Provided by: libglobus-xio-doc_3.6-2_all bug

NAME

       Globus XIO Driver - Globus XIO introduces a notion of a driver stack to its API.

       With in globus_xio every IO operation must occur on a globus_xio handle. Associated with
       each handle is a stack of drivers. A driver is a module piece of code that implements the
       globus_xio driver interface. The purpose of a driver is manipulate data passed in by the
       user in someway. Each driver in a stack will serve its own unique purpose.
       IO operations pass from driver to driver, starting at the top of the stack and ending at
       the bottom. When the bottom layer driver finishes with the operation it signals globus_xio
       that it has completed. Completion notification then follows the driver stack up to the
       top.

       Driver Types:

           Transport driver:
            A transport driver is one that is responsible for actually putting bytes onto the
           wire. For example: A TCP driver or a UDP driver would be an example of transport
           drivers.

            Per driver stack there must be exactly one transport driver and must be at the bottom
           of the stack. A transform driver is defined by its lack of passing an operation to the
           next driver in the stack. This type of driver does not rely on globus_xio for further
           completion of an operation, rather it is self sufficent in this task.

           Transform driver:
            A tranform driver is any intermediate driver in the stack. These drivers are
           indentified by there reliance on the driver stack to complete the operation. These
           drivers must pass the operation down the stack because they cannot complete it
           themselves. An example of a transform driver would be a gsi driver. This driver would
           wrap and unwrap messages, but would not be able to complete the transport itself, so
           it would rely on the remaining drivers in the stack.

       Driver API
           The globus xio driver api is a set of functions and interfaces to allow a developer to
           create a backend driver for globus_xio. To create a driver the user must implement all
           of the interface functions in the driver specification. There are also a set of
           functions provide to assist the driver author in implemention.

           Quick Start:
            Four basic driver needs the user will have to pay attention to a few new structures
           and concepts.

           globus_xio_operation_t
            This structure represents a request for an operation. If the driver can service the
           operation it does so and the calls the appropriate finish_operation() function. If the
           driver cannot completely service the operation it can pass() it along to the next
           driver in the stack. As soon as the operation structure is either finished or passed
           it is no longer valid for use in any other function.

           globus_xio_driver_handle_t
            A driver_handle represents a open handle to the driver stack for xio. The driver
           obtains a driver_handle by calling globus_xio_driver_open(). When the open operation
           completes (it callback is called) the driver then has a driver_handle. The
           driver_handle allows the user to do some complex things that will be described later.

           globus_xio_stack_t
            This structure provides the driver with information about the driver stack It is
           mainly used for creating driver_handle as a parameter to lobus_xio_driver_open()..

       Typical Sequence:
           Here is a typcial sequence of events for a globus_xio transform driver:

             ar
               <tt>Open</tt>
                globus_xio_driver_open_t is called.  The user calls
                globus_xio_driver_open() passing it the operation and
                the stack and a callback.  When the open callback is called
                the driver is given a new operation as a parameter.  The driver
                will then call globus_xio_driver_finished_open() passing
                it the now initialized driver_handle and the newly received
                operation.
                The call to globus_xio_driver_finished_open() does two things:
                1) it tells globus_xio that this driver has finished its open
                operation, and 2) it gives xio the driver_handle (which contains
                information on the drivers below it).

             ar
               <tt>Read/Write</tt>
                 The read or write interface funcion is called.  It receives
                 a operation as a parameter.  The driver then calls the
                 approriate pass operation and waits for the callback.  When
                 the callback is received the driver calls finished_operation
                 passing in the operation structure it received in the callback

             ar
                 <tt>Close</tt>
                 The close interface function is called and is passed an
                 operation and a driver_handle.  The driver will call
                 globus_xio_driver_close() passing it the operation.
                 When the close callback is received the driver calls
                 globus_xio_driver_finished_close() passing it the
                 operation received in the close callback and the driver_handle
                 received in the interface function.  At this point the
                 driver_handle
                 is no longer valid..

       Advanced Driver Programming
           The typical driver implementatin is describe above. However globus_xio allows driver
           authors to do more advanced things. Some of these things will be explored here.

           Read Ahead
           Once a driver_handle is open a driver can spawn operation structures from it. This
           gives the driver the ability to request io from the driver stack before it receives a
           call to its own interface io interface function. So if a driver wishes to read ahead
           it does the following:

       · it creats an operation by calling globus_xio_driver_create_operation() and passing it
         the driver_handle it is intereesting in using.
       · call globus_xio_driver_read() using this operations. When the read callback is received
         the driver may call finished_operation() on the op it receives (this ultimitely results
         in very little, since this operation was started by this driver, but it is good practice
         and will free up resources that would otherwise leak).
       · Now when the user finally does receive a read interface call from globus_xio it can
         imediately finish it using the operation it just received as a parameter and updating
         the iovec structure to represent the read that already occured.

           Preopening handles.
           Once the driver has received a globus_xio_driver_stack_t it can open a driver_handle.
           The globus_xio_driver_stack_t comes in the call to the interface function
           globus_xio_server/client_init_t(). The driver uses this structure in a call to
           globus_xio_driver_open(). When this functionality completes the driver has an
           initialized driver_handle and can use it to create operations as described above. The
           driver can now hang onto this driver_handle until it receives an open interface
           function call. At which time it can call globus_xio_driver_finished_open() passing in
           the driver_handle and thereby glueing the pre opened driver_handle with the requested
           globus_xio operation.

Author

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