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Manual Reference Pages  -  NET::OPENSSH::PARALLEL (3)

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Net::OpenSSH::Parallel - Run SSH jobs in parallel



  use Net::OpenSSH::Parallel;

  my $pssh = Net::OpenSSH::Parallel->new();
  $pssh->add_host($_) for @hosts;

  $pssh->push(*, scp_put => /local/file/path, /remote/file/path);
  $pssh->push(*, command => gurummm,
              /remote/file/path, /tmp/output);
  $pssh->push($special_host, command => prumprum, /tmp/output);
  $pssh->push(*, scp_get => /tmp/output, logs/%HOST%/output);



Run this here, that there, etc.

Net::OpenSSH::Parallel is an scheduler that can run commands in parallel in a set of hosts through SSH. It tries to find a compromise between being simple to use, efficient and covering a good part of the problem space of parallel process execution via SSH.

Obviously, it is build on top of Net::OpenSSH!

Common usage of the module is as follows:
o create a Net::OpenSSH::Parallel object
o register the hosts where you want to run commands with the add_host method
o queue the actions you want to run (commands, file copy operations, etc.) using the push method.
o call the run method and let the parallel scheduler take care of everything!

    Labeling hosts

Every host is identified by an unique label that is given when the host is registered into the parallel scheduler. Usually, the host name is used also as the label, but this is not required by the module.

The rationale behind using labels is that a hostname does not necessarily identify unique remote processors (for instance, sometimes your logical remote processors may be user accounts distributed over a set of hosts: foo1@bar1, foo2@bar1, foo3@bar2, ...; a set of hosts that are accessible behind an unique IP, listening in different ports; etc.)

    Selecting hosts

Several of the methods of this module (well, currently, just push) accept a selector string to determine which of the registered hosts should be affected by the operation.

For instance, in...

  $pssh->push(*, command => ls)

the first argument is the selector. The one used here, *, selects all the registered hosts.

Other possible selectors are:

  bar*                # selects everything beginning by bar
  foo1,foo3,foo6      # selects the hosts of the given names
  bar*,foo1,foo3,foo6 # both
  *doz*               # everything containing doz

Note: I am still considering how the selector mini-language should be, do not hesitate to send your suggestions!

    Local resource usage

When the number of hosts managed by the scheduler is too high, the local node can become overloaded.

Roughly, every SSH connection requires two local ssh processes (one to run the SSH connection and another one to launch the remote command) that results in around 5MB of RAM usage per host.

CPU usage varies greatly depending on the tasks carried out. The most expensive are short remote tasks (because of the local process creation and destruction overhead) and tasks that transfer big amounts of data through SSH (because of the encryption going on).

In practice, CPU usage does not matter too much (mostly because the OS would be able to manage it but also because there is not too many things we can do to reduce it) and usually it is RAM about what we should be more concerned.

The module accepts two parameters to limit resource usage:
o workers

is the maximum number of remote commands that can be running concurrently.

o connections

is the maximum number of SSH connections that can be active concurrently.

In practice, limiting the maximum number of connections indirectly limits RAM usage and limiting the the maximum number of workers indirectly limits CPU usage.

The module requires the maximum number of connections to be at least equal or bigger than the maximum number of workers, and it is recommended that maximum_connections >= 2 * maximum_workers (otherwise the scheduler will not be able to reuse connections efficiently).

You will have to experiment to find out which combinations give the best results in your particular scenarios.

Also, for small sets of hosts you can just let these parameters unlimited.

    Variable expansion

This module activates Net::OpenSSH variable expansion by default. That way, it is possible to easily customize the actions executed on every host in base to some of its properties.

For instance:

  $pssh->push(*, scp_get => "/var/log/messages", "messages.%HOST%");

copies the log files appending the name of the remote hosts to the local file names.

The variables HOST, USER, PORT and LABEL are predefined.

    Error handling

When something goes wrong (for instance, some host is unreachable, some connection dies, some command fails, etc.) the module can handle the error in several predefined ways as follows:

Error policies

To set the error handling police, new, add_host and push methods support and optional on_error argument that can take the following values (these constants are available from Net::OpenSSH::Parallel::Constants):
OSSH_ON_ERROR_IGNORE Ignores the error and continues executing tasks in the host queue as it had never happened.
OSSH_ON_ERROR_ABORT Aborts the processing on the corresponding host. The error will be propagated to other hosts joining it at any later point once the join is reached.

In other words, this police aborts the queued jobs for this host and any other that has a dependency on it.

OSSH_ON_ERROR_DONE Similar to OSSH_ON_ERROR_ABORT but will not propagate errors to other hosts via joins.
OSSH_ON_ERROR_ABORT_ALL Causes all the host queues to be aborted as soon as possible (and that usually means after currently running actions end).
OSSH_ON_ERROR_REPEAT The module will try to perform the current task again and again until it succeeds. This police can lead to an infinite loop and so its direct usage is discouraged (but see the following point about setting the policy dynamically).
The default policy is OSSH_ON_ERROR_ABORT.

Setting the policy dynamically

When a subroutine reference is used as the policy instead of the any of the constants previously described, the given subroutine will be called on error conditions as follows:

  $on_error->($pssh, $label, $error, $task)

$pssh is a reference to the Net::OpenSSH::Parallel object, $label is the label associated to the host where the error happened. $error is the error type as defined in Net::OpenSSH::Parallel::Constants and $task is a reference to the task that was being carried out.

The return value of the subroutine must be one of the described constants and the corresponding policy will be applied.

Retrying connection errors

If the module fails when trying to establish a new SSH connection or when an existing connection dies unexpectedly, the option reconnections can be used to instruct the module to retry the connection until it succeeds or the given maximum is reached.

reconnections is accepted by both the new and add_host methods.


  $pssh->add_host(foo, reconnections => 3);

Note that the reconnections maximum is not per host but per queued task.


These are the available methods:
$pssh = Net::OpenSSH::Parallel->new(%opts) creates a new object.

The accepted options are:
workers => $maximum_workers sets the maximum number of operations that can be carried out in parallel (see Local resource usage).
connections => $maximum_connections sets the maximum number of SSH connections that can be established simultaneously (see Local resource usage).

$maximum_connections must be equal or bigger than $maximum_workers

reconnections => $maximum_reconnections when connecting to some host fails, this argument tells the module the maximum number of additional connection attempts that it should perform before giving up. The default value is zero.

See also Retrying connection errors.

on_error => $policy Sets the error handling policy (see Error handling).

$pssh->add_host($label, %opts)
$pssh->add_host($label, $host, %opts) registers a new host into the $pssh object.

$label is the name used to refer to the registered host afterwards.

When the hostname argument is omitted, the label is used also as the hostname.

The accepted options are:
on_error => $policy Sets the error handling policy (see Error handling).
reconnections => $maximum_reconnections See Retrying connection errors.

Any additional option will be passed verbatim to the Net::OpenSSH constructor later. For instance:

  $pssh->add_host($host, user => $user, password => $password);

$pssh->push($selector, $action, \%opts, @action_args)
$pssh->push($selector, $action, @action_args) pushes a new action into the queues selected by $selector.

The supported actions are:
command => @cmd queue the given shell command on the selected hosts.


  $self->push(*, command
              { stdout_fh => $find_fh, stderr_to_stdout => 1 },
              find, /my/dir);

scp_get => @remote, $local
scp_put => @local, $remote These methods queue a scp remote file copy operation in the selected hosts.
rsync_get => @remote, $local
rsync_put => @local, $remote These methods queue an rsync remote file copy operation in the selected hosts.
sub => sub { ... }, @extra_args
sub { ... }, @extra_args Queues a call to a perl subroutine that will be executed locally.

Note that subroutines are executed synchronously in the same process, so no other task will be scheduled while they are running.

The sub is called as

  $sub->($pssh, $label, @extra_args)

where $pssh is the current Net::OpenSSH::Parallel object.

parsub => sub { ... }, @extra_args Queues a call to a perl subroutine that will be executed locally on a forked process.

The sub is called as

  $sub->($label, $ssh, @extra_args)

Where $ssh is an Net::OpenSSH object that can be used to interact with the remote machine.

Note that the interface is different to that of the sub action.

An example of usage:

  sub sudo_install {
      my ($label, $ssh, @pkgs) = @_;
      my ($pty) = $ssh->open2pty(sudo, apt-get, install, @pkgs);
      my $expect = Expect->init($pty);
      $expect->expect($timeout, ":");
      $expect->expect($timeout, "\n");
      while(<$expect>) { print };
      close $expect;

  $pssh->push(*, parsub => \&sudo_install, scummvm);

If the subroutine dies or calls _exit with a non zero return code, the error handling code will be triggered (see Error handling).

The parsub action accepts the additional option no_ssh indicating that the $ssh object is not going to be used. For instance:

  $pssh->push(*, parsub => { no_ssh => 1 },
              sub {
                    my $label = shift;
                    { exec "gzip", "/tmp/file-$label" };
                    die "exec failed: $!";

That can make the script faster when the maximum number of simultaneous connections is limited. See Local resource usage.

join => $selector Joins allow to synchronize jobs between different servers.

For instance:

  $ssh->push(server_B, scp_get => /tmp/foo, foo);
  $ssh->push(server_A, join => server_B);
  $ssh->push(server_A, scp_put => foo, /tmp/foo);

The join makes server_A to wait for the scp_get operation queued in server_B to finish before proceeding with the scp_put.

In general the join will make the selected servers wait for any task queued on the servers matched by $selector to finish before proceeding with the next queued tasks.

One common usage is to synchronize all servers at some point:

  $ssh->push(*, join => *);

By default, errors are propagated at joins. For instance, in the example above, if the scp_get operation queued on server_B failed, it would abort any further operation queued on server_B and any further operation queued after the join in server_A. See also Error handling.

here => $tag Push a tag in the stack that can be used as a target for goto operations.
goto => $target Jumps forward until the given here tag is reached.

Joins to other hosts queues will be ignored, and joins from other queues to this one will be successfully fulfilled. For instance:

  $pssh->add_host(A => ...);
  $pssh->add_host(B => ...);
  $pssh->push(*, cmd  => echo "hello from %HOST");
  $pssh->push(A, goto => there);
  $pssh->push(A, join => B);                     # ignored by A on goto
  $pssh->push(B, join => A);                     # fulfilled by A on goto
  $pssh->push(*, cmd  => echo "hello from %HOST% again");
  $pssh->push(*, here => there);
  $pssh->push(*, cmd  => echo "bye bye from %HOST%");

Note that it is not possible to jump backwards.

There is an special target END that can be used to jump to the end of the queue.

stop Discards any additional operations queued. Any pending joins will be successfully fulfilled.

It is equivalent to

  $pssh->push(*, goto => END);

connect Just ensures that connecting to the remote machine is possible without doing any other action.

When given, %opts can contain the following options:
on_error => $fail_mode
on_error => sub { ... } See Error handling.
or_goto => $tag Supported for command, scp_get, scp_put, rsync_get and rsync_put, when the command, scp or rsync operation fails a goto to the given target is performed.

For instance:

  $pssh->all(command => { or_goto => no_file },
                        "test -f /etc/foo");
  $pssh->all(scp_get => "/etc/foo", "/tmp/foo-%LABEL%");
  $pssh->all(here    => "no_file");

Failures related to SSH errors do not trigger the goto but the error handling code.

timeout => $seconds not implemented yet!
on_done => sub { ... } not implemented yet!

Any other option will be passed to the corresponding Net::OpenSSH method (spawn, scp_put, etc.).

$pssh->all($action => @args)
$pssh->all($action => \%opts, @args) Shortcut for...

  $pssh->push(*, $action, \%opts, @args);

$pssh->run Runs the queued operations.

It returns a true value on success and false otherwise.

$pssh->get_error($label) Returns the last error associated to the host of the given label.
$pssh->get_errors In list context returns a list of pairs $label => $error for the failed queues.

In scalar context returns the number of failed queues.

FAQ - Frequently Asked Questions

Running remote commands with sudo <B>QB>: I need to run the remote commands with sudo that asks for a password. How can I do it?

<B>AB>: First read the answer given to a similar question on Net::OpenSSH FAQ.

The problem is that Net::OpenSSH::Parallel methods do not support the <stdin_data> option, so you will have to use an external file.

  $pssh->push(*, cmd => { stdin_file => $passwd_file },
                   sudo, -Skp, , --, @cmd);

One trick you can use if you only have one password is to use the DATA file handle:

  $pssh->push(*, cmd => { stdin_fh => \*DATA},
              sudo, -Skp, , --, @cmd);
  # and at the end of your script

Or you can also use the parsub action:

  my %sudo_passwords = (host1 => "foo", ...);

  sub sudo {
    my ($label, $ssh, @cmd) = @_;
    $ssh->system({stdin_data => "$sudo_passwords{$label}\n"},
                 sudo, -Skp, , --, @cmd);

  $pssh->push(*, parsub => \&sudo, @cmd);


o run N processes per host concurrently

allow running more than one process per remote server concurrently

o delay before reconnect

when connecting fails, do not try to reconnect immediately but after some predefined period

o rationalize debugging

currently it is a mess

o add logging support

log the operations performed in a given file

o stdio redirection

add support for better handling of the Net::OpenSSH stdio redirection facilities

o configurable valid return codes

Non zero exit code is not always an error.


This module should be considered beta quality, everything seems to work but it may yet contain critical bugs.

If you find any, report it via <> or by email (to, please.

Feedback and comments are also welcome!

The ’sub’ and ’parsub’ features should be considered experimental and its API or behavior could be changed in future versions of the module.

    Reporting bugs

In order to report a bug, write a minimal program that triggers it and place the following line at the beginning:

  $Net::OpenSSH::Parallel::debug = -1;

Then, send me (via RT or email) the debugging output you get when you run it. Include also the source code of the script, a description of what is going wrong and the details of your OS and the versions of Perl, Net::OpenSSH and Net::OpenSSH::Parallel you are using.

    Development version

The source code for this module is hosted at GitHub: <>.

    Commercial support

Commercial support, professional services and custom software development around this module are available through my current company. Drop me an email with a rough description of your requirements and we will get back to you ASAP.

    My wishlist

If you like this module and you are feeling generous, take a look at my Amazon Wish List: <>

Also consider contributing to the OpenSSH project this module builds upon: <>.


Net::OpenSSH is used to manage the SSH connections to the remote hosts.

SSH::Batch has a similar focus as this module. In my opinion it is simpler to use but rather more limited.

GRID::Machine allows to run perl code distributed in a cluster via SSH.

If your application requires orchestrating work-flows more complex than those supported by Net::OpenSSH::Parallel, you should probably consider some POE or AnyEvent based solution (check POE::Component::OpenSSH).

App::MrShell is another module allowing to run the same command in several host in parallel.

Some people find easier to use Net::OpenSSH combined with Parallel::ForkManager, threads or Coro.

Net::SSH::Mechanize is another framework written on top of AnyEvent that allows to run remote commands through SSH in parallel.


Copyright X 2009-2012, 2015 by Salvador Fandinõ (

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.10.0 or, at your option, any later version of Perl 5 you may have available.

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perl v5.20.3 NET::OPENSSH::PARALLEL (3) 2015-12-04

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