Using the VCL XML-RPC interface with Python

Apache VCL is a cloud computing platform that NC State uses to provide on-demand access to specific software or environments. It’s neat because you can quickly and easily get a virtual machine to try things out with, or a cluster to experiment with networking. The remote API for VCL is provided via an XML-RPC interface. Python has a nice XML-RPC library called xmlrpclib that makes building clients comically easy.

For instance, here’s a short example, with in-line comments explaining the four lines that need any explanation:

import xmlrpclib
import getpass

# We're connecting over SSL so subclass SafeTransport
class AuthenticatedTransport(xmlrpclib.SafeTransport): 

    def __init__(self, username, password):
        xmlrpclib.SafeTransport.__init__(self)
        self.password = password
        self.username = username

    def send_content(self, connection, request_body):
        # X- headers are user-defined in HTTP
        # These next few are used by VCL
        connection.putheader("X-User", self.username)
        connection.putheader("X-Pass", self.password)
        connection.putheader("X-APIVERSION: 2")
        # That's the end of the VCL-specific headers

        xmlrpclib.SafeTransport.send_content(self, connection, request_body)

username = raw_input("Username: ")
password = getpass.getpass("Password: ")

auth_transport = AuthenticatedTransport(username, password)
server = xmlrpclib.Server("https://vcl.ncsu.edu/scheduling/index.php?mode=xmlrpccall", transport=auth_transport)

try:
    result = server.XMLRPCtest("foo")
except xmlrpclib.ProtocolError as e:
    print "URL: {}".format(e.url)
    print "Headers: {}".format(e.headers)
    print "Error code: {}".format(e.errcode)
    print "Error message: {}".format(e.errmsg)
except xmlrpclib.Fault as f:
    print "Fault Code: {}".format(f.faultCode)
    print "Fault: {}".format(f.faultString)
    print "Args: {}".format(f.args)
    print "Message: {}".format(f.message)
else:
    print result

If all goes well, then you should receive a response of the form:

{'status': 'success', 'message': 'RPC call worked successfully', 'string': 'foo'}

Then you can use the API reference to construct a program that will, for instance, spin up a small cluster of instances, run a student’s code, and then grade it.