Switch load files

The switch load file is a shared library (a DLL on Windows systems) that is loaded by the code in the IBM MQ application and the queue manager. Its purpose is to simplify the loading of the database's client shared library, and to return the pointers to the XA functions.

The details of the switch load file must be specified before the queue manager is started. The details are placed in the qm.ini file on Windows, UNIX and Linux systems.

• On Windows and Linux (x86 and x86-64 platforms) systems, use the IBM MQ Explorer to update the qm.ini file.
• On all other systems edit the file, qm.ini, directly.

The C source for the switch load file is supplied with the IBM MQ installation if it supports Scenario 1 global units of work. The source contains a function called MQStart. When the switch load file is loaded, the queue manager calls this function, which returns the address of a structure called an XA switch.

The XA switch structure exists in the database client shared library, and contains a number of function pointers, as described in Table 1:

Function pointer name	XA function	Purpose
xa_open_entry	xa_open	Connect to database
xa_close_entry	xa_close	Disconnect from database
xa_start_entry	xa_start	Start a branch of a global unit of work
xa_end_entry	xa_end	Suspend a branch of a global unit of work
xa_rollback_entry	xa_rollback	Roll back a branch of a global unit of work
xa_prepare_entry	xa_prepare	Prepare to commit a branch of a global unit of work
xa_commit_entry	xa_commit	Commit a branch of a global unit of work
xa_recover_entry	xa_recover	Discover from the database whether it has an in-doubt unit of work
xa_forget_entry	xa_forget	Allow a database to forget a branch of a global unit of work
xa_complete_entry	xa_complete	Complete a branch of a global unit of work

During the first MQBEGIN call in the application, the IBM MQ code that executes as part of MQBEGIN loads the switch load file, and calls the xa_open function in the database shared library. Similarly, during queue manager startup, and on other subsequent occasions, some queue manager processes load the switch load file and call xa_open.

We can reduce the number of xa_* calls by using dynamic registration. For a complete description of this optimization technique, see XA dynamic registration.

Parent topic: Database coordination