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<META NAME="Author" CONTENT="James CE Johnson">
<META NAME="Description" CONTENT="A first step towards using ACE productively">
<TITLE>ACE Tutorial 007</TITLE>
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<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#000FFF" VLINK="#FF0F0F">
<CENTER><B><FONT SIZE=+2>ACE Tutorial 007</FONT></B></CENTER>
<CENTER><B><FONT SIZE=+2>Creating a thread-pool server</FONT></B></CENTER>
<P>
<HR WIDTH="100%">
<P>Two new files this time. The first is <A HREF="thread_pool.h">thread_pool.h</A>
where we declare our Thread_Pool object. This is responsible for
abstracting away the thread pool implementation details and allowing us
to make so few changes to the rest of the code.
<P>
<HR WIDTH="100%"><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">// $Id: thread_pool.h,v 1.1 1998/08/30
16:04:12 jcej Exp $</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">#ifndef THREAD_POOL_H</FONT>
<BR><FONT FACE="Arial,Helvetica">#define THREAD_POOL_H</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> In order to implement a thread
pool, we have to have an object that can create</FONT>
<BR><FONT FACE="Arial,Helvetica"> a thread. The ACE_Task<>
is the basis for doing just such a thing.</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT>
<BR><FONT FACE="Arial,Helvetica">#include "ace/Task.h"</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> We need a forward reference
for ACE_Event_Handler so that our enqueue() method</FONT>
<BR><FONT FACE="Arial,Helvetica"> can accept a pointer to one.</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT>
<BR><FONT FACE="Arial,Helvetica">class ACE_Event_Handler;</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">/*</FONT>
<BR><FONT FACE="Arial,Helvetica"> Although we modified the
rest of our program to make use of the thread pool</FONT>
<BR><FONT FACE="Arial,Helvetica"> implementation, if you look
closely you'll see that the changes were rather</FONT>
<BR><FONT FACE="Arial,Helvetica"> minor. The "ACE way"
is generally to create a helper object that abstracts</FONT>
<BR><FONT FACE="Arial,Helvetica"> away the details not relevant
to your application. That's what I'm trying</FONT>
<BR><FONT FACE="Arial,Helvetica"> to do here by creating the
Thread_Pool object.</FONT>
<BR><FONT FACE="Arial,Helvetica"> */</FONT>
<BR><FONT FACE="Arial,Helvetica">class Thread_Pool : public ACE_Task<ACE_MT_SYNCH></FONT>
<BR><FONT FACE="Arial,Helvetica">{</FONT>
<BR><FONT FACE="Arial,Helvetica">public:</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Provide an enumeration for the default pool size. By doing this,
other objects</FONT>
<BR><FONT FACE="Arial,Helvetica">
can use the value when they want a default.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
enum size_t</FONT>
<BR><FONT FACE="Arial,Helvetica">
{</FONT>
<BR><FONT FACE="Arial,Helvetica">
default_pool_size_ = 5</FONT>
<BR><FONT FACE="Arial,Helvetica">
};</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
// Basic constructor</FONT>
<BR><FONT FACE="Arial,Helvetica">
Thread_Pool(void);</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Opening the thread pool causes one or more threads to be activated.
When activated,</FONT>
<BR><FONT FACE="Arial,Helvetica">
they all execute the svc() method declared below.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
int open( int _pool_size = default_pool_size_ );</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
When you're done wit the thread pool, you have to have some way to shut
it down.</FONT>
<BR><FONT FACE="Arial,Helvetica">
This is what close() is for.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
int close(void);</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
To use the thread pool, you have to put some unit of work into it.
Since we're</FONT>
<BR><FONT FACE="Arial,Helvetica">
dealing with event handlers (or at least their derivatives), I've chosen
to provide</FONT>
<BR><FONT FACE="Arial,Helvetica">
an enqueue() method that takes a pointer to an ACE_Event_Handler.
The handler's</FONT>
<BR><FONT FACE="Arial,Helvetica">
handle_input() method will be called, so your object has to know when it
is being</FONT>
<BR><FONT FACE="Arial,Helvetica">
called by the thread pool.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
int enqueue( ACE_Event_Handler * _handler );</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">protected:</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Our svc() method will dequeue the enqueued event handler objects and invoke
the</FONT>
<BR><FONT FACE="Arial,Helvetica">
handle_input() method on each. Since we're likely running in more
than one thread,</FONT>
<BR><FONT FACE="Arial,Helvetica">
idle threads can take work from the queue while other threads are busy
executing</FONT>
<BR><FONT FACE="Arial,Helvetica">
handle_input() on some object.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
int svc(void);</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
Another handy ACE template is ACE_Atomic_Op<>. When parameterized,
this allows</FONT>
<BR><FONT FACE="Arial,Helvetica">
is to have a thread-safe counting object. The typical arithmetic
operators are</FONT>
<BR><FONT FACE="Arial,Helvetica">
all internally thread-safe so that you can share it across threads without
worrying</FONT>
<BR><FONT FACE="Arial,Helvetica">
about any contention issues.</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
typedef ACE_Atomic_Op<ACE_Mutex,int> counter_t;</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">
/*</FONT>
<BR><FONT FACE="Arial,Helvetica">
We use the atomic op to keep a count of the number of threads in which
our svc()</FONT>
<BR><FONT FACE="Arial,Helvetica">
method is running. This is particularly important when we want to
close() it down!</FONT>
<BR><FONT FACE="Arial,Helvetica">
*/</FONT>
<BR><FONT FACE="Arial,Helvetica">
counter_t active_threads_;</FONT>
<BR><FONT FACE="Arial,Helvetica">};</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P><FONT FACE="Arial,Helvetica">#endif // THREAD_POOL_H</FONT><FONT FACE="Arial,Helvetica"></FONT>
<P>
<HR WIDTH="100%">
<P>Well, that doesn't look too complex. What about the implementation?
<P>
<HR WIDTH="100%">
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