3.3.5. DataWriterListener

DataWriterListener is an abstract class defining the callbacks that will be triggered in response to state changes on the DataWriter. By default, all these callbacks are empty and do nothing. The user should implement a specialization of this class overriding the callbacks that are needed on the application. Callbacks that are not overridden will maintain their empty implementation.

DataWriterListener defines the following callbacks:

• on_publication_matched(): The DataWriter has found a DataReader that matches the Topic and has a common partition and a compatible QoS, or has ceased to be matched with a DataReader that was previously considered to be matched.

• on_offered_deadline_missed(): The DataWriter failed to provide data within the deadline period configured on its DataWriterQos. It will be called for each deadline period and data instance for which the DataWriter failed to provide data.

• on_offered_incompatible_qos(): The DataWriter has found a DataReader that matches the Topic and has a common partition, but with a requested QoS that is incompatible with the one defined on the DataWriter.

• on_liveliness_lost(): The DataWriter did not respect the liveliness configuration on its DataWriterQos, and therefore, DataReader entities will consider the DataWriter as no longer active.

• on_unacknowledged_sample_removed(): The Datawriter has removed a sample that has not been acknowledged by every matched DataReader.

3.3.5.1. on_unacknowledged_sample_removed callback

on_unacknowledged_sample_removed() non-standard callback notifies the user when a sample has been removed without being sent/received by the matched DataReaders. This could happen in constrained networks or if the publication throughput is too demanding. This callback can be used to detect these situations so the publishing application can apply some solution to ease this issue like reducing the publication rate.

The criteria to consider that a sample has been removed without being acknowledged depends on the ReliabilityQosPolicy:

• BEST_EFFORT_RELIABILITY_QOS DataWriters will notify that a sample has been removed while unacknowledged if the sample has not been sent through the transport.

• RELIABLE_RELIABILITY_QOS DataWriters consider samples to have been removed unacknowledged if not every matched DataReader has confirmed its reception by sending the corresponding meta-traffic ACK message. Consequently, a sample that is notified as removed unacknowledged might be received by one or more DataReaders, but not by every matched one, or at least, the ACK message has not been received at the moment of sample removal. A race condition is inevitable in this case, because when the sample is removed, the ACK from some matched DataReader is missing, but that means that it might have been lost in the transmission or that the message is still coming through and it will be received after the sample removal. Thus, this criteria may include false positives, but from the user’s point of view, it is more meaningful to know when the sample has not been acknowledged by every matched DataReader even if some samples are erroneously notified.

A specific case must be considered for reliable DataWriters with DisablePositiveACKsQosPolicy enabled. This policy disables the sending of positive ACK messages, unless the sample has been lost in which case the matched DataReader notifies the loss with a negative NACK message. If no NACK has been received in the time defined in the QoS policy, the sample is considered to be received. Again, this is prone to race conditions because the NACK message might be on its way or have been lost in the network. For this specific case, where ACK messages are not going to be received, the reliable DataWriter uses the same criteria as the best effort DataWriter.

class CustomDataWriterListener : public DataWriterListener
{

public:

    CustomDataWriterListener()
        : DataWriterListener()
    {
    }

    virtual ~CustomDataWriterListener()
    {
    }

    void on_publication_matched(
            DataWriter* writer,
            const PublicationMatchedStatus& info) override
    {
        static_cast<void>(writer);
        if (info.current_count_change == 1)
        {
            std::cout << "Matched a remote Subscriber for one of our Topics" << std::endl;
        }
        else if (info.current_count_change == -1)
        {
            std::cout << "Unmatched a remote Subscriber" << std::endl;
        }
    }

    void on_offered_deadline_missed(
            DataWriter* writer,
            const OfferedDeadlineMissedStatus& status) override
    {
        static_cast<void>(writer);
        static_cast<void>(status);
        std::cout << "Some data could not be delivered on time" << std::endl;
    }

    void on_offered_incompatible_qos(
            DataWriter* writer,
            const OfferedIncompatibleQosStatus& status) override
    {
        std::cout << "Found a remote Topic with incompatible QoS (QoS ID: " << status.last_policy_id <<
            ")" << std::endl;
    }

    void on_liveliness_lost(
            DataWriter* writer,
            const LivelinessLostStatus& status) override
    {
        static_cast<void>(writer);
        static_cast<void>(status);
        std::cout << "Liveliness lost. Matched Subscribers will consider us offline" << std::endl;
    }

    void on_unacknowledged_sample_removed(
            DataWriter* writer,
            const InstanceHandle_t& instance) override
    {
        static_cast<void>(writer);
        static_cast<void>(instance);
        std::cout << "Sample removed unacknowledged" << std::endl;
    }

};