TKLBAM hooks

This document page is based on this blog post.

TKLBAM has a nifty, general purpose hooks mechanism you can use to trigger useful actions on backup and restore.

Examples of hooks:

• Cleaning up temporary files
• Stopping/starting services to increase data consistency
• Encoding/decoding data from non-supported databases
• Using LVM to create/restore a snapshot of a fast changing volume

The hooks mechanism was originally developed so we could fix a few issues indirectly related to the usability of TKLBAM. In particular, our very first beta users reported that sometimes tklbam-restore would fail to find any backup volumes. When we investigated this turned out to be a clock discrepancy. The obvious solution was to sync the clock before starting the restore, but the more I thought about it the more the idea of hardwiring that ntpdate stuff rubbed me the wrong way. For a few reasons:

• It's an auxiliary problem, not a core issue with TKLBAM's logic
• I'm offline much of the time during development so I needed some way to turn this off, but I don't want to add more testing-specific code unless it's absolutely necessary.
• It's OK if a specific server (e.g., pool.ntp.org) is the default, but there should be some way to configure it if a user, for example, wants to use an internal NTP server.

I tried to think of a clean way to achieve these simple goals in a clean way (e.g., cli options, environment variables, configuration files), but everything I came up with was just so darn ugly.

Then I realized that a hooks mechanism would solve this problem in a simple, generic way.

import os
import sys
import executil
from string import Template

NTPSERVER = os.environ.get("NTPSERVER", "pool.ntp.org")

ERROR_TPL = """\
##########################
## FIXCLOCK HOOK FAILED ##
##########################

Amazon S3 and Duplicity need a UTC synchronized clock so we invoked the
following command::

    $COMMAND

Unfortunately, something went wrong...

$ERROR
"""

def fixclock():
    command = "ntpdate -u " + NTPSERVER

    try:
        executil.getoutput(command)
    except executil.ExecError, e:
        msg = Template(ERROR_TPL).substitute(COMMAND=command,
                                             ERROR=e.output)

        print >> sys.stderr, msg,
        sys.exit(1)

def main():
    op, state = sys.argv[1:]

    if op in ('restore', 'backup') and state == 'pre':
        fixclock()

if __name__ == "__main__":
    main()