evank / rcron

Python reimplementation of rcron

Rcron

Cron job redundancy and failover for a group of machines.

This is a python reimplementation of Benjamin Pineau's rcron tool, and is intended as a drop-in replacement.

Requirements

A POSIX system with a Python 2.4+ interpreter, and a separate tool to maintain state across machines -- such as keepalived.

Installation

The rcron script can live anywhere in the path. For our purposes, we'll expect it to live in /bin:

cp ./src/rcron.py /bin/rcron
chown root:root /bin/rcron
chmod 0755 /bin/rcron

It expects a configuration file, either at /etc/rcron/rcron.conf or specified with the --conf flag:

mkdir -p /etc/rcron
cp ./etc/rcron.conf.example /etc/rcron/rcron.conf
chown root:root /etc/rcron/rcron.conf
chmod 0644 /etc/rcron/rcron.conf

For Debian and Ubuntu systems, there is an included init.d script to generate a state file upon boot:

cp ./etc/debian.initd.sh /etc/init.d/rcron
chown root:root /etc/init.d/rcron
chmod 0755 /etc/init.d/rcron
update-rc.d rcron defaults

Configuration and Usage

The rcron config file supports the following directives:

• cluster_name
  • An arbitrary name for your group of machines
  • Should be identical across machines within the same group
• state_file
  • A file indicating the current machine's state
  • Defaults to /var/run/rcron.state
  • For backward compatibility, falls back to a default of /var/run/rcron/state
• default_state
  • Rcron's default state if the state file cannot be read, either "active" or "passive"
  • Defaults to active
• syslog_facility
  • The syslog facility from which messages should be generated
  • Defaults to LOG_CRON
• syslog_level
  • The severity level at which messages should be generated
  • Defaults to LOG_INFO
• nice_level
  • Job priority per the nice utility
  • Defaults to 19

To ensure a job is run only on the active machine in a group, prefix it with your rcron path:

# Run daily at 3am
0 3 * * * /bin/rcron /home/jdoe/bin/my-job

Maintaining State

An external tool (eg, keepalived) would maintain state across machines such that only one in the group would have an "active" state and actually run said job(s).

Two machines, one active and one passive

Both machines can have an identical rcron configuration with a default state of "passive".

The first machine should have keepalived configured with a higher priority, so that it can promote itself to a MASTER state:

# /etc/keepalived/keepalived.conf on machine 1
vrrp_instance VI_1 {
    state BACKUP
    interface eth0
    virtual_router_id 31
    priority 100 # higher than my siblings!
    advert_int 1
    authentication {
        auth_type PASS
        auth_pass 1111
    }
    notify_backup "/bin/echo passive > /var/run/rcron.state"
    notify_master "/bin/echo active > /var/run/rcron.state"
    notify_fault  "/bin/echo passive > /var/run/rcron.state"
    notify_stop   "/bin/echo passive > /var/run/rcron.state"
}

The second machine should have a lower priority than the first, so that it defaults to a BACKUP state:

# /etc/keepalived/keepalived.conf on machine 2
vrrp_instance VI_1 {
    state BACKUP
    interface eth0
    virtual_router_id 31
    priority 99 # lower than my first sibling!
    advert_int 1
    authentication {
        auth_type PASS
        auth_pass 1111
    }
    notify_backup "/bin/echo passive > /var/run/rcron.state"
    notify_master "/bin/echo active > /var/run/rcron.state"
    notify_fault  "/bin/echo passive > /var/run/rcron.state"
    notify_stop   "/bin/echo passive > /var/run/rcron.state"
}

Three or more machines

To ensure that only one machine is active, every machine should be configured with a different priority.

For example, if we want to add a third machine to the example above, we would configure a lower priority than the second so that it only promotes itself to a MASTER state when all higher priority machines are unavailable:

# /etc/keepalived/keepalived.conf on machine 3
vrrp_instance VI_1 {
    state BACKUP
    interface eth0
    virtual_router_id 31
    priority 98 # lower than my second sibling!
    advert_int 1
    authentication {
        auth_type PASS
        auth_pass 1111
    }
    notify_backup "/bin/echo passive > /var/run/rcron.state"
    notify_master "/bin/echo active > /var/run/rcron.state"
    notify_fault  "/bin/echo passive > /var/run/rcron.state"
    notify_stop   "/bin/echo passive > /var/run/rcron.state"
}

Machines across disparate networks

By default, keepalived uses multicast for communication between machines. If you have multiple machines in the same group that live across two or more separate internal networks, they may not be able to communicate with each other. In such a case, you would need to use unicast (added in keepalived v1.2.8), where each machine in the group is configured with the external ip of every other machine:

# /etc/keepalived/keepalived.conf on machine 1 via unicast
vrrp_instance VI_1 {
    state BACKUP
    interface eth1
    virtual_router_id 31
    priority 100
    advert_int 1
    unicast_src_ip 30.100.200.101 # this is me!
    unicast_peer {
        30.100.200.102 # this is my first sibling
        30.100.200.103 # this is my second sibling
        # et al...
    }
    authentication {
        auth_type PASS
        auth_pass 1111
    }
    notify_backup "/bin/echo passive > /var/run/rcron.state"
    notify_master "/bin/echo active > /var/run/rcron.state"
    notify_fault  "/bin/echo passive > /var/run/rcron.state"
    notify_stop   "/bin/echo passive > /var/run/rcron.state"
}

Test Drive

You can use the included Vagrantfile & Ansible configuration to create a number of Ubuntu-based virtual machines, all preconfigured with rcron and keepalived v1.2.13 over unicast:

vagrant up
vagrant ssh rcl1 -c "sudo tail -f /var/log/syslog | grep -i --color=always keepalived"

License

As with the original C implementation, this is released under the MIT license.