EXTENDING

[Overview]
Developers can extend Snap by defining new snapshot targets, or abstract
entities which snapshots can be taken of / restored, and backends implementing
those targets. 

Examples of targets and specific backends

  -repos: yum and apt-get

  -packages: rpm, deb, installed windows software

  -files: the files modified outside the package system
          (in the case of windows, the files not owned
           by the operating system)

  -services: postgresql, mysql, iptables, apache httpd, iis

[Targets]
To create a new snapshot target, register it with the list in
snap/snapshottarget.py and add a corresponding directory to snap/backends/
(containing an empty __init__.py)


[Backends]
In the target directory a backend will need to be written for every os-dependent
implementation of the operations needed to backup/restore that snapshot target

Furthermore configure which backend will be used by default on a per os basis
by modifying DEFAULT_BACKENDS in snap/options.py

Each backend will need to implement the following methods:

  -backup(self, basedir, include=[], exclude=[])
   performs the actual backup operation, taking in the following params:
     -self: an instance of the backend class
     -basedir: the directory to write any files containing the backup records
                (eg the actual data and any metadata files)
     -include: list of specific entities to backup, an empty list should be
               assumed to mean to run the default / all-encompassing backup
     -exclude: list of specific entities to exclude when backing up

  -restore(self, basedir)
   performs the actual restoration operation
     -self: an instance of the backend class
     -basedir: the directory to read files containing the backup records

From there you are done, snap will automatically pickup the target and
any backends, and present those to the user via the command line tool.

Any configuration options set in /etc/snap.conf will be automatically picked up
and made available to your backend via the global 'snap.config.options' dictionary