From: Charles Comstock <cc1@...>
Date: 2004-02-09T18:30:04+09:00
Subject: Re: Multi-threading lesson wanted

Robert Klemme wrote:

> "Tim Bates" <tim@bates.id.au> schrieb im Newsbeitrag
> news:4025CA70.9090202@bates.id.au...
> <snip>
> 
>>Below is the code I have written. The idea is that other application
>>code calls SAMS::Database.get_handle{ |handle| handle.execute... } and
>>this module takes care of the rest. Also, the application's shutdown
>>code will call SAMS::Database.destroy to cleanly disconnect all the
> 
> handles.
> 
> You definitely need some kind of synchronization mechanism that makes
> accesses to the shared connection pool thread safe.  In your case a
> ConditionVariable will help you with the max connection logic.
> 
> See section "Logging from multiple threads" for example usage of a
> ConditionVariable at
> http://www.rubygarden.org/ruby?MultiThreading
> 
> I'd implement a Semaphore using a ConditionVariable and a Mutex; that way
> you can initialize the semaphore with the max connection value and count
> the semaphore down when you take a connection from the pool and increment
> it when you put it back.  If the semaphore is zero, the next thread trying
> to decrease the semaphore is put to sleep and will wake up when another
> thread increments the semaphore.
> 
> You can as well use the semaphore implementation in the RAA:
> http://raa.ruby-lang.org/list.rhtml?name=semaphore
> 
> <snip>
> 
> As an additional note: I would not use module methods for the pool
> handling.  Instead I'd instantiate an instance of the pool.  This is IMHO
> better since the pool is not necessarily a singleton: Just think of an
> application that needs to access more than one database and hence use more
> than one connection configuration.
> 
> Kind regards
> 
>     robert
> 
Why doesn't the counting semaphore library come standard with Ruby?  I 
haven't done anything multithreaded in a little while, but I remember 
being annoyed I didn't have a counting semaphore.  Is there a particular 
reason it's not part of the standard library?  Obviously not everything 
should come standard but this seems to be the sort of thing that should.

	Charles Comstock