Understanding service internals

Registering synchronous services

What exactly happens when a method is decorated with @service?

The @service decorator simply marks the method as a service by setting the attribute _morse_service to True on the function.

Before actually registering the service, a mapping between the component and one or several middlewares that will manage incoming requests must be defined (for instance, we may want the goto service of the morse.actuators.waypoint.Waypoint to be managed by the YARP middleware for the component MainRobot).

These mapping are defined by your builder script (it is so simulation-dependant).

At start up, morse.blender.main.init()...

  1. Reads the configuration,
  2. Instantiates classes associated to each component,
  3. Registers the mappings (with morse.core.services.MorseServices.register_request_manager_mapping()),
  4. Calls morse.core.abstractobject.AbstractObject.register_services() on each component instance.

morse.core.abstractobject.AbstractObject.register_services() iterates over every methods marked as MORSE service within the class, and call morse.core.services.do_service_registration().

This method finds the middleware(s) in charge of managing services for this component, and calls morse.core.request_manager.RequestManager.register_service().

It is up to each middleware to manage registration of new services. They may have to, for instance, expose the new service into a shared directory (yellow pages), etc.

Upon incoming request...

When a new request comes in, the middleware-specific part receives it, deserializes it and hands it over to morse.core.request_manager.RequestManager.on_incoming_request(). This method dispatches the request to the correct component, and executes it (for synchronous services) or starts the execution and returns an internal request ID (for asynchronous services).

This internal request ID can be used by the middleware to track the status of a request.

On completion, the morse.core.request_manager.RequestManager.on_service_completion() callback is invoked, and the final result can be sent back to the client.

Asynchronous services

Registration of asynchronous services is mostly identical to synchronous services. The @async_service decorator simply calls the @service decorator with the async parameter set to True, which leads to wrap the original method in a new method that takes one more parameter (a callback) and calls morse.core.abstractobject.AbstractObject.set_service_callback().

Simplified version of the @service decorator:

def service(fn, async=False):
  dfn = fn
  if async:
     def decorated_fn(self, callback, *param):
        self._set_service_callback(callback)
        fn(self, *param)
     dfn = decorated_fn
     dfn.__name__ = fn.__name__

  dfn._morse_service = True
  dfn._morse_service_is_async = async

  return dfn

However, asynchronous services behaviour differs when a request comes in:

Manually registering services

While usually not necessary, you may have sometimes to manually register services (i.e. without using decorators).

This first code snippet shows how to register a synchronous service that uses sockets as communication interface:

from morse.middleware.socket_request_manager import SocketRequestManager

def add(a, b):
    return a + b

req_manager = SocketRequestManager()
req_manager.register_service("test_component", add)

while True:
    # This calls the middleware specific part, in charge of reading
    # incoming request and writing back pending results.
    req_manager.process()
    # In a real case, you don't want to block on such a loop, and MORSE
    # takes care itself to call req_manager.process()

If you run this sample code, you can test it with a simple Telnet session:

> telnet localhost 4000
Connected to localhost.
> req1 test_component add (1,2)
req1 OK 3

Note

The socket-based protocol is fairly simple: you provide a request id, the name of the component that offers the service, the name of the service and (if necessary) parameters. Parameters must be contained in a valid Python iterable (a tuple, like in the example, or an array).

Here, req1 is the custom request id, chosen by the client.

For asynchronous services, a callback function is passed to the service. It allows the service to notify when it is complete.

This second code snippet shows an example of asynchronous service:

from morse.core import status
from morse.middleware.socket_request_manager import SocketRequestManager

result_cb = None
run_computation = False
value = None

# We start here our asynchronous service.
# an arbitrary amount of parameters can be passed, but the first one
# must be the callback to set the service result upon completion.
def start_computation(result_setter, start_value):
     global result_cb, value, run_computation

     result_cb = result_setter
     value = start_value
     run_computation = True

     # the service must return true if the task was successfully started
     return True

# This is the actual code called at each simulation step in our component
def complex_computation(a):
    global run_computation

    if a == 0:
        # At the end of the computation, we set the result.
        # the result can be any valid Python object
        result_cb((status.SUCCESS, "done!"))
        run_computation = False

    morse.sleep(1)
    return a - 1

req_manager = SocketRequestManager()

# here we explicitely register an asynchronous service.
# the optional 'service_name' argument allows to define a custom service
# name.
req_manager.register_async_service("test_component", start_computation, service_name = "compute")

while True:
    req_manager.process()

    if run_computation:
       value = complex_computation(value)
       print("Value is now %i" % value)

If you test the code with Telnet:

> telnet localhost 4000
Connected to localhost.
> req2 test_component compute (5,)
[after 5 seconds]
req2 OK done!

Note

When passing a single parameter, you still need to pass a valid Python iterable, with only one element. Hence the (5,).

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