windows-server-2003/admin/netui/macprint/spooler/cdd.txt

MACPRINT SPOOLER CDD



Design Goals:

MacPrint is ported from the OS/2 implementation.  The primary design
goal is to simplify the porting process as much as possible.  The
original structure of MacPrint was retained and modified only where
necessary.  There are 3 main areas of modification:

* configuration
* synchronization
* removal of the despooler component
* support for cached dictionaries

configuration changes -

Where the OS/2 implementation depended on being able to determine
at service start time the configuration for all possible shared
print queues (this was read from lanman.ini), the NT implementation
allows for the dynamic addition and removal of printer objects
without specifying configuration information in a data file.  This
forced the modification of the main queue list structure to be
a single linked list instead of an array.  In the NT implementation,
there is now a single data structure for NT printer objects, and
being in this structure implies the existence of a validly configured
NT Printer Object.

All of the code to read and parse configuration information from
LANMAN.INI at service start time was removed and replaced by code
to get configuration information from the EnumPrinters() API as new
printers are discovered.


synchronization changes -

NT provides a more robust synchronization mechanism than OS/2.
WaitForMultipleObjects() actually works correctly under NT, and
it is possible to wait on objects besides semaphores.  Mutual
exclusion to data is achieved through critical sections as opposed
to mutexes or semaphores.  Critical sections require fewer resources
than either, but can only be shared between threads of the same
process.  Events are used to process jobs asynchronously.  Instead
of clearing a semaphore when a read or open occurs, an event is
signaled.  Semaphores in NT are used as a gating mechanism,
and are not designed to be used as a signaling mechanism.  Syncronization
with the termination of threads is achieved by waiting on the thread
object as opposed to waiting on an event that the thread signals
just before it terminates.


removal of the despooler -

The spooler and despooler were somewhat integrated in the OS/2
implementation - especially with regards to configuration information.
Data structures pertinent to the despooler only were removed, and
the configuration routines are no longer contained in a DLL shared
by the spooler and despooler.  The only connection between the two
components is now through a PostScript comment that is included by
the spooler to indicate if ctrl-d's should be stripped by the
despooler.


support for cached dictionaries -

As the caching of PostScript dictionaries achieved only modest performance
improvements at best, and since it added significant complexity to the
code, support for cached dictionaries will most likely be removed in
the NT implementation.  If at coding time it becomes clear that more
work is involved to remove the feature than to port it, it may stay
in.




Thread/Process Structure:

MacPrint is an NT Service, hence it follows the NT Service Model.  main()
is the service control dispatcher for the MacPrint service.  This thread
is used by the NT Service Controller to communicate with the MacPrint
Service.

MacPrintMain() is the main thread for MacPrint.  It is responsible for
monitoring the NT Print Manager for configuration changes (creation
and removal of printer objects) and for starting and stopping threads
to manage each NT Printer Object.  Essentially, MacPrintMain polls
until a service stop request is received.  While polling, it enumerates
printers from the NT Print Manager and either creates a control thread
for a newly discovered printer or terminates the control thread for
each printer no longer configured.

QueueServiceThread() is the thread for each active NT Printer Object.
This thread listens for jobs directed to the NT printer and processes
the job.  One thread supports multiple jobs for the printer.  This is
done by using two events per queue - ReadEvent and OpenEvent.  The
PAP API have the property that they take an address to store a completion
code when the event completes, and this address is specified to be in
a per job data structure.  The event is common for all jobs in the
queue.  When a particular event is signaled, the list of pending jobs
in that queue is walked to find one that is in the correct state
(read_pending or open_pending) and that has it's completion code set.
The event is processed for that job, and the job is moved to the end
of the list so that events for jobs are processed in round-robin
order.