Sending messages Receiving messages Replying to the sender

Using the messaging APIs

The messaging APIs are contained within the SimSpace Weaver app SDK (minimum version 1.16.0). Messaging is supported in C++, Python, and our integrations with Unreal Engine 5 and Unity.

There are two functions that handle message transactions: SendMessage and ReceiveMessages. All sent messages contain a destination and a payload. The ReceiveMessages API returns a list of messages currently in an app’s inbound message queue.

Sending messages

Messages consist of a transaction (similar to other Weaver API calls), a payload, and a destination.

Message payload

The message payload is a flexible data structure of up to 256 bytes. We recommend the following as a best practice for creating your message payloads.

To create the message payload

Create a data structure (such as a struct in C++) that defines the contents of the message.
Create the message payload that contains the values to send in your message.
Create the MessagePayload object.

Message destination

The destination of a message is defined by the MessageEndpoint object. This includes both an endpoint type and an endpoint ID. The only endpoint type currently supported is Partition, which enables you to address messages to other partitions in the simulation. The endpoint ID is the partition ID of your target destination.

You can only provide 1 destination address in a message. Create and send multiple messages if you want to send messages to more than 1 partition at the same time.

For guidance on how to resolve a message endpoint from a position, see Tips when working with messaging.

Send the message

You can use the SendMessage API after you create the destination and payload objects.

Full example of sending messages

The following example demonstrates how you can construct and send a generic message. This example sends 16 individual messages. Each message contains a payload with a value betwen 0 and 15, and the current simulation tick.

C++


// Message struct definition
struct MessageTickAndId
{
    uint32_t id;
    uint32_t tick;
};

Aws::WeaverRuntime::Result<void> SendMessages(Txn& txn) noexcept
{
     // Fetch the destination MessageEndpoint with the endpoint resolver
    WEAVERRUNTIME_TRY(
        Api::MessageEndpoint destination,
        Api::Utils::MessageEndpointResolver::ResolveFromPosition(
        txn,
            "MySpatialSimulation",
            Api::Vector2F32 {231.3, 654.0}
        )
    );
    Log::Info("destination: ", destination);

    WEAVERRUNTIME_TRY(auto tick, Api::CurrentTick(txn));

    uint16_t numSentMessages = 0;
    for (std::size_t i=0; i<16; i++)
    {
        // Create the message that'll be serialized into payload
        MessageTickAndId message {i, tick.value};
        
        // Create the payload out of the struct
        const Api::MessagePayload& payload = Api::Utils::CreateMessagePayload(
            reinterpret_cast<const std::uint8_t*>(&message), 
            sizeof(MessageTickAndId)
        );
        
        // Send the payload to the destination
        Result<void> result = Api::SendMessage(txn, payload, destination);
        if (result.has_failure())
        {
            // SendMessage has failure modes, log them
            auto error = result.as_failure().error();
            std::cout<< "SendMessage failed, ErrorCode: " << error << std::endl;
            continue;
        }
        
        numSentMessages++;
    }

    std::cout << numSentMessages << " messages is sent to endpoint" 
       << destination << std::endl;
    return Aws::WeaverRuntime::Success();
}

Python


# Message data class
@dataclasses.dataclass
class MessageTickAndId:
    tick: int = 0
    id: int = 0
    
# send messages
def _send_messages(self, txn):
    tick = api.current_tick(txn)
    num_messages_to_send = 16

    # Fetch the destination MessageEndpoint with the endpoint resolver
    destination = api.utils.resolve_endpoint_from_domain_name_position(
       txn,
       "MySpatialSimulation",
       pos
   )
    Log.debug("Destination_endpoint = %s", destination_endpoint)

   for id in range(num_messages_to_send):
       # Message struct that'll be serialized into payload
        message_tick_and_id = MessageTickAndId(id = id, tick = tick.value)
        
       # Create the payload out of the struct
        message_tick_and_id_data = struct.pack(
           '<ii',
           message_tick_and_id.id,
           message_tick_and_id.tick
       )
        payload = api.MessagePayload(list(message_tick_and_id_data))

        # Send the payload to the destination
        Log.debug("Sending message: %s, endpoint: %s",
           message_tick_and_id,
           destination
       )
        api.send_message(
           txn,
           payload,
           destination,
           api.MessageDeliveryType.BestEffort
       )

    Log.info("Sent %s messages to %s", num_messages_to_send, destination)
    return True

Receiving messages

SimSpace Weaver delivers messages into a partition’s inbound message queue. Use the ReceiveMessages API to get a MessageList object that contains the messages from the queue. Process each message with the ExtractMessage API to get the message data.

Replying to the sender

Every received message contains a message header with information about the message’s original sender. You can use the message.header.source_endpoint to send a reply.

C++


Result<void> ReceiveMessages(Txn& txn) noexcept
{
     // Fetch all the messages sent to the partition owned by the app
    WEAVERRUNTIME_TRY(auto messages, Api::ReceiveMessages(txn));
    std::cout << "Received" << messages.messages.size() << " messages" << std::endl;
    for (Api::Message& message : messages.messages)
    {
        std::cout << "Received message: " << message << std::endl;

         // Deserialize payload to the message struct
        const MessageTickAndId& receivedMessage 
            = Api::Utils::ExtractMessage<MessageTickAndId>(message);
        std::cout << "Received MessageTickAndId, Id: " << receivedMessage.id 
            <<", Tick: " << receivedMessage.tick << std::endl;
        
        // Get the sender endpoint and payload to bounce the message back
        Api::MessageEndpoint& sender = message.header.source_endpoint;
        Api::MessagePayload& payload = message.payload;
        Api::SendMessage(txn, payload, sender);
    }

    return Aws::WeaverRuntime::Success();
}

Python


# process incoming messages
def _process_incoming_messages(self, txn):
    messages = api.receive_messages(txn)
    for message in messages:
        payload_list = message.payload.data
        payload_bytes = bytes(payload_list)
        message_tick_and_id_data_struct 
           = MessageTickAndId(*struct.unpack('<ii', payload_bytes))

        Log.debug("Received message. Header: %s, message: %s", 
                    message.header, message_tick_and_id_data_struct)
       # Get the sender endpoint and payload 
       # to bounce the message back
       sender = message.header.source_endpoint
       payload = payload_list
       api.send_message(
           txn,
           payload_list,
           sender,
           api.MessageDeliveryType.BestEffort

    Log.info("Received %s messages", len(messages))
    return True

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Document Conventions

Messaging

When to use messaging