#
#  LMS Interface to external queueing system (xqueue)
#
import json
import requests

# TODO: Collection of parameters to be hooked into rest of edX system
XQUEUE_SUBMIT_URL = 'http://xqueue.edx.org/xqueue/submit/'

def upload_files_to_s3():
    print '  THK: xqueue_interface.upload_files_to_s3'


def make_xheader(lms_callback_url, lms_key, queue_name):
    '''
    Generate header for delivery and reply of queue request.

    Xqueue header is a JSON-serialized dict:
        { 'lms_callback_url': url to which xqueue will return the request (string),
          'lms_key': secret key used by LMS to protect its state (string), 
          'queue_name': designate a specific queue within xqueue server, e.g. 'MITx-6.00x' (string)
        }
    '''
    return json.dumps({ 'lms_callback_url': lms_callback_url,
                        'lms_key': lms_key,
                        'queue_name': queue_name })


def send_to_queue(header, body, xqueue_url=None):
    '''
    Submit a request to xqueue.
    
    header: JSON-serialized dict in the format described in 'xqueue_interface.make_xheader'

    body: Serialized data for the receipient behind the queueing service. The operation of
            xqueue is agnostic to the contents of 'body'

    '''
    if xqueue_url is None:
        xqueue_url = XQUEUE_SUBMIT_URL

    # Contact queue server
    payload = {'xqueue_header': header,
               'xqueue_body'  : body}
    try:
        r = requests.post(xqueue_url, data=payload)
    except Exception as err:
        msg = 'Error in xqueue_interface.send_to_queue %s: Cannot connect to server url=%s' % (err, xqueue_url)
        raise Exception(msg)

    #print r.text