comment on

First of all, let me say this is an excellent module that is rapidly getting better. I have spent a couple of hours testing and thinking about this module and hopefully come up with some constructive comments on its current functionality. Please keep in mind these are only suggestions and very open to debate.

General interface

IPC::Shareable has the following basic interface:

use IPC::Shareable;

# simple method
tie( $scalar, 'IPC::Shareable', 'varid' );

# complex method
tie( $scalar, 'IPC::Shareable', 'varid', { ...options... } );  # OR
tie( $scalar, 'IPC::Shareable', { key => 'varid', ...options... } );
[download]

where you can do a simple 3-arg tie() and that's all, or optionally pass a hashref of options. Could i suggest a similar interface for Win32::MMF::Shareable:

use Win32::MMF::Shareable;

# simple method
tie( $scalar, 'Win32::MMF::Shareable', 'varid' );
# automatically creates a namespace using default options

# complex method
tie( $scalar, 'Win32::MMF::Shareable', 'varid', { ...options... } );  
+# OR
tie( $scalar, 'Win32::MMF::Shareable', { key => 'varid', ...options...
+ } );
# for compatibility
[download]

Rather than having to call Win32::MMF::Shareable::Init to set things like namespace (see following section), size, autolock, etc, use defaults for everything and pass a hashref of option overrides to tie(). To avoid having to set the same options for each tie call every time, change Init() to setdefaults() so it works something like this:

use Win32::MMF::Shareable;

# BAD

tie( $bigvar1, 'Win32::MMF::Shareable', 'bigvar1', { autolock => 0, si
+ze => 1024 ** 2 } );
tie( $bigvar2, 'Win32::MMF::Shareable', 'bigvar2', { autolock => 0, si
+ze => 1024 ** 2 } );
tie( $bigvar3, 'Win32::MMF::Shareable', 'bigvar3', { autolock => 0, si
+ze => 1024 ** 2 } );

# GOOD

Win32::MMF::Shareable->setdefaults( { autolock => 0, size => 1024 ** 2
+ } );

tie( $bigvar1, 'Win32::MMF::Shareable', 'bigvar1' );
tie( $bigvar2, 'Win32::MMF::Shareable', 'bigvar2' );
tie( $bigvar3, 'Win32::MMF::Shareable', 'bigvar3' );
[download]

If you tie without any options you get the default values for each option. Optionally, you can call Win32::MMF::Shareable->setdefaults() to change the defaults for any future calls to tie. This way you have the best of simplicity and flexibility.

Thoughts on namespaces

IPC::Shareable uses a single identifier to reference a shared variable (refereed to as GLUE or key in the docs), where as Win32::MMF has two levels, a namespace and an identifier within the namespace. While this works well in Win32::MMF, as the namespace can be the object and you can have multiple items within the namespace, it becomes somewhat redundant when you tie a single variable with Win32::MMF::Shareable.

tie( $scalar, 'Win32::MMF::Shareable', { namespace => 'ns1', key => 'f
+oo' } );
tie( $scalar, 'Win32::MMF::Shareable', { namespace => 'ns1', key => 'b
+ar' } );
tie( $scalar, 'Win32::MMF::Shareable', { namespace => 'ns2', key => 'f
+oo' } );
tie( $scalar, 'Win32::MMF::Shareable', { namespace => 'ns2', key => 'b
+ar' } );
[download]

My suggestion would be to make the namespace option in Win32::MMF::Shareable optional, and have it transparently create and use the default namespace 'shareable'. This eliminates redundancy, still allows the user to override it if necessary, and allows users to mix-n-match the two modules:

# Process 1
use Win32::MMF::Shareable;

my @queue;
tie( @queue, 'Win32::MMF::Shareable', 'queue' );
@queue = qw( 8265 6201 7548 2165 7892 3546 3426 6246 );


# Process 2
use Win32::MMF;

my $ns = Win32::MMF->new( -namespace => "my_namespace" );
$ns->setvar( fredsname => 'fred' );
my $fredsname = $ns->getvar( 'fredsname' );
# etc... as normal

# get the current value of another processes tied variable!
my $server = Win32::MMF->new( -namespace => "shareable", reuse => 1 );
+ 
my @current_queue = $server->getvar( 'queue' );
[download]

Options

The two modules use different flags to control when a particular 'varid' is created or mapped to an existing one. IPC::Shareable uses create & exclusive flags, while Win32::MMF has a single reuse flag, and they behave as follows (bold is the default):

IPC::Shareable

create flag exclusive flag varid doesn't exist varid already exists

0 (unused) croaks maps to varid

1 0 creates new varid maps to varid

1 1 creates new varid croaks

Win32::MMF

reuse flag varid doesn't exist varid already exists

0 creates new varid maps to varid

1 fails silently maps to varid

Currently you can't specify for Win32::MMF to create exclusively – failing if the varid already exists. How about something like the following, which handles all 3 cases simply:

connect flag varid doesn't exist varid already exists

undef (or "auto"?) creates new varid maps to varid

1 fails silently maps to varid

0 creates new varid fails silently

tie( $var, 'Win32::MMF::Shareable', 'var' );  # OR
tie( $var, 'Win32::MMF::Shareable', 'var', { connect => 'auto' } );  #
+ map if already available, otherwise create
tie( $var, 'Win32::MMF::Shareable', 'var', { connect => 1 } ); # must 
+already exist
tie( $var, 'Win32::MMF::Shareable', 'var', { connect => 0 } ); # must 
+not already exist
[download]

To maintain compatibility with IPC::Shareable you would still need to accept 'create' and 'exclusive' flags (but overridden by 'connect' if present) and convert them to the appropriate connect flag.

The other IPC::Shareable options are:

key – same as your 'varid', suggest you use this, or at least accept it as an alias

mode – redundant under win32, accept but ignore

destroy – removes shared data after the process exits, wether another process is using it or not. Not sure why one would use this, accept but ignore (unless you feel like implementing it)

size – same as your size

Tied object

Now that we tie our shared variable, we no longer have access to an object which means we lose functionality of lock(), unlock(), debug(), and anything else you decide to implement in the future. Other tie() modules overcome this by returning an object from the call to tie(), which would allow things like:

$data = tie( %data, 'Win32::MMF::Shareable', 'data', { timeout => 60 }
+ );
if( $data->{error} ) { die( “can't tie data: ” . $data->{errstr} ) }  
+# maybe? or just return undef

$data{min} = 0;
$data{max} = 500;
$data{current} = 0;

$data->debug() if $debug_flag;

# each process must do blocks of 10 at a time
{
  $data->lock();  # timeout 60 seconds

  for( $data{end} = $data{current} + 10; $data{current} < $data{end}; 
+$data{current}++ )
  {
    # do job number $data{current}
  }

  $data->unlock();
}
[download]

Other thoughts

IPC::Shareable uses Storable to serialise its data – you may wish to consider switching from Data::Dumper to maintain consistancy. Not to mention Data::Dumper has some issues in what it can serialise. Maybe even provide an option for the user so they can choose.

The two modules Win32::MMF and Win32::MMF::Shareable, while implemented in a similar way, behave quiet differently to the user. It may be a good idea to document each separately to avoid confusion.

In the pod: "Because memory and variable management are managed internally by the Win32::MMF module, you do not need to specify how much memory is required by the variable." Is this correct? You don't have to specify a size for a tied variable? Will it automatically expand if the data grows? If so, why not have this for non-tied variables as well?

How does your module handle the issue of when to clear the data in shared memory? I assume that it's when the last 'connected' process exits or tied variable falls out of scope, correct? What about if the process is killed (via a SIGTERM)? Any possible memory leaks? (I assume you've already covered these issues, just interested to know).

*phew* I thinks that's about it... feedback welcome!

- ><iper

use japh; print;
[download]

In reply to Re: Re: Re: Win32::MMF - Memory Mapped File Support for Perl by xiper
in thread (Updated Again) Win32::MMF - Memory Mapped File Support for Perl by Roger

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