Two user-defined version control routines provide a safety mechanism to guard against a collation routine being used on the wrong global, or an attempt being made to modify a collation routine for an existing global. Either of these situations could cause incorrect collation or damage to subscripts.

When a global is assigned an alternative collation sequence, GT.M invokes a user-supplied routine that returns a numeric version identifier for the set of collation routines, which was stored with the global. The first time a process accesses the global, GT.M determines the assigned collation sequence, then invokes another user-supplied routine. The second routine matches the collation sequence and version identifier assigned to the global with those of the current set of collation routines.

When you write the code that matches the type and version, you can decide whether to modify the version identifier and whether to allow support of globals created using a previous version of the routine.