IPv6 Use Case 9: distributed command/script synchronization, dynamic network mode (cloud friendly)

This use case shows extends Use Case 9 and shows as dynamic network mode feature works.

Before you can start:

Download and install flom on two different systems: they must be able to contact each other using IP network; it is suggested to play with Use Case 7 and Use Case 8 before trying this one.

UDP/IP multicast and firewalls

Please pay attention some Linux distros configure a default firewall that silently drop multicast datagrams: disable your firewall or configure it to not drop multicast datagrams.
openSUSE 12.2 is such a distribution.

If you did not already try IPv6 Use Case 8, you should take a look to it, especially to the paragraph that explains how to verify there are not system and network issues related to UDP/IPv6 datagrams.

Activate a flom network daemon and check it’s up & running:

System centos71-64 will be our flom network daemon (server) system, so we have to activate it with the following commands:

[tiian@centos71-64 usr]$ pgrep flom
[tiian@centos71-64 usr]$ flom -A ff02::1 -d -1 -- true
[tiian@centos71-64 usr]$ pgrep flom
2025
[tiian@centos71-64 usr]$ ps -ef | grep flom | grep -v grep
tiian     2025     1  0 21:50 ?        00:00:00 flom -A ff02::1 -d -1 -- true
[tiian@centos71-64 usr]$

There was no a running daemon before we started it, now there’s exactly one flom running process with process id 2025.

Check the daemon is serving local requests using auto-discovery feature (UDP/IP multicast):

[tiian@centos71-64 usr]$ flom -A ff02::1 -d 0 -- ls
bin  etc  games  include  lib  lib64  libexec  local  sbin  share  src	tmp
[tiian@centos71-64 usr]$

Switch to the second terminal (second system) and try the same command using the auto-discovery feature:

tiian@ubuntu1404-64:/usr$ flom -A ff02::1 -d 0 -- ls
bin  games  include  lib  local  sbin  share  src
tiian@ubuntu1404-64:/usr$

Summary

This example of command flom allows you to synchronize commands/scripts running on different systems and you don’t have to start flom daemon on a specific system: it may be started on any system that can answer to UDP/IPv6 multicast queries.
With dynamic network mode all the information needed to start and reach the flom daemon is the UDP/IPv6 multicast address and port (multicast group): this can save you a lot of configuration pains, but exposes you to serious issues in case of network partitions (see distributed features for details).
The dynamic mode is cloud friendly because a pool of systems can automatically elige a daemon without a special node.