plugin_module>

    <plugin_config UDR_config>
        path $(this)/udr
    </plugin_config>

When Firebird wants to load an external routine (function, procedure or trigger) into its metadata
cache, it gets (if not already done for the database*) the external engine through the plugin
external engine factory and asks it for the routine. The plugin used is the one referenced by the
attribute plugin_module of the external engine.

* This is in Super-Server. In [Super-]Classic, different attachments to one database create
multiple metadata caches and hence multiple external engine instances.


----------------------------------
UDR - User Defined Routines engine
----------------------------------

The UDR (User Defined Routines) engine adds a layer on top of the FirebirdExternal interface with
these objectives:
 - Establish a way to place external modules into server and make them available for usage;
 - Create an API so that external modules can register their available routines;
 - Make routines instances per-attachment, instead of per-database like the FirebirdExternal does
   in SuperServer mode.

External names of the UDR engine are defined as following:
    '<module name>!<routine name>!<misc info>'

The <module name> is used to locate the library, <routine name> is used to locate the routine
registered by the given module, and <misc info> is an user defined string passed to the routine
and can be read by the user. "!<misc info>" may be ommitted.

Modules available to the UDR engine should be in a directory listed through the path attribute of
the correspondent plugin_config tag. By default, UDR modules should be on <fbroot>/plugins/udr,
accordingly to its path attribute in <fbroot>/plugins/udr_engine.conf.

The user library should include FirebirdUdr.h (or FirebirdUdrCpp.h) and link with the udr_engine
library. Routines are easily defined and registered using some macros, but nothing prevents you from
doing things manually. An example routine library is implemented in examples/plugins, showing how to
write functions, selectable procedures and triggers. Also it shows how to interact with the current
database through the ISC API.

The UDR routines state (i.e., member variables) are shared between multiple invocations of the same
routine until it's unloaded from the metadata cache. But note that it isolates the instances per
session, different from the raw interface that shares instances by multiple sessions in SuperServer.

By default, UDR routines use the same character set specified by the client. They can modify it
overriding the getCharSet method. The chosen character set is valid for communication with the ISC
library as well as the communications done through the FirebirdExternal API.