OpenDDS and notification of publisher presence

Question

Problem: How can I get liveliness notifications of booth publisher connect and disconnect?

Background: I'm working with a OpenDDS implementation where I have a publisher and a subscriber of a data type (dt), using the same topic, located on separate computers.

The reader on the subscriber side has overridden implementations of on_data_available(...)and on_liveliness_changed(...). My subscriber is started first, resulting in a callback to on_liveliness_changed(...) which says that there are no writers available. When the publisher is started I get a new callback to telling me there is a writer available, and when the publisher publishes, on_data_available(...) is called. So far everything is working as expected.

The writer on the publisher has a overridden implementation of on_publication_matched(...). When starting the publisher, on_publication_matched(...) gets called since we already have a subscriber started.

The problem is that when the publisher disconnects, I get no callback to on_liveliness_changed(...) on the reader side, nor do I get a new callback when the publisher is started again.

I have tried to change the readerQos by setting the readerQos.liveliness.lease_duration. But the result is that the on_data_available(...) never gets called, and the only callback to on_liveliness_changed(...) is at startup, telling me that there are no publishers.

DDS::DataReaderQos readerQos;
DDS::StatusKind mask = DDS::DATA_AVAILABLE_STATUS | DDS::LIVELINESS_CHANGED_STATUS |  DDS::LIVELINESS_LOST_STATUS ; 
m_subscriber->get_default_datareader_qos( readerQos );
DDS::Duration_t t = { 3, 0 };
readerQos.liveliness.lease_duration = t;
m_binary_Reader = static_cast<binary::binary_tdatareader( m_subscriber->create_datareader(m_Sender_Topic,readerQos,this, mask, 0, false) );

/Kristofer

Answer 1

Ok, guess there aren't many DDS users here.

After some research I found that a reader/writer match occurs only if this compatibility criterion is satisfied: offered lease_duration <= requested lease_duration

The solution was to set the writer QoS to offer the same liveliness. There is probably a way of checking if the requested reader QoS could be supplied by the corresponding writer, and if not, use a "lower" QoS, all thou I haven't tried it yet.

In the on_liveliness_changed callback method I simply evaluated the alive_count in the from the LivelinessChangedStatus.

/Kristofer

Answer 2

Hi - fellow DDS user here. :) Sorry for not finding your posting earlier or I could have replied. It seems the dds tag has been hijacked by other topics. Maybe we can use data-distribution-service in the future?