def create_request(
    liveaction, action_db=None, runnertype_db=None, validate_params=True
):
    """
    Create an action execution.

    :param action_db: Action model to operate one. If not provided, one is retrieved from the
                      database using values from "liveaction".
    :type action_db: :class:`ActionDB`

    :param runnertype_db: Runner model to operate one. If not provided, one is retrieved from the
                          database using values from "liveaction".
    :type runnertype_db: :class:`RunnerTypeDB`
    :param validate_params: Whether to validate parameters against schema. Default to True, but
                          set to False when raising a request to report an error.
    :type validate_params: ``bool``

    :return: (liveaction, execution)
    :rtype: tuple
    """
    # We import this here to avoid conflicts w/ runners that might import this
    # file since the runners don't have the config context by default.
    from st2common.metrics.base import get_driver

    # Use the user context from the parent action execution. Subtasks in a workflow
    # action can be invoked by a system user and so we want to use the user context
    # from the original workflow action.
    parent_context = executions.get_parent_context(liveaction) or {}
    parent_user = parent_context.get("user", None)

    if parent_user:
        liveaction.context["user"] = parent_user

    # Validate action
    if not action_db:
        action_db = action_utils.get_action_by_ref(liveaction.action)

    if not action_db:
        raise ValueError('Action "%s" cannot be found.' % liveaction.action)
    if not action_db.enabled:
        raise ValueError(
            'Unable to execute. Action "%s" is disabled.' % liveaction.action
        )

    if not runnertype_db:
        runnertype_db = action_utils.get_runnertype_by_name(
            action_db.runner_type["name"]
        )

    if not hasattr(liveaction, "parameters"):
        liveaction.parameters = dict()

    # For consistency add pack to the context here in addition to RunnerContainer.dispatch() method
    liveaction.context["pack"] = action_db.pack

    # Validate action parameters.
    schema = util_schema.get_schema_for_action_parameters(action_db, runnertype_db)
    validator = util_schema.get_validator()
    if validate_params:
        util_schema.validate(
            liveaction.parameters,
            schema,
            validator,
            use_default=True,
            allow_default_none=True,
        )

    # validate that no immutable params are being overriden. Although possible to
    # ignore the override it is safer to inform the user to avoid surprises.
    immutables = _get_immutable_params(action_db.parameters)
    immutables.extend(_get_immutable_params(runnertype_db.runner_parameters))
    overridden_immutables = [
        p for p in six.iterkeys(liveaction.parameters) if p in immutables
    ]
    if len(overridden_immutables) > 0:
        raise ValueError(
            "Override of immutable parameter(s) %s is unsupported."
            % str(overridden_immutables)
        )

    # Set notification settings for action.
    # XXX: There are cases when we don't want notifications to be sent for a particular
    # execution. So we should look at liveaction.parameters['notify']
    # and not set liveaction.notify.
    if not _is_notify_skipped(liveaction) and not _is_notify_empty(action_db.notify):
        liveaction.notify = action_db.notify

    # Write to database and send to message queue.
    liveaction.status = action_constants.LIVEACTION_STATUS_REQUESTED
    liveaction.start_timestamp = date_utils.get_datetime_utc_now()

    # Set the "action_is_workflow" attribute
    liveaction.action_is_workflow = action_db.is_workflow()

    # Publish creation after both liveaction and actionexecution are created.
    liveaction = LiveAction.add_or_update(liveaction, publish=False)
    # Get trace_db if it exists. This could throw. If it throws, we have to cleanup
    # liveaction object so we don't see things in requested mode.
    trace_db = None
    try:
        _, trace_db = trace_service.get_trace_db_by_live_action(liveaction)
    except db_exc.StackStormDBObjectNotFoundError as e:
        _cleanup_liveaction(liveaction)
        raise trace_exc.TraceNotFoundException(six.text_type(e))

    execution = executions.create_execution_object(
        liveaction=liveaction,
        action_db=action_db,
        runnertype_db=runnertype_db,
        publish=False,
    )

    if trace_db:
        trace_service.add_or_update_given_trace_db(
            trace_db=trace_db,
            action_executions=[
                trace_service.get_trace_component_for_action_execution(
                    execution, liveaction
                )
            ],
        )

    get_driver().inc_counter("action.executions.%s" % (liveaction.status))

    return liveaction, execution

    Create an action execution.

    :param action_db: Action model to operate one. If not provided, one is retrieved from the
                      database using values from "liveaction".
    :type action_db: :class:`ActionDB`

    :param runnertype_db: Runner model to operate one. If not provided, one is retrieved from the
                          database using values from "liveaction".
    :type runnertype_db: :class:`RunnerTypeDB`
    :param validate_params: Whether to validate parameters against schema. Default to True, but
                          set to False when raising a request to report an error.
    :type validate_params: ``bool``

    :return: (liveaction, execution)
    :rtype: tuple
    
st2common/st2common/services/action.py