Christian Ferrari

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Use Case 7: distributed command/script synchronization

This use case shows a basic example of synchronization happening between commands running on different systems.

Before you can start:

Download and install flom on two different systems: they must be able to contact each other using IP network. I have two systems named “mojan” and “presanella”, this is a network check example:

Terminal 1 (mojan) output

tiian@mojan:/usr$ flom --version
FLoM: Free Lock Manager
Copyright (c) 2013-2020, Christian Ferrari; all rights reserved.
License: GPL (GNU Public License) version 2
Package name: flom; package version: 1.5.0-dev; release date: 2020-02-25
TLS supported protocol(s): TLSv1
Access https://github.com/tiian/flom for project community activities
Documentation is available at http://www.tiian.org/flom/

tiian@mojan:/usr$ ping -c 3 presanella
PING presanella (192.168.1.3) 56(84) bytes of data.
64 bytes from presanella (192.168.1.3): icmp_seq=1 ttl=64 time=7.30 ms
64 bytes from presanella (192.168.1.3): icmp_seq=2 ttl=64 time=7.23 ms
64 bytes from presanella (192.168.1.3): icmp_seq=3 ttl=64 time=7.92 ms

--- presanella ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 7.230/7.486/7.921/0.309 ms
tiian@mojan:/usr$

Terminal 2 (presanella) output

tiian@presanella:/usr$ flom --version
FLoM: Free Lock Manager
Copyright (c) 2013-2020, Christian Ferrari; all rights reserved.
License: GPL (GNU Public License) version 2
Package name: flom; package version: 1.5.0-dev; release date: 2020-02-25
TLS supported protocol(s): TLSv1
Access https://github.com/tiian/flom for project community activities
Documentation is available at http://www.tiian.org/flom/

tiian@presanella:/usr$ ping -c 3 mojan
PING mojan (192.168.1.4) 56(84) bytes of data.
64 bytes from mojan (192.168.1.4): icmp_req=1 ttl=64 time=73.4 ms
64 bytes from mojan (192.168.1.4): icmp_req=2 ttl=64 time=95.3 ms
64 bytes from mojan (192.168.1.4): icmp_req=3 ttl=64 time=116 ms

--- mojan ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 73.458/95.202/116.794/17.693 ms
tiian@presanella:/usr$

If anything looks good, you can go on with the experiment…

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

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

tiian@mojan:/usr$ pgrep flom
tiian@mojan:/usr$ flom -a 192.168.1.4 -d -1 -- true
tiian@mojan:/usr$ pgrep flom
2198
tiian@mojan:/usr$  There was no a running daemon before we started it, now there's exactly one *flom* running process with *process id 2198*.

Check the daemon is serving local requests:

tiian@mojan:/usr$ flom -a 192.168.1.4 -d 0 -- ls
bin  games  include  lib  lib64  local	sbin  share  src
tiian@mojan:/usr$

Switch to the second terminal (second system) and try the same command using the IP address or the network name of flom daemon:

tiian@presanella:/usr$ flom -a 192.168.1.4 -d 0 -- ls
bin  games  include  lib  local  sbin  share  src
tiian@presanella:/usr$ flom -a mojan -d 0 -- ls
bin  games  include  lib  local  sbin  share  src
tiian@presanella:/usr$

Experiment synchronization:

  1. inside the first terminal write this command at prompt, but do not press “enter”: “flom -a mojan -d 0 -- ls”
  2. inside the second terminal write this command at prompt: “flom -a mojan -d 0 -- sleep 10”
  3. now press “enter” key at the second terminal (where you have written “flom -a mojan -d 0 -- sleep 10”)
  4. switch to first terminal and press “enter” key

Expected result:

  1. the second terminal (system presanella in the above example) pauses for 10 seconds
  2. the first terminal (system mojan in the above example) pauses and displays the output of command “ls” after the second terminal sleeping terminates

Terminal 1 output:

tiian@mojan:/usr$ flom -a 192.168.1.4 -d 0 -- ls
bin  games  include  lib  lib64  local	sbin  share  src
tiian@mojan:/usr$

Terminal 2 output:

tiian@presanella:/usr$ flom -a mojan -d 0 -- sleep 10
tiian@presanella:/usr$

Explanation:

command “sleep 10” and command “ls” synchronized: “ls” executed after “sleep 10” completion. flom command protects (synchronizes) the execution of the command (or script) specified after the -- separator on the command line.

Special attention:

Pay attention to local names resolution: many times they point to loopback address. See the following examples

How names are resolved by mojan system:

tiian@mojan:/usr$ ping -c 1 mojan
PING mojan (127.0.1.1) 56(84) bytes of data.
64 bytes from mojan (127.0.1.1): icmp_seq=1 ttl=64 time=0.050 ms

--- mojan ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.050/0.050/0.050/0.000 ms
tiian@mojan:/usr$ ping -c 1 presanella
PING presanella (192.168.1.3) 56(84) bytes of data.
64 bytes from presanella (192.168.1.3): icmp_seq=1 ttl=64 time=3.31 ms

--- presanella ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 3.319/3.319/3.319/0.000 ms
tiian@mojan:/usr$

How the same names are resolved by presanella system:

tiian@presanella:/usr$ ping -c 1 mojan
PING mojan (192.168.1.4) 56(84) bytes of data.
64 bytes from mojan (192.168.1.4): icmp_req=1 ttl=64 time=90.6 ms

--- mojan ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 90.642/90.642/90.642/0.000 ms
tiian@presanella:/usr$ ping -c 1 presanella
PING presanella (127.0.1.1) 56(84) bytes of data.
64 bytes from presanella (127.0.1.1): icmp_req=1 ttl=64 time=0.102 ms

--- presanella ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 0.102/0.102/0.102/0.000 ms
tiian@presanella:/usr$

This is a common behavior and can lead to unexpected result:

  1. activating a daemon using its network name is equivalent to activate the daemon on address 127.0.0.1 (loopback): it can be reached only by processes inside the same system
  2. trying to reach a daemon using its network name from the same system hosting the daemon may not work if the daemon was not started using network name.

Extra considerations related to -d option:

-d (--daemon-lifespan) is the parameter used to specify how long a flom daemon must run.
-d -1 is used in the above example with the meaning “start a daemon and don’t stop it”
-d 0 is used in the above example with the meaning “don’t start a daemon”
First usage is typically server oriented, while the second usage is typically client oriented.
If you are not happy to specify “-d 0” for every client command, you can set Lifespan option in a configuration file.

To terminate a running flom daemon, simply use kill or pkill command:

tiian@mojan:/usr$ pgrep flom
2198
tiian@mojan:/usr$ pkill flom
tiian@mojan:/usr$ pgrep flom
tiian@mojan:/usr$

Summary

This simple usage form of command flom allows you to synchronize commands/scripts running on different systems.