Adding new service

MORSE has a concept of services: services are remote procedure call commonly used to configure or control the behaviour of the simulation.

Each component can register services that are made publicly available to the outside world through middleware-specific channels.

Services can be either synchronous or asynchronous. Synchronous services block the simulation until they complete. They must remain fast to execute. Asynchronous services may span computations on several simulation steps (but each individual cycle must remain fast).

Exposing methods as services

Most of the time, adding a new service is as easy as adding @service in front of a function declared within a component.

Note

Arguments coming from remote caller are passed to service using string, so be sure to convert your data in the expected type.

Warning

Do not use eval() to convert your data from string to expected type, as this would cause a serious security threat. Use type specific functions (like int(), float(),...) instead.

Lets have a look at human.py, the component that allows us to control a human character in the simulation.

import morse.core.robot
from morse.core.services import service

class HumanClass(morse.core.robot.Robot):

    def __init__(self, obj, parent=None):
        [...]

    @service
    def move(self, speed, rotation):

        human = self.bge_object

        human.applyMovement( [float(speed), 0, 0], True )
        human.applyRotation( [0, 0 , float(rotation)], True )

    [...]

By adding the @service decorator to the move method, we expose move as a MORSE service.

During the simulation initialization, MORSE registers these services for each instances of the component, maps them to one (or several) middlewares (as specified by the scene description), and starts listening for incoming request.

Each middleware has its own naming scheme for services, but one can expect the services to be available as component_name.service_name.

The example below shows a simple Python client that would use the HumanClass.move service as declared above:

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("localhost", 4000))
s.send("id1 human move (1.0, 1.6)\n")

In this example, we assume that human is the name of the Blender object that instantiates a HumanClass.

Note

The value of the id (here id1) has no importance at all: it is specific to the implementation of the services with sockets and used only by the client to track requests and responses.

Using pymorse, the previous code can be rewrite as:

from pymorse import Morse
with Morse() as morse:
    morse.rpc('human', 'move', 1.0, 1.6)

Returning values

A service can return any valid Python object (None, a string, a dictionary, a complex object...). The serialization is left to the middleware.

If the service call fails, you are expected to raise a morse.core.exceptions.MorseRPCInvokationError exception (or any custom exception inheriting from it) with a useful error message:

import morse.core.robot
from morse.core.exceptions import MorseRPCInvokationError
from morse.core.services import service

class HumanClass(morse.core.robot.Robot):

    def __init__(self, obj, parent=None):
        [...]

    @service
    def move(self, speed, rotation):

        if float(speed) < 0:
            raise MorseRPCInvokationError("Our human can not walk backward!")

        human = self.bge_object

        human.applyMovement( [float(speed), 0, 0], True )
        human.applyRotation( [0, 0, float(rotation)], True )

    [...]

MORSE will answer the request with a morse.core.status.FAILED status.

Free functions

Synchronous services can also be declared outside classes (on free-functions).

In this case, the decorator takes one parameter, the (pseudo) component.

For instance, morse.services.supervision_services declares such services. The following example shows the list_robots service that returns the list of robot declared in the simulation:

from morse.core import blenderapi
from morse.core.services import service

@service(component = "simulation")
def list_robots():
    return [obj.name for obj in blenderapi.persistantstorage().robotDict.keys()]

The pseudo-component simulation is used as namespace for the service: this one is accessible as simulation.list_robots.

Asynchronous services

RPC calls may be used to start the execution of a task that may take a long time to complete.

In such cases, asynchronous services can be used to initialize and start the task. MORSE automatically notifies the client when the task is completed.

Declaring new asynchronous services is slightly more complex: we need first an initialization method and secondly, a way to tell when the task is achieved.

Declaring an initialization method is very similar to synchronous services. For instance, the waypoint actuator defines an asynchronous goto service:

import morse.core.actuator
from morse.core.services import async_service

class Waypoint(morse.core.actuator.Actuator):

    def __init__(self, obj, parent=None):
        [...]

    @async_service
    def goto(self, x, y):
        self.local_data['x'] = float(x)
        self.local_data['y'] = float(y)
        self.local_data['z'] = 0

    [...]

The @service decorator is simply replaced by @async_service. By doing so, MORSE automatically registers a callback that is used to monitor the status of the task and notify the client upon completion.

In this example we simply set a new target position in the actuator local_data dictionary, but any kind of initialization can be started here. It must only remain short (since the simulator blocks until the initialization method returns).

The execution of the task itself takes place at each simulation step in the component morse.core.object.Object.default_action() method. Each execution step should remain short since the simulator blocks on the default_action as well.

When the task is achieved, the component must notify it by calling self.completed(status, result).

status is one of the status defined in morse.core.status (mainly SUCCESS and FAILED), result is any valid Python object.

Note

As you may have noticed, at a given time, only one asynchronous request can be handled by a component. If a second asynchronous request is received, the behaviour may vary, as explained below.

Note

Asynchronous services can normally only exist inside components (i.e., they must be declared within a class inheriting from morse.core.abstractobject.AbstractObject). The section Manually registering services explains how to overcome this constraint.

Interruption policy for asynchronous services

As of morse-1.2, only one asynchronous service may run at a given time.

You can define the behaviour of the simulator when a second request is received either at the middleware level (global policy) or at the individual service level (local policy).

To set a local policy, simply decorate your services with the @interruptible and @noninterruptible decorators (morse.core.services.interruptible() and morse.core.services.noninterruptible()). These decorators must appear before the @async_service decorator.

An interruptible service is preempted when a new asynchronous service is started by calling the interrupt method. The interrupt method is defined in morse.core.abstractobject.AbstractObject to send back to the caller the status morse.core.status.PREEMPTED. It is advised to overload this behaviour in the component class to ensure the service is actually interrupted (do not forget however to call overloaded interrupt method, as shown in the example below).

import morse.core.actuator

class Waypoint(morse.core.actuator.Actuator):

     def interrupt(self):
         self.local_data['x'] = self.bge_object.worldPosition[0]
         self.local_data['y'] = self.bge_object.worldPosition[1]
         self.local_data['z'] = self.bge_object.worldPosition[2]
         morse.core.actuator.Actuator.interrupt(self)

Note

It is recommended to always implement the interrupt method even if the default policy is non-interruptible, as a caller can decide to manually interrupt the service.

A non-interruptible service triggers a failure (status morse.core.status.FAILED) when someone attempts to start a new asynchronous service.

To set a global policy, you need to catch a morse.core.exceptions.MorseServiceAlreadyRunningError exception when invoking the morse.core.request_manager.RequestManager.on_incoming_request() method.

This exception has a special member service that points to the asynchronous service currently running:

try:
    is_synchronous, value = self.on_incoming_request(component, service, params)
except MorseServiceAlreadyRunningError as e:
    logger.warning(e.service.__name__ + " is already running!")

Note

A service with a local policy defined (i.e. decorated with either @interruptible or @noninterruptible) will never trigger a MorseServiceAlreadyRunningError exception, and thus, the local policy always overrides the global policy.