def _client_id(request):
    """
    Gets the value of the Client-ID header from the request.

    We know the Client-ID must be decodable as ASCII because
    Cloud Queues requires it to be submitted as a UUID. Mimic
    is not so strict, but requiring ASCII is ok.
    """
    return request.requestHeaders.getRawHeaders(b'client-id')[0].decode('ascii'
        )
