Cluster::Init - Clusterwide "init", spawn cluster applications


  use Cluster::Init;

  unless (fork())
    exit 0;

  my $client = Cluster::Init->client;

  # spawn all apps for resource group "foo", runlevel "run"

  # spawn all apps for resource group "foo", runlevel "runmore"
  # (this stops everything started by runlevel "run")

  # spawn all apps for resource group "bar", runlevel "3"
  # (this does *not* stop or otherwise affect anything in "foo")


This module provides basic init functionality, giving you a single inittab-like file to manage initialization and daemon startup across a cluster or collection of machines.

This module is used by OpenMosix::HA, for instance, to provide high availability with failure detection, automatic migration, and restart of applications running in a cluster. OpenMosix::HA provides you with the ability to build 24x7 mission-critical, high-performance server farms using only commodity hardware. See OpenMosix::HA.

I wrote the original version of this module to provide a more flexible interface between IBM's AIX HACMP cluster manager and managed applications. This provided a cleaner configuration, much faster configuration changes, and respawn ability for individual daemons.

Other uses are possible, including non-cluster environments -- use your imagination. Generically, what you get in this package is an application-level "init" written in Perl, with added ability to configure resource groups, status file output, and a 'test' runmode (see below).

Commercial support for this module is available: see "SUPPORT".


See http://www.Infrastructures.Org for cluster management techniques, including clean ways to install, replicate, and update nodes.

See "CONCEPTS" for an explanation of terms.

Much of the following work is done for you if you're running OpenMosix::HA on an openMosix cluster -- see OpenMosix::HA.

To use Cluster::Init (without OpenMosix::HA) to manage your cluster-hosted processes, on either a high-throughput computing cluster or a high-availability cluster:

  • Install Cluster::Init on each node.

  • Create "etc/cltab"" in ".

  • Replicate "etc/cltab"" in " to all nodes.

  • Run 'clinit -d' on each node. Putting this in /etc/inittab as a "respawn" process would be a good idea, or you could have it started as a managed process under HACMP, VCS, Linux-HA etc.

  • Run 'clinit my_group my_level' on each node where you want resource group my_group to be running at runlevel my_level.

  • Check current status in "var/run/clinit/clstat"" in " on each node. (Or use OpenMosix::HA, which collates this for you across all nodes.)


Use Perl's normal sequence:

  perl Makefile.PL
  make test
  make install

You'll need to install this module on each node in the cluster.

This module includes a script, "clinit", which will be installed when you run 'make install'. See the output of perl -V:installscript to find out which directory the script is installed in.



A group of machines administered as a single unit and offering a common set of services. See enterprise cluster, high-availability cluster, and high-throughput cluster.

Computing Cluster

See High-Throughput Cluster.

Enterprise Cluster

A well-administered enterprise infrastructure (see http://www.Infrastructures.Org), in which each machine, whether desktop or server, provides scalable commodity services. Any machine or group of machines can be easily and quickly replaced, with minimal user impact, without restoring from backups, with no advance notice or unique preparation. May include elements of both high availability and high throughput clusters.

High-Availability Cluster

(Also HA Cluster.) A cluster of machines optimized for providing high uptime and minimal user impact in case of hardware failure, in return for increased per-node expense and complexity. Normally includes shared disk, unattended failover of filesystem mounts and IP and MAC addresses, and automatic daemon restart on the surviving node(s). Suitable for applications such as NFS and database servers, and other services which normally cannot be replicated easily.

Examples of HA cluster platforms include OpenMosix::HA, Linux-HA, AIX HACMP, and Veritas VCS.

Due to the expense of providing the per-node redundancy required for high availability, HA clusters are normally not scalable to the hundreds of nodes typically needed for high-throughput applications. OpenMosix::HA is the exception to this rule; it provides an HA layer on top of a high-throughput openMosix cluster.

High-Throughput Cluster

A cluster of machines optimized for cheaply delivering large quantities of work in a short time, in return for reduced per-process reliability. May include features such as process checkpointing and migration, high-speed interconnects, or distributed shared memory. Some high-throughput clusters are optimized for scavenging unused cycles on desktop machines. Most high-throughput clusters are suitable for supercomputing-class applications which can be parallellized across dozens, hundreds, or even thousands of nodes.

Examples of high-throughput cluster platforms include OpenMosix::HA, openMosix itself, Linux Beowulf, and Condor.

Due to the internode dependencies inherent in distributed shared memory or migration of interactive processes, high-throughput clusters normally do not meet the needs of high availability -- they are intended for brute-force problem solving where the death of a single process out of thousands is not significant. High-throughput clusters are not typically designed to provide mission-critical interactive services to the public.

The one (known) exception is OpenMosix::HA -- it provides high availability for both interactive and batch processes running on a high-throughput openMosix cluster.

Resource Group

A collection of applications and physical resources (like filesystem mounts) which need to execute together on the same cluster node. Resource groups allow easy migration of applications between nodes.

Cluster::Init supports resource groups explicitly. Resource groups are configured in "etc/cltab"" in ".

For example, sendmail, /etc/, and the /var/spool/mqueue directory might make up a resource group -- they all need to be present on the same node. From "etc/cltab"" in ", you could spawn the scripts which update, mount mqueue, and then start sendmail itself.

Another example; Apache, a virtual IP address, and the filesystem containing the HTML document tree might together constitute a resource group. To start this resource group, you might need to mount the filesystem, ifconfig the virtual IP, and start httpd. This sequence can easily be specified in /etc/cltab.



Cluster::Init includes clinit, a script which is intended to be a bolt-in cluster init tool. The script is called like init or telinit, with the addition of a new "resource group" argument. See the output of clinit -h.

The first time you execute clinit you will need to use the -d flag only, to start the Cluster::Init daemon. This flag does not automatically background the daemon though -- this is so it will work as a "respawn" entry in /etc/inittab. If you're testing from the command line or running from a shell script, use 'clinit -d &'.

Once you have the daemon running, use clinit without the -d flag. This will cause it to run as a client only, talking to the daemon via a UNIX domain socket. At this point you will use clinit in roughly the same way you would use the UNIX telinit, in this case commanding resource groups to switch to different runlevels. That's it!

Use the -k flag to tell the daemon and all child processes to shut down gracefully.



  # start a Cluster::Init server daemon
  Cluster::Init->daemon (
      'cltab' => '/etc/cltab',
      'socket' => '/var/run/clinit/clinit.s'
      'clstat' => '/var/run/clinit/clstat'

The server-side constructor. You'll likely want to fork before calling this method -- it does not return until you issue a "shutdown" from a "client()" process. See the "clinit" source code for an example.

Accepts an optional hash containing the paths to the configuration file, socket, and status output file. You can also specify 'socket' and 'clstat' locations in ""/etc/cltab".

The daemon opens and listens on a UNIX domain socket, "/var/run/clinit/clinit.s" by default. The "client()" will communicate with the daemon via this socket.


  # create a Cluster::Init client object
  my $client = Cluster::Init->client (
      'cltab' => '/etc/cltab',
      'socket' => '/var/run/clinit/clinit.s'
      'clstat' => '/var/run/clinit/clstat'

The client-side constructor.

Accepts an optional hash containing the paths to the configuration file, socket, and status output file. You can also specify 'socket' and 'clstat' locations in ""/etc/cltab".

Returns a Cluster::Init object. You'll normally call the resulting object's "tell()" method one or more times after this. See the "clinit" source code for example usage.

The client looks for the "daemon()" on a UNIX domain socket, "/var/run/clinit/clinit.s" by default.


  # tell resource group "mygroup" to change to runlevel "newlevel"
  $client->tell("mygroup", "newlevel");

  # cause Cluster::Init daemon to re-read cltab
  $client->tell(":::ALL:::", ":::REREAD:::");

Tells a running "daemon()" to change a resource group to a new runlevel. Called as a method on an object returned by "client()". See the "clinit" source code for example usage.

At this time, this method returns a string containing a success or failure message. I don't use this string in OpenMosix::HA, so it isn't very refined -- it doesn't give you much you can use to detect failure programmatically, for example. For a better solution, see "status()".

The tell(":::ALL:::", ":::REREAD:::") usage is only a convention; in fact, any call to tell() with true values for group and level will cause a re-read, regardless of whether the values provided match any actual group or runlevel.


  # return status of all running groups
  my $text=$client->status();

  # filter by group and level
  my $text=$client->status(group=>'foo',level=>'bar');

  # provide nonstandard path to clstat
  my $text=$client->status(group=>'foo',level=>'bar',clstat=>'/tmp/clstat');

This method will read "var/run/clinit/clstat"" in " for you, dumping it to stdout. All arguments are optional. If you provide 'group' or 'level', then output will be filtered accordingly. If you specify 'clstat', then the status file at the given pathname will be read (this is handy if you need to query multiple Cluster::Init status files in a shared cluster filesystem, and is what OpenMosix::HA does).

In addition to the usual $obj->status() syntax, the status() method can also be called as a class function, as in Cluster::Init::status(clstat=>'/tmp/clstat'). The 'clstat' argument is required in this case. Again, this is handy if you want to query a running Cluster::Init on another machine via a shared filesystem, without creating an Cluster::Init object or daemon here.


  # tell daemon to gracefully stop all child processes and exit

Causes daemon to stop all child processes and exit. Processes will be sent SIGINT, SIGTERM, then SIGKILL at intervals of several seconds; the daemon will not exit until the last process has stopped -- this method will always return sooner.



The main Cluster::Init configuration file. Identical in format to /etc/inittab, with a new "resource group" column added. See t/cltab in the Cluster::Init distribution for an example.

The path and name of this file can be changed: see "daemon()" and "client()".

This file must be replicated across all hosts in the cluster by some means of your own. On openMosix clusters, OpenMosix::HA will replicate this file for you. See http://www.Infrastructures.Org for ways to do this in other environments.

You can specify tests to be performed during startup of a resource group: In addition to the init-style runmodes of 'once', 'wait', 'respawn', and 'off', Cluster::Init supports a 'test' runmode. If the return code of a 'test' command is anything other than zero, then the resource group as a whole is marked as 'FAILED' in "/var/run/clinit/clstat". For example, the 'test' runmode is used by OpenMosix::HA to test a node for eligibility before attempting to start a resource group there.

You can specify different locations for "/var/run/clinit/clinit.s" and "/var/run/clinit/clstat" in ""/etc/cltab", like this:

  # location of socket
  # location of status file

Settings found in ""/etc/cltab" override those found in the "daemon()" or "client()" constructor arguments.


Plain-text file showing the status of all running resource groups. Any time Cluster::Init changes the runlevel of a resource group, it will update this file. This file can be read directly or via the "status()" method.

The path and name of this file can be changed: see "daemon()", "client()", and "/etc/cltab".


A UNIX domain socket used by "client()" to communicate with "daemon()".

The path and name of this file can be changed: see "daemon()", "client()", and "/etc/cltab".


See TODO file for a more comprehensive and current list. The most significant outstanding bugs right now are:

  • Perl 5.8 incompatibility -- blows chunks with a scalar dereference error. This module won't work at all on 5.8 until I get a chance to fix this.

  • Runlevel of '0' (zero) is broken right now; groups named '0' will probably never be supported either. If you pass a '0' as an argument to tell() (either group or level), then whatever you intended to happen is not going to happen.

    If you're just trying to force a re-read of cltab, then use some nonexistent group and level; I use tell('::ALL::','::REREAD::') or somesuch, as mentioned in "tell()".

    If you're just trying to shut a single group off, use something like tell("realgroupname",999). This will stop all of that group's processes gracefully, assuming that there is no real runlevel '999' configured for that group.

  • Deleting a group from cltab without stopping it first will cause the group's processes to be sent SIGKILL -- they will not be stopped gracefully with SIGINT or SIGTERM. Better to send tell("group",999) to stop it gracefully first, as mentioned above.

  • Duplicate tags in cltab are detected but not enough useful exceptions are generated.

  • Intermittent failure line 35 t/0232stop.t -- indicator error as far as I can tell; just re-run make test for now.


Commercial support for this module is available at http://clusters.TerraLuna.Org. On that web site, you'll also find pointers to the latest version, a community mailing list, other cluster management software, etc. You can also find help for general infrastructure (and cluster) administration at http://www.Infrastructures.Org.


        Steve Traugott


Copyright (c) 2003 Steve Traugott. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

The full text of the license can be found in the LICENSE file included with this module.


OpenMosix::HA, http://clusters.TerraLuna.Org, http://www.Infrastructures.Org, init(8), telinit(8), perl(1).