Offering SOME/IP Methods

SOME/IP Methods

Methods implement a request-response communication in the SOME/IP protocol. A server offers a method. A client calls the method by sending a request message to the server. The server answers the request message with a response message. In contrast to SOME/IP events, no subscription of the offered service has to be set up in order to make a method request call.

Request and response messages can carry a serialized payload. Typically, the payload is either the argument to the method or the return value of the method. For defining the service interface data types which can be used in methods and serializing structured data into bytes follow the article SOME/IP Service Interface Data Types.

In SOME/IP there is also the possibility of fire&forget communication. The client sends a request message and does not expect a response from the server. The fire&forget communication is not implemented in someipy yet.

Step 1: Connect to the someipy Daemon

The first step is to connect to the someipy daemon. The daemon is a separate process communicating with the application using someipy via a Unix Domain Socket (UDS). The daemon is responsible for handling all communication with the SOME/IP network, including service discovery and message sending/receiving.

someipy_daemon = await connect_to_someipy_daemon()

In case, a non-default Unix Domain Socket path is used, a config dictionary can be passed to the connect_to_someipy_daemon function.

Step 2: Define the Data Types for Request and Response

In this example, we will offer a SOME/IP service that calculates the sum of two signed integers and returns the result back to the client. Therefore, we need to define two data types: one type Addends is passed as an argument in the request and the second data type Sum is used for transmitting the result in the response message.

@dataclass
class Addends(SomeIpPayload):
    addend1: Sint16
    addend2: Sint16

    def __init__(self, addend1: int = 0, addend2: int = 0):
        self.addend1 = Sint16(addend1)
        self.addend2 = Sint16(addend2)

@dataclass
class Sum(SomeIpPayload):
    value: Sint32

    def __init__(self):
        self.value = Sint32()

Details on defining data types can be found here.

Step 3: Implementing the Method Handler

In the next step, we will implement the method handler. This function will receive a bytes object and a Tuple with the caller’s IP address and port. The received bytes object will be deserialized into an Addends object. After calculating the sum, the Sum object will be serialized and returned. The method handler has to return a MethodResult object which has the following members:

  • message_type (MessageType): The MessageType is an enum and can be either MessageType.RESPONSE or MessageType.ERROR. You can use MessageType.ERROR if you want to indicate an application-specific error and send an explicit error message. Then you can also add a payload, e.g., with an error message. You could also send a MessageType.RESPONSE with an appropriate ReturnCode.

  • return_code (ReturnCode): The ReturnCode enum reflects the return codes defined in the SOME/IP protocol specification. For indicating a successful method call to the client, E_OK is returned. For indicating other errors, one of the other return codes has to be chosen.

  • payload (bytes): The payload to be returned is a bytes object, i.e., the method handler has to serialize structured messages used for the result. someipy will not internally serialize the data.

For details about error handling in SOME/IP, read chapter 4.2.6 in the SOME/IP protocol specification.

The method handler is an asynchronous function (async def). This allows for I/O-bound calls using await inside the method handler, e.g., calling another SOME/IP method or querying from a database, without blocking other tasks or method handlers from running.

async def add_method_handler(input_data: bytes, addr: Tuple[str, int]) -> MethodResult:
    print(
        f"Received data: {' '.join(f'0x{b:02x}' for b in input_data)} from IP: {addr[0]} Port: {addr[1]}"
    )

    result = MethodResult()

    try:
        # Deserialize the input data
        addends = Addends()
        addends.deserialize(input_data)
    except Exception as e:
        print(f"Error during deserialization: {e}")

        # Set the return code to E_MALFORMED_MESSAGE and return
        result.message_type = MessageType.RESPONSE
        result.return_code = ReturnCode.E_MALFORMED_MESSAGE
        return result

    # Perform the addition
    sum = Sum()
    sum.value = Sint32(addends.addend1.value + addends.addend2.value)
    print(f"Send back: {' '.join(f'0x{b:02x}' for b in sum.serialize())}")

    result.message_type = MessageType.RESPONSE
    result.return_code = ReturnCode.E_OK
    result.payload = sum.serialize()
    return result

Step 4: Definition of the Service

In order to offer a service containing a SOME/IP method, we will instantiate a Method and a Service object. The Method class holds the method ID and the reference to the method handler function. The Service object contains the Method objects and is used afterwards to instantiate a ServerServiceInstance. The Service will contain a single method with ID 0x1234. The ServiceBuilder class is used to create the Service object.

It’s also possible to define multiple Method``s and add them all to the ``Service. The with_method function can be called multiple times on the ServiceBuilder object.

SAMPLE_SERVICE_ID = 0x1234

addition_method = Method(
     id=SAMPLE_METHOD_ID,
     protocol=TransportLayerProtocol.UDP,
     method_handler=add_method_handler,
 )

 addition_service = (
     ServiceBuilder()
     .with_service_id(SAMPLE_SERVICE_ID)
     .with_major_version(1)
     .with_method(addition_method)
     .build()
 )

Step 5: Instantiate the Service

The previously defined Service can be instantiated as one or multiple service instances. Since we are offering a method as a server, a ServerServiceInstance object is created.

The constructor of the ServerServiceInstance class requires several parameters:

  • daemon: The someipy_daemon object (defined above)

  • service: The Service object (defined above)

  • instance_id: A service instance ID (0x5678 in this example)

  • endpoint_ip: The IP address of the network interface on which the service is offered (127.0.0.1 in this example)

  • endpoint_port: The port on which the service is offered (3000 in this example)

  • ttl: The time-to-live for the service discovery entries (5 seconds in this example)

  • cyclic_offer_delay_ms: The period of the cyclic offer service SD messages (2000 ms in this example)

SAMPLE_INSTANCE_ID = 0x5678

service_instance_addition = ServerServiceInstance(
    daemon=someipy_daemon,
    service=addition_service,
    instance_id=SAMPLE_INSTANCE_ID,
    endpoint_ip=interface_ip,
    endpoint_port=3000,
    ttl=5,
    cyclic_offer_delay_ms=2000,
)

Step 6: Announce the Service via Service Discovery

The next step is to use start_offer to announce the service instance to potential clients. The start_offer function will communicate with the someipy daemon which will take care of periodically sending service discovery messages with offer entries.

await service_instance_addition.start_offer()

Step 7: Shutdown the Application

At the end of your application, make sure to stop offering the service instance and disconnect from the someipy daemon to ensure a clean shutdown of the application.

await service_instance_temperature.stop_offer()
await someipy_daemon.disconnect_from_daemon()