path: root/docs/tutorials/007/thread_pool.cpp

// $Id$

#include "thread_pool.h"

/*
   We need this header so that we can invoke handle_input() on the objects we dequeue.
 */
#include "ace/Event_Handler.h"


/*
   All we do here is initialize our active thread counter.
 */
Thread_Pool::Thread_Pool(void)
 : active_threads_(0)
{
}

/*
   Our open() method is a thin disguise around the ACE_Task<> activate() method.  By
   hiding activate() in this way, the users of Thread_Pool don't have to worry about
   the thread configuration flags.
 */
int Thread_Pool::open( int _pool_size )
{
  return this->activate(THR_NEW_LWP,_pool_size);
}

/*
   Closing the thread pool can be a tricky exercise.  I've decided to take an easy approach
   and simply enqueue a secret message for each thread we have active.
 */
int Thread_Pool::close(void)
{
	/*
	   Find out how many threads are currently active
	 */
	int counter = active_threads_.value();

	/*
	   For each one of the active threads, enqueue a "null" event handler.  Below, we'll
	   teach our svc() method that "null" means "shutdown".
	 */
	while( counter-- )
	{
		this->enqueue( 0 );
	}

	/*
	   As each svc() method exits, it will decrement the active thread counter.  We just wait
	   here for it to reach zero.  Since we don't know how long it will take, we sleep for
	   a quarter-second or so between tries.
	 */
	while( active_threads_.value() )
	{
		ACE_OS::sleep( ACE_Time_Value(0.25) );
	}

	return(0);
}

/*
   When an object wants to do work in the pool, it should call the enqueue() method.
   We introduce the ACE_Message_Block here but, unfortunately, we seriously missuse it.
 */
int Thread_Pool::enqueue( ACE_Event_Handler * _handler )
{
	/*
	   An ACE_Message_Block is a chunk of data.  You put them into an ACE_Message_Queue.
	   ACE_Task<> has an ACE_Message_Queue built in.  In fact, the parameter to ACE_Task<>
	   is passed directly to ACE_Message_Queue.  If you look back at our header file you'll
	   see that we used ACE_MT_SYNCH as the parameter indicating that we want MultiThread
	   Synch safety.  This allows us to safely put ACE_Message_Block objects into the
	   message queue in one thread and take them out in another.
	 */

	/*
	   An ACE_Message_Block wants to have char* data.  We don't have that.  We could
	   cast our ACE_Event_Handler* directly to a char* but I wanted to be more explicit.
	   Since casting pointers around is a dangerous thing, I've gone out of my way here
	   to be very clear about what we're doing.

	   First:  Cast the handler pointer to a void pointer.  You can't do any useful work
	           on a void pointer, so this is a clear message that we're making the
			   pointer unusable.

	   Next:   Cast the void pointer to a char pointer that the ACE_Message_Block will accept.
	 */
	void * v_data = (void*)_handler;
	char * c_data = (char*)v_data;

	/*
	   Construct a new ACE_Message_Block.  For efficiency, you might want to preallocate a
	   stack of these and reuse them.  For simplicity, I'll just create what I need as I need it.
	 */
	ACE_Message_Block * mb = new ACE_Message_Block( c_data );

	/*
	   Our putq() method is a wrapper around one of the enqueue methods of the ACE_Message_Queue
	   that we own.  Like all good methods, it returns -1 if it fails for some reason.
	 */
	if( this->putq(mb) == -1 )
	{
	  /*
	     Another trait of the ACE_Message_Block objects is that they are reference counted.
		 Since they're designed to be passed around between various objects in several threads
		 we can't just delete them whenever we feel like it.  The release() method is similar
		 to the destroy() method we've used elsewhere.  It watches the reference count and will
		 delete the object when possible.
	   */
	  mb->release();
	  return(-1);
	}

	return(0);
}

/*
   The "guard" concept is very powerful and used throughout multi-threaded applications.
   A guard normally does some operation on an object at construction and the "opposite"
   operation at destruction.  For instance, when you guard a mutex (lock) object, the guard
   will acquire the lock on construction and release it on destruction.  In this way, your
   method can simply let the guard go out of scope and know that the lock is released.

   Guards aren't only useful for locks however.  In this application I've created two guard
   objects for quite a different purpose.
 */

/*
   The Counter_Guard is constructed with a reference to the thread pool's active thread
   counter.  The guard increments the counter when it is created and decrements it at
   destruction.  By creating one of these in svc(), I know that the counter will be decremented
   no matter how or where svc() returns.
 */
class Counter_Guard
{
public:
	Counter_Guard( Thread_Pool::counter_t & _counter )
	 : counter_(_counter)
	{
		++counter_;
	}

	~Counter_Guard(void)
	{
		--counter_;
	}

protected:
	Thread_Pool::counter_t & counter_;
};

/*
   My Message_Block_Guard is also a little non-traditional.  It doesn't do anything in the
   constructor but it's destructor ensures that the message block's release() method is called.
   This is a cheap way to prevent a memory leak if I need an additional exit point in svc().
 */
class Message_Block_Guard
{
public:
	Message_Block_Guard( ACE_Message_Block * & _mb )
	 : mb_(_mb)
	{
	}

	~Message_Block_Guard( void )
	{
		mb_->release();
	}

protected:
	ACE_Message_Block * & mb_;
};

/*
   Now we come to the svc() method.  As I said, this is being executed in each thread of the
   Thread_Pool.  Here, we pull messages off of our built-in ACE_Message_Queue and cause them
   to do work.
 */
int Thread_Pool::svc(void)
{
	/*
	   The getq() method takes a reference to a pointer.  So... we need a pointer to give it
	   a reference to.
	 */
	ACE_Message_Block * mb;

	/*
	   Create the guard for our active thread counter object.  No matter where we choose to
	   return() from svc(), we no know that the counter will be decremented.
	 */
	Counter_Guard counter_guard(active_threads_);

	/*
	   Get messages from the queue until we have a failure.  There's no real good reason
	   for failure so if it happens, we leave immediately.
	 */
	while( this->getq(mb) != -1 )
	{
		/*
		   A successful getq() will cause "mb" to point to a valid refernce-counted
		   ACE_Message_Block.  We use our guard object here so that we're sure to call
		   the release() method of that message block and reduce it's reference count.
		   Once the count reaches zero, it will be deleted.
		 */
		Message_Block_Guard message_block_guard(mb);

		/*
		   As noted before, the ACE_Message_Block stores it's data as a char*.  We pull that
		   out here and later turn it into an ACE_Event_Handler*
		 */
		char * c_data = mb->base();

		/*
		   We've chosen to use a "null" value as an indication to leave.  If the data we got
		   from the queue is not null then we have some work to do.
		 */
		if( c_data )
		{
			/*
			   Once again, we go to great lengths to emphasize the fact that we're casting pointers
			   around in rather impolite ways.  We could have cast the char* directly to an
			   ACE_Event_Handler* but then folks might think that's an OK thing to do.

			   (Note:  The correct way to use an ACE_Message_Block is to write data into it.
			    What I should have done was create a message block big enough to hold an
				event handler pointer and then written the pointer value into the block.  When
				we got here, I would have to read that data back into a pointer.  While politically
				correct, it is also a lot of work.  If you're careful you can get away with casting
				pointers around.)
			 */
			void * v_data = (void*)c_data;
	
			ACE_Event_Handler * handler = (ACE_Event_Handler*)v_data;
	
			/*
			   Now that we finally have an event handler pointer, invoke it's handle_input() method.
			   Since we don't know it's handle, we just give it a default.  That's OK because we
			   know that we're not using the handle in the method anyway.
			 */
			if( handler->handle_input(ACE_INVALID_HANDLE) == -1 )
			{
			  /*
			     Tell the handler that it's time to go home.  The "normal" method for shutting
				 down a handler whose handler failed is to invoke handle_close().  This will
				 take care of cleaning it up for us.
				 Notice how we use the handler's get_handle() method to populate it's "handle"
				 parameter.  Convenient isn't it?
			   */
			  handler->handle_close(handler->get_handle(),0);

			  /*
			     Also notice that we don't exit the svc() method here!  The first time I did
				 this, I was exiting.  After a few clients disconnect you have an empty
				 thread pool.  Hard to do any more work after that...
			   */
			}
		}
		else
		{
			/*
			   If we get here, we were given a message block with "null" data.  That is our
			   signal to leave, so we return(0) to leave gracefully.
			 */
			return(0);		// Ok, shutdown request
		}

		// message_block_guard goes out of scope here
		// and releases the message_block instance.
	}

	return(0);
}