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Posted to modperl@perl.apache.org by "Tesfaiesus, Mesel" <me...@hp.com> on 2005/08/12 15:40:12 UTC

Apache::Singleton (sharing one object)

Hi, 

I'm trying to write an application, that allows the user to generate
complex SQL Queries step by step.  
Therefore I need to make the same SQL-object available over many
requests. 

I'm using the Aapache::Singleton Module and mod_perl 2.x/Apache2.x but I
always seem to get a new Object as the properties are set to default
every time.


My Testcode is printing out a $counter - which it saved as an attribute
of the object - that's supposed to increment with every request:

###############   MyClass Module ###################

 package MyClass;

 use base qw (Apache::Singleton);

 {
 	Sub _new_instance {
		my $class = shift;
				
		my $self = {};
		bless $self, $class;

		$self->setProperty( 'counter', 1);

	}	
 }

#####################################################




################   Script: index.pl  ################

 Use MyClass;

 my $instance = MyClass->instance();

 my $counter = 0;

 $counter = $instance->getProperty('counter');

 print $counter;

 $counter++;

 $instance->setProperty('counter', $counter);

######################################################



The result is always 1. it doesn't increment. What am I doing wrong?

Any comment is appreciated.

Cheers,
mesel

Re: Apache::Singleton (sharing one object)

Posted by Perrin Harkins <pe...@elem.com>.
On Fri, 2005-08-12 at 15:40 +0200, Tesfaiesus, Mesel wrote:
> I'm using the Aapache::Singleton Module and mod_perl 2.x/Apache2.x but I
> always seem to get a new Object as the properties are set to default
> every time.

It defaults to request scope, as described in the documentation.  To
make it switch to process scope, use this:

use base qw (Apache::Singleton::Process);

However, this will only persist in the one process (it's like putting
the object in a global).  If what you need is to make it available in
all processes, create it during startup and put it in a global.  If you
need that, but you need the data between processes to be in synch (e.g.
the counter in this example to be shared between all processes) you
should use a database or something like Cache::FastMmap to share the
state of it.

- Perrin

Re: Apache::Singleton (sharing one object)

Posted by Praveen Ray <pr...@yahoo.com>.
Shouldn't you be using the database itself to store the
state? Will Apache::Singleton instance replicate itself if
you were to run your application on multiple servers behind
load balancers (for scalability and fail-over purposes)?
If yes, you probably should usw Storage Module to save your
SQL object and retrieve in each request. That would be
'true' singleton, in my opinion.


--- "Tesfaiesus, Mesel" <me...@hp.com> wrote:

> Hi, 
> 
> I'm trying to write an application, that allows the user
> to generate
> complex SQL Queries step by step.  
> Therefore I need to make the same SQL-object available
> over many
> requests. 
> 
> I'm using the Aapache::Singleton Module and mod_perl
> 2.x/Apache2.x but I
> always seem to get a new Object as the properties are set
> to default
> every time.
> 
> 
> My Testcode is printing out a $counter - which it saved
> as an attribute
> of the object - that's supposed to increment with every
> request:
> 
> ###############   MyClass Module ###################
> 
>  package MyClass;
> 
>  use base qw (Apache::Singleton);
> 
>  {
>  	Sub _new_instance {
> 		my $class = shift;
> 				
> 		my $self = {};
> 		bless $self, $class;
> 
> 		$self->setProperty( 'counter', 1);
> 
> 	}	
>  }
> 
> #####################################################
> 
> 
> 
> 
> ################   Script: index.pl  ################
> 
>  Use MyClass;
> 
>  my $instance = MyClass->instance();
> 
>  my $counter = 0;
> 
>  $counter = $instance->getProperty('counter');
> 
>  print $counter;
> 
>  $counter++;
> 
>  $instance->setProperty('counter', $counter);
> 
> ######################################################
> 
> 
> 
> The result is always 1. it doesn't increment. What am I
> doing wrong?
> 
> Any comment is appreciated.
> 
> Cheers,
> mesel
> 


  - Praveen  

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