Alex White (wu) > Proc-Launcher > Proc::Launcher

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Module Version: 0.0.35   Source  

NAME ^

Proc::Launcher - yet another forking process controller

VERSION ^

version 0.0.35

SYNOPSIS ^

    use Proc::Launcher;

    # define a method to start your application if it isn't already running
    use MyApp;
    my $start_myapp = sub { MyApp->new( context => $some_shared_data )->run() };

    # create a new launcher object
    my $launcher = Proc::Launcher->new( start_method => $start_myapp,
                                        daemon_name  => 'myapp',
                                      );

    # an alternate version of the same thing without the subroutine reference
    my $launcher = Proc::Launcher->new( class        => 'MyApp',
                                        start_method => 'run'
                                        context      => $some_shared_data,
                                        daemon_name  => 'myapp',
                                      );

    # check if the process was already running
    if ( $launcher->is_running() ) { warn "Already running!\n" }

    # start the process if there isn't already one running
    $launcher->start();

    # shut down the process if it is already running.  start a new process.
    $launcher->restart();

    # get the process pid
    my $pid = $launcher->pid();

    # kill -HUP
    $launcher->stop();

    # kill -9
    $launcher->force_stop();

    # get the process log file path
    my $log = $launcher->log_file;

DESCRIPTION ^

This library is designed to fork one or more long-running background processes and to manage them. This includes starting, stopping, and automatically restarting processes--even those that don't behave well.

The pid of the forked child processes are written to pid files and persist across multiple restarts of the launcher. This means that stdout/stderr/stdin of the children are not directly connected to the launching process. All stdout and stderr from the child processes is written to a log file.

For more useful functions (e.g. a supervisor to restart processes that die), see Proc::Launcher::Manager.

RELATED WORK ^

There are a large number of modules already on CPAN for forking and managing child processes, and also for managing daemon processes (apachectl style). After doing a lot of reading and experimentation, I unfortunately ended up deciding to write yet another one. Here is a bit of information on related modules and how this one differs.

While it is possible to exec() and manage external executables in the child processes, that is merely an afterthought in this module. If you are looking for a module to start and manage external executables, you might also want to check out Server::Control, App::Control, App::Daemon, or Supervisor on CPAN, or ControlFreak on github.

If you are looking to investigate and/or kill running processes IDs, see Unix::PID, Unix::PID::Tiny, or Proc::ProcessTable. This module only manages processes that have been forked by this module.

If you only want to read and write a PID file, see Proc::PID::File.

On the other hand, if you're looking for a library to fork dependent child processes that maintain stdout/stderr/stdin connected to the child, check out IPC::Run, IPC::ChildSafe, Proc::Simple, Proc::Reliable, Proc::Fork, etc. This module assumes that all child processes will close stdin/stdout/stderr and will continue to live even after the launching process has exited. Furthermore the launched process will be manageable by launchers that are started after the launched process is already running.

This library does not do anything like forking/pre-forking multiple processes for a single daemon (e.g. for a high-volume server, see Net::Server::PreFork) or limiting the maximum number of running child processes (see Proc::Queue or Parallel::Queue). Instead it is assumed that you are dealing with is a fixed set of named daemons, each of which is associated with a single process to be managed. Of course any managed processes could fork it's own children. Note that only the process id of the immediate child will be managed--any child processes created by child process (grandchildren of the launcher) are not tracked.

Similarly your child process should never do a fork() and exit() or otherwise daemonize on it's own. When the child does this, the launcher will lose track of the grandchild pid and assume it has shut down. This may result in restarting your service while it is already running.

This library does not handle command line options--that is left to your application/script. It does not export any methods nor require you to inherit from any classes or to build any subclasses. This means that you can launch a process from any normal perl subroutine or object method--the launched class/method/subroutine does not have to be modified to be daemonized.

This library does not use or require an event loop (e.g. AnyEvent, POE, etc.), but is fully usable from with an event loop since objects of this class avoid calling sleep() (doing so inside a single-threaded event loop causes everything else running in the event loop to wait). This does mean that methods such as stop() will return immediately without providing a status. See more about this in the note below in rm_zombies().

For compatibility with the planned upcoming GRID::Launcher module (which uses GRID::Machine), this module and it's dependencies are written in pure perl.

The intended use for this library is that a supervising process will acquire some (immutable) global configuration data which is then passed (at fork time) to one or more long-running component daemon processes. In the Panoptes project, this library is used for starting and managing the various Panoptes components on each node (Panoptes::Monitor, Panoptes::Rules, Panoptes::Util::WebUI, etc.) and also for managing connections to the remote agents.

If you are aware of any other noteworthy modules in this vein, please let me know!

CONSTRUCTOR OPTIONS ^

The constructor supports the following options, e.g.:

  my $launcher = Proc::Launcher->new( debug => 1, ... );

Each of these attributes will also have a getter, e.g.:

  if ( $launcher->debug ) { print "DEBUGGING ENABLED!\n" }

All attributes are read-only unless otherwise specified. Read/write attributes may be set like so:

  $launcher->debug( 1 );
debug => 0

Enable debugging messages to STDOUT. This attribute is read/write.

daemon_name => 'somename'

Specify the name of the daemon. This will be used as the prefix for the log file, pid file, etc.

context => $data

Context data to be passed to the forked processes. This could be any complex data structure and may contain things like configuration data.

class => 'My::App'

Name of the class where the start method is located. An object of this class will be created, passing in your context data. Then the start_method will be called on the object.

start_method => 'start_me'

If a class is specified, this is the name of the start method in that class.

If no class is specified, this must be a subroutine reference.

pid_dir => "$ENV{HOME}/logs"

Specify the directory where the pid file should live.

pid_file => "$pid_dir/$daemon_name.pid"

Name of the pid file.

disable_file => "$pid_dir/$daemon_name.disabled"

Location to the 'disable' file. If this file exists, the daemon will not be started when start() is called.

log_file => "$pid_dir/$daemon_name.log"

Path to the daemon log file. The daemon process will have both stdout and stderr redirected to this log file.

pipe => 0

If set to true, specifies that the forked process should create a named pipe. The forked process can then read from the named pipe on STDIN. If your forked process does not read from and process STDIN, then there's no use in enabling this option.

pipe_file => "$pid_dir/$daemon_name.cmd"

Path to the named pipe.

METHODS ^

start( $data )

Fork a child process. The process id of the forked child will be written to a pid file.

The child process will close STDIN and redirect all stdout and stderr to a log file, and then will execute the child start method and will be passed any optional $data that was passed to the start method.

Note that there is no ongoing communication channel set up with the child process. Changes to $data in the supervising process will never be available to the child process(es). In order to force the child to pick up the new data, you must stop the child process and then start a new process with a copy of the new data.

If the process is found already running, then the method will immediately return null. If a process was successfully forked, success will be returned. Success does not imply that the daemon was started successfully--for that, check is_running().

stop()

If the process id is active, send it a kill -HUP.

restart( $data, $sleep )

Calls the stop() method, followed by the start() method, optionally passing some $data to the start() method.

This method is not recommended since it doesn't check the status of stop(). Instead, call stop(), wait a bit, and then check that the process has shut down before trying to start it again.

WARNING: this method calls sleep to allow a process to shut down before trying to start it again. If sleep is set to 0, the child process won't have time to exit, and thus the start() method will never run. As a result, this method is not recommended for use in a single-threaded cooperative multitasking environments such as POE.

is_running()

Check if a process is running by sending it a 'kill -0' and checking the return status.

Before checking the process, rm_zombies() will be called to allow any child processes that have exited to be reaped. See a note at the rm_zombies() method about the leaky abstraction here.

If the pid is not active, the stopped() method will also be called to ensure the pid file has been removed.

rm_zombies()

Calls waitpid to clean up any child processes that have exited.

waitpid is called with the WNOHANG option so that it will always return instantly to prevent hanging.

Normally this is called when the is_running() method is called (to allow child processes to exit before polling if they are still active). If you are using a Proc::Launcher in a long-lived process, after stopping a daemon you should always call is_running() until you get a false response (i.e. the process has successfully stopped). If you do not call is_running() until the process exits, you will create zombies.

force_stop()

If the process id is active, then send a 'kill -9'.

stopped()

This method is called when a process has been detected as successfully shut down. The pidfile will be removed if it still exists.

pid()

Read and return the pid from the pidfile.

The pid is validated to ensure it is a number. If an invalid pid is found, 0 is returned.

write_my_pid()

Write the pid to the pid file.

This operation involves checking a couple of times to make sure that no other process is running or trying to start another process at the same time. The pid is actually written to a temporary file and then renamed. Since rename() is an atomic operation on most filesystems, this serves as a basic but effective locking mechanism that doesn't require OS-level locking. This should prevent two processes from both starting a daemon at the same time.

remove_pidfile

Remove the pidfile. This should only be done after the process has been verified as shut down.

Failure to remove the pidfile is not a fatal error.

read_log

Return any new log data since the last offset was written. If there was no offset, set the offset to the current end of file.

You may want to call this before performing any operation on the daemon in order to set the position to the end of the file. Then perform your operation, wait a moment, and then call read_log() to get any output generated from your command while you waited.

write_pipe( $string )

Write text to the process's named pipe. The child can then read this data from it's STDIN.

Simply returns false if a named pipe was not enabled.

disable()

Create the disable flag file unless it already exists.

enable()

If the disable flag file exists, remove it.

is_enabled()

Check if the launcher is enabled.

If the disable flag file exists, then the launcher will be considered disabled.

SUPPORT ^

You can find documentation for this module with the perldoc command.

    perldoc Proc::Launcher

You can also look for information at:

LICENCE AND COPYRIGHT ^

Copyright (c) 2009, VVu@geekfarm.org All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

- Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

- Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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