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Unix Guest Agent for Openstack

This guest agent provides functionality such as configuring the networking for a guest.

Layout:

src/       -- The main agent daemon code, written in C, which embeds the
              python interpreter
include/   -- Include files for src/
lib/       -- Supporting code for the main agent daemon, along with a
              python module wrapper
plugins/   -- Python plugin modules (for communication and command parsing)
commands/  -- Python modules that implement the real code for commands
tests/     -- Unit tests
scripts/   -- Startup and misc scripts

Plug-in Info

There are currently 2 types of plugins, exchanges and parsers. Exchange plugins are those that handle the low level communication between a client and the daemon. Parser plugins are those that can decode the communication protocol.

Currently, there's 1 exchange plugin, xscomm.py, and 1 parser plugin, jsonparser.py. xscomm.py handles communication via XenStore and the parser plugin will decode/encode json for the requests/responses.

Exchange Plug-in

Needs to define a class that contains the following methods:

  • get_request():
  • Has no arguments
  • Returns some sort of request object
  • put_response(request, response):
  • Has request and response arguments
  • Returns None (return value is ignored)

Parser Plug-in

Needs to define a class that contains the following methods:

  • parse_request(request):
  • Takes a request object returned from an Exchange plugin's get_request()
  • Returns a response object that will be passed to an Exchange plugin's put_response()

~

Command Modules

commands/init.py implements a class called 'CommandBase' that is used to create commands by subclassing it. This automatic registering of commands via the subclassing occurs via a metaclass.

To create a new command:

  • create a class that derrives from commands.CommandBase
  • define a method in your class that uses this decorator:
  • @commands.command_add('<command_name>')
  • (obviously replace the decorator argument with the right command name)

Misc.

jsonparser.py requires a class instance to be passed on init which defines a 'run_command' method.

When importing 'commands', it replaces the 'commands' module with a wrapper, so you can use the 'commands' attribute directly instead of having to use commands.CommandBase

Call commands.init() to init all of the command classes Pass the result to JsonParser

Example Configuration File

  • Needed to register the exchange/parser plugin combiniation with the
  • main daemon
  • import agentlib
  • To get jsonparser and xscomm
  • import plugins
  • Loads 'commands' plus all modules that contain command classes
  • import commands.command_list
  • Not required, as the default is False
  • test_mode = False
  • Inits all command classes
  • c = commands.init()
  • Creates instance of JsonParser, passing in available commands
  • parser = plugins.JsonParser(c)
  • Create the XSComm intance
  • xs = plugins.XSComm()
  • Register an exchange/parser combination with the main daemon
  • agentlib.register(xs, parser)