DataWriters send DDS data samples and heartbeats to DataReaders. A DataReader responds to a heartbeat by sending an acknowledgement that tells the DataWriter what the DataReader has received so far and what it is missing. If there are many DataReaders, all sending ACKNACKs to the same DataWriter at the same time, a message storm can result. To prevent this, you can set a delay for each DataReader, so they don’t all send ACKNACKs at the same time. This delay is set in the DATA_READER_PROTOCOL QosPolicy (DDS Extension).
If you have several DataReaders per DataWriter, varying this delay for each one can avoid ACKNACK message storms to the DataWriter. If you are not concerned about message storms, you do not need to change this QosPolicy.
Example:
reader_qos.protocol.rtps_reliable_reader.min_heartbeat_response_delay.sec = 0; reader_qos.protocol.rtps_reliable_reader.min_heartbeat_response_delay.nanosec = 0; reader_qos.protocol.rtps_reliable_reader.max_heartbeat_response_delay.sec = 0; reader_qos.protocol.rtps_reliable_reader.max_heartbeat_response_delay.nanosec =
0.5 * 1000000000UL; // 0.5 sec
As the name suggests, the minimum and maximum response delay bounds the random wait time before the response. Setting both to zero will force immediate response, which may be necessary for the fastest recovery in case of lost DDS samples.
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