I just found this discussion when looking for keyed topic support. At Houston Mechatronics we have been using a long ago branched version of ROS2. We are now moving to Ardent and saw that keys still aren't there. We use keys for several different things. One of the things we have implemented is a parameter server purely in DDS which uses keyed topics so there is only one Parameter topic. The key is the name of the parameter and the value is in the other part of the topic data. This gives us great flexibility with the underlying DDS software because it handles the history. If I put my history at keeping only 1 the DDS vendor software we are using (RTI) will intelligently give me the last of each instance of the key. I only need to worry about one topic, but I can get all of the latest values and nothing else.
RTI at least has suggested using keyed topics as one of the primary ways to efficiently use DDS in numerous parts of our system. We've had to fork rosidl and rosidl_dds to have this DDS feature on our end. It supports the RTI version of keys in IDLs, but something similar could likely be extended for the other vendor solutions if they differ.
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