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Casiano Rodriguez-Leon
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GRID::Machine::REMOTE - The server that runs on the other side of the SSH link


The Structure of the Remote Server

As with most servers, the server side of the GRID::Machine object consists of an infinite loop waiting for requests:

  while( 1 ) {
     my ( $operation, @args ) = $server->read_operation();

     if ($server->can($operation)) {

     $server->send_error( "Unknown operation $operation\nARGS: @args\n" );

The Protocol

The protocol simply consists of the name of the method to execute and the arguments for such method. The programmer - using inheritance - can extend the protocol with new methods (see the section "EXTENDING THE PROTOCOL"). The following operations are currently supported:

  • GRID::Machine::EVAL

    Used by the local method eval

  • GRID::Machine::STORE

    Used by the local methods compile and sub to install code on the remote side.

  • GRID::Machine::EXISTS

    Used by the local method exists

  • GRID::Machine::CALL

    Used by the local method call

  • GRID::Machine::MODPUT

    Used by the modput method. A list of pairs (Module::Name, code for Module::Name) is sent to the remote machine. For each pair, the remote side writes to disk a file Module/Name.pm with the contents of the string code for Module::Name. The file is stored in the directory referenced by the prefix attribute of the GRID::Machine object.

  • GRID::Machine::OPEN

    Used by the open method. As arguments receives a string defining the way the file will be accessed.

  • GRID::Machine::QUIT

    Usually is automatically called when the GRID::Machine object goes out of scope

  • GRID::Machine::GPRINT

    Most requests go from the local machine to the remote Perl server. However, this and the next go in the other direction. This request is generated in the remote machine and served by the local machine. It is used when inmediate printing is required (see section "Functions gprint and gprintf")

  • GRID::Machine::GPRINTF

    This request is generated in the remote machine and served by the local machine. It is used when inmediate printing is required (see section "Functions gprint and gprintf")

  • GRID::Machine::CALLBACK

    Used to implement callbacks

The SERVER function

The SERVER function is available on the remote machine. Returns the object representing the remote side of the GRID::Machine object. This way code on the remote side can gain access to the GRID::Machine object. See an example:

    my $m = GRID::Machine->new( host => 'beowulf');

    $m->sub(installed => q { return  keys %{SERVER->stored_procedures}; });
    my @functions = $m->installed()->Results;
    local $" = "\n";
    print "@functions\n";

The stored_procedures method returns a reference to the hash containing the subroutines installed via the sub and compile methods. The keys are the names of the subroutines, the values are the CODE references implementing them. When executed the former program produces the list of installed subroutines:

                    $ accessobject.pl

The read_operation Method


     my ( $operation, @args ) = $server->read_operation( );

Reads from the link. Returns the type of operation/tag and the results of the operation.

The send_error Method


     $server->send_error( "Error message" );

Inside code to be executed on the remote machine we can use the function send_error to send error messages to the client

The send_result Method


        stdout  => $stdout,
        stderr  => $stderr,
        errmsg  => $errmsg,
        results => [ @results ],

Inside code to be executed on the remote machine we can use the function send_result to send results to the client


Let us see a simple example. We will extend the protocol with a new tag MYTAG. We have to write a module that will be used in the remote side of the link:

  $ cat -n MyRemote.pm      
     1  package GRID::Machine;
     2  use strict;
     4  sub MYTAG {
     5    my ($server, $name) = @_;
     7    $server->send_operation("RETURNED", "Hello $name!\n") if defined($name); 
     8    $server->send_operation("DIED", "Error: Provide a name to greet!\n");
     9  }
    11  1;

This component will be loaded on the remote machine via the ssh link. The name of the handling method MYTAG must be the same than the name of the tag (operation type) used to send the request. Here is a client program using the new tag:

  $ cat -n extendprotocol.pl
     1  #!/usr/local/bin/perl -w
     2  use strict;
     3  use GRID::Machine;
     5  my $name = shift;
     6  my $host = 'user@remote.machine';
     8  my $machine = GRID::Machine->new(host => $host, remotelibs => [ qw(MyRemote) ]);
    10  $machine->send_operation( "MYTAG", $name);
    11  my ($type, $result) = $machine->read_operation();
    13  die $result unless $type eq 'RETURNED';
    14  print $result;

When the program is executed we get the following output:

                          $ extendprotocol.pl Larry
                          Hello Larry!
                          $ extendprotocol.pl
                          Error: Provide a name to greet!


Functions gprint and gprintf

When running a RPC the output generated during the execution of the remote subroutine isn't available until the return of the RPC. Use gprint and gprintf if what you want is inmediate output (for debugging purposes, for instance). They work as print and printf respectively.

See an example:

  $ cat -n gprint.pl
     1  #!/usr/local/bin/perl -w
     2  use strict;
     3  use GRID::Machine;
     5  my $host = $ENV{GRID_REMOTE_MACHINE};
     7  my $machine = GRID::Machine->new(host => $host, uses => [ 'Sys::Hostname' ]);
     9  my $r = $machine->sub(
    10    rmap => q{
    11      my $f = shift; # function to apply
    12      die "Code reference expected\n" unless UNIVERSAL::isa($f, 'CODE');
    15      print "Inside rmap!\n"; # last message
    16      my @result;
    17      for (@_) {
    18        die "Array reference expected\n" unless UNIVERSAL::isa($_, 'ARRAY');
    20        gprint hostname(),": Processing @$_\n";
    23        push @result, [ map { $f->($_) } @$_ ];
    24      }
    26      gprintf "%12s:\n",hostname();
    27      for (@result) {
    28        my $format = "%5d"x(@$_)."\n";
    29        gprintf $format, @$_
    30      }
    31      return @result;
    32    },
    33  );
    34  die $r->errmsg unless $r->ok;
    36  my $cube = sub { $_[0]**3 };
    37  $r = $machine->rmap($cube, [1..3], [4..6], [7..9]);
    38  print $r;

When executed the program produces the following output:

          $ gprint.pl
          orion: Processing 1 2 3
          orion: Processing 4 5 6
          orion: Processing 7 8 9
              1    8   27
             64  125  216
            343  512  729
          Inside rmap!

Observe how the message 'Inside rmap!' generated at line 15 using print is the last (actually is sent to STDOUT in line 38). The messages generated using gprint and gprintf (lines 20, 26 and 29) were inmediately sent to STDOUT.


To run the remote side under the control of the perl debugger use the debug option of new. The associated value must be a port number higher than 1024:

     my $machine = GRID::Machine->new(
        host => $host,
        debug => $port,
        includes => [ qw{SomeFunc} ],

Before running the example open a SSH session to the remote machine in a different terminal and execute netcat to listen (option -l) in the chosen port:

  pp2@nereida:~/LGRID_Machine$ ssh beowulf 'netcat -v -l -p 12345'
  listening on [any] 12345 ...

Now run the program in the first terminal:

  pp2@nereida:~/LGRID_Machine/examples$ debug1.pl beowulf:12345
  Debugging with 'ssh beowulf PERLDB_OPTS="RemotePort=beowulf:12345" perl -d'
  Remember to run 'netcat -v -l -p 12345' in beowulf

The program looks blocked. If you go to the other terminal you will find the familiar perl debugger prompt:

  casiano@beowulf:~$ netcat -v -l -p 12345
  listening on [any] 12345 ...
  connect to [] from beowulf.pcg.ull.es [] 38979

  Loading DB routines from perl5db.pl version 1.28
  Editor support available.

  Enter h or `h h' for help, or `man perldebug' for more help.

  33:     1;
  auto(-1)  DB<1> c GRID::Machine::main
  490:      my $server = shift;

From now on you can execute almost any debugger command. Unfortunately you are now inside GRID::Machine code and - until you gain some familiarity with GRID::Machine code - it is a bit difficult to find where your code is and where to put your breakpoints. Future work: write a proper debugger front end.



Casiano Rodriguez Leon <casiano@ull.es>


This work has been supported by CEE (FEDER) and the Spanish Ministry of Educacion y Ciencia through Plan Nacional I+D+I number TIN2005-08818-C04-04 (ULL::OPLINK project http://www.oplink.ull.es/). Support from Gobierno de Canarias was through GC02210601 (Grupos Consolidados). The University of La Laguna has also supported my work in many ways and for many years.

I wish to thank Paul Evans for his IPC::PerlSSH module: it was the source of inspiration for this module. To Alex White, Dmitri Kargapolov, Eric Busto and Erik Welch for their contributions. To the Perl Monks, and the Perl Community for generously sharing their knowledge. Finally, thanks to Juana, Coro and my students at La Laguna.


Copyright (c) 2007 Casiano Rodriguez-Leon (casiano@ull.es). All rights reserved.

These modules are free software; you can redistribute it and/or modify it under the same terms as Perl itself. See perlartistic.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.