def call_req(obj_name, method, params):
    """Construct message used when sending API calls to CtrlServer.

    :param obj_name: Name of API-exported system that owns the given method.
    :type obj_name: string
    :param method: API-exported method to call on the given object.
    :type method: string
    :param params: Params to pass to the given method.
    :type params: dict
    :returns: Constructed call_req dict, ready to be sent over the wire.

    """
    return {
        "type": "call_req",
        "obj_name": obj_name,
        "method": method,
        "params": params
    }