How to advertise multiple endpoints for reader/writers without duplicating messages

[Hugal31]

I'm trying to have a topic available on TCP. However, I want the topic to still be available for clients not configured to use TCP, so I also add an UDP endpoint.

However, when I do that:

• I don't have a way to choose which endpoint to prefer when both are available.
• The messages are actually duplicated.

Here is my sample config:

<?xml version="1.0" encoding="UTF-8" ?>
<dds xmlns="http://www.eprosima.com/XMLSchemas/fastRTPS_Profiles"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://www.eprosima.com https://raw.githubusercontent.com/eProsima/Fast-DDS/refs/tags/v2.6.9/resources/xsd/fastRTPS_profiles.xsd">

    <profiles>
        <transport_descriptors>
            <transport_descriptor>
                <transport_id>shm_transport</transport_id>
                <type>SHM</type>
            </transport_descriptor>
            <transport_descriptor>
                <transport_id>tcp_transport</transport_id>
                <type>TCPv4</type>
                <listening_ports><port>5000</port></listening_ports>
            </transport_descriptor>
            <transport_descriptor>
                <transport_id>udp_transport</transport_id>
                <type>UDPv4</type>
            </transport_descriptor>
        </transport_descriptors>

        <participant profile_name="default_participant" is_default_profile="true">
            <rtps>
                <userTransports>
                    <transport_id>shm_transport</transport_id>
                    <transport_id>tcp_transport</transport_id>
                    <transport_id>udp_transport</transport_id>
                </userTransports>

                <useBuiltinTransports>false</useBuiltinTransports>
            </rtps>
        </participant>

        <data_reader profile_name="default_reader" is_default_profile="true">
            <unicastLocatorList>
                <locator><tcpv4/></locator>
                <locator><udpv4/></locator>
            </unicastLocatorList>
        </data_reader>
        <data_writer profile_name="default_publisher" is_default_profile="true">
            <unicastLocatorList>
                <locator><tcpv4/></locator>
                <locator><udpv4/></locator>
            </unicastLocatorList>
        </data_writer>
    </profiles>
</dds>

How can I ensure only one endpoint is chosen, and it is TCP if available?

I am using Fast-DDS 2.6.9.

[Carlosespicur]

Hi @Hugal31,
Fast DDS is designed to be redundant if multiple transport locators are defined. In your case, you're configuring DataReaders and DataWriters with both TCP and UDP locators. This results in all messages being sent over TCP and UDP, causing the problem you describe. If you only want to use TCP (or UDP), you could create DataReaders configured with TCP (or UDP) locators only.

[Hugal31]

That would work until a client wants to publish on the topic. But that's might be good enough for my use case, provided I solve #5658 as well. Thank you.