Detailed Description

<<interface>> Abstract base class for all Listener interfaces.

Entity:: DDSEntity

QoS:: QoS Policies

Status:: Status Kinds

All the supported kinds of concrete DDSListener interfaces (one per concrete DDSEntity type) derive from this root and add methods whose prototype depends on the concrete Listener.

Listeners provide a way for RTI Connext to asynchronously alert the application when there are relevant status changes.

Almost every application will have to implement listener interfaces.

Each dedicated listener presents a list of operations that correspond to the relevant communication status changes to which an application may respond.

The same DDSListener instance may be shared among multiple entities if you so desire. Consequently, the provided parameter contains a reference to the concerned DDSEntity.

Access to Plain Communication Status

The general mapping between the plain communication statuses (see Status Kinds) and the listeners' operations is as follows:

For each communication status, there is a corresponding operation whose name is on_<communication_status>(), which takes a parameter of type <communication_status> as listed in Status Kinds.

on_<communication_status> is available on the relevant DDSEntity as well as those that embed it, as expressed in the following figure:

Hierarchical

listener processing. The most specific relevant enabled listener is called."

When the application attaches a listener on an entity, it must set a mask. The mask indicates to RTI Connext which operations are enabled within the listener (cf. operation DDSEntity set_listener() ).

When a plain communication status changes, RTI Connext triggers the most specific relevant listener operation that is enabled. In case the most specific relevant listener operation corresponds to an application-installed 'nil' listener the operation will be considered handled by a NO-OP operation that does not reset the communication status.

This behavior allows the application to set a default behavior (e.g., in the listener associated with the DDSDomainParticipant) and to set dedicated behaviors only where needed.

Access to Read Communication Status

The two statuses related to data arrival are treated slightly differently. Since they constitute the core purpose of the Data Distribution Service, there is no need to provide a default mechanism (as is done for the plain communication statuses above).

The rule is as follows. Each time the read communication status changes:

First, RTI Connext tries to trigger the DDSSubscriberListener::on_data_on_readers with a parameter of the related DDSSubscriber;

If this does not succeed (there is no listener or the operation is not enabled), RTI Connext tries to trigger DDSDataReaderListener::on_data_available on all the related DDSDataReaderListener objects, with a parameter of the related DDSDataReader.

The rationale is that either the application is interested in relations among data arrivals and it must use the first option (and then get the corresponding DDSDataReader objects by calling DDSSubscriber::get_datareaders on the related DDSSubscriber and then get the data by calling FooDataReader::read or FooDataReader::take on the returned DDSDataReader objects), or it wants to treat each DDSDataReader independently and it may choose the second option (and then get the data by calling FooDataReader::read or FooDataReader::take on the related DDSDataReader).

Note that if DDSSubscriberListener::on_data_on_readers is called, RTI Connext will not try to call DDSDataReaderListener::on_data_available. However, an application can force a call to the DDSDataReader objects that have data by calling DDSSubscriber::notify_datareaders.

Operations Allowed in Listener Callbacks

The operations that are allowed in DDSListener callbacks depend on the DDS_ExclusiveAreaQosPolicy QoS policy of the DDSEntity to which the DDSListener is attached -- or in the case of a DDSDataWriter of DDSDataReader listener, on the DDS_ExclusiveAreaQosPolicy QoS of the parent DDSPublisher or DDSSubscriber. For instance, the DDS_ExclusiveAreaQosPolicy settings of a DDSSubscriber will determine which operations are allowed within the callbacks of the listeners associated with all the DataReaders created through that DDSSubscriber.

Note: these restrictions do not apply to builtin topic listener callbacks.

Regardless of whether DDS_ExclusiveAreaQosPolicy::use_shared_exclusive_area is set to DDS_BOOLEAN_TRUE or DDS_BOOLEAN_FALSE, the following operations are not allowed:

Within any listener callback, deleting the entity to which the DDSListener is attached
Within a DDSTopic listener callback, any operations on any subscribers, readers, publishers or writers

An attempt to call a disallowed method from within a callback will result in DDS_RETCODE_ILLEGAL_OPERATION.

If DDS_ExclusiveAreaQosPolicy::use_shared_exclusive_area is set to DDS_BOOLEAN_FALSE, the setting which allows more concurrency among RTI Connext threads, the following are not allowed:

Within any listener callback, creating any entity
Within any listener callback, deleting any entity
Within any listener callback, enabling any entity
Within any listener callback, setting the QoS of any entities
Within a DDSDataReader or DDSSubscriber listener callback, invoking any operation on any other DDSSubscriber or on any DDSDataReader belonging to another DDSSubscriber.
Within a DDSDataReader or DDSSubscriber listener callback, invoking any operation on any DDSPublisher (or on any DDSDataWriter belonging to such a DDSPublisher) that has DDS_ExclusiveAreaQosPolicy::use_shared_exclusive_area set to DDS_BOOLEAN_TRUE.
Within a DDSDataWriter of DDSPublisher listener callback, invoking any operation on another Publisher or on a DDSDataWriter belonging to another DDSPublisher.
Within a DDSDataWriter of DDSPublisher listener callback, invoking any operation on any DDSSubscriber or DDSDataReader.

An attempt to call a disallowed method from within a callback will result in DDS_RETCODE_ILLEGAL_OPERATION.

The above limitations can be lifted by setting DDS_ExclusiveAreaQosPolicy::use_shared_exclusive_area to DDS_BOOLEAN_TRUE on the DDSPublisher or DDSSubscriber (or on the DDSPublisher or DDSSubscriber of the DDSDataWriter or DDSDataReader) to which the listener is attached. However, the application will pay the cost of reduced concurrency between the affected publishers and subscribers.

Best Practices with Listeners

Note that all the issues described below are avoided by using DDSWaitSet.

Avoid blocking or performing a lot of processing in Listener callbacks

Listeners are invoked by internal threads that perform critical functions within the middleware and need to run in a timely manner. By default, Connext DDS creates a few threads to use to receive data and only a single thread to handle periodic events.

Because of this, user applications installing Listeners should never block in a Listener callback. There are several negative consequences of blocking in a listener callback:

The application may lose data for the DataReader the listener is installed on, because the receive thread is not removing it from the socket buffer and it gets overwritten.
The application may receive strictly reliable data with a delay, because the receive thread is not removing it from the socket buffer and if it gets overwritten it must be re-sent.
The application may lose or delay data for other DataReaders, because by default all DataReaders created with the same DomainParticipant share the same threads.
The application may not be notified of periodic events on time

If the application needs to make a blocking call when data is available, or when another event occurs, the application should use DDSWaitSet.

Avoid taking application mutexes/semaphores in Listener callbacks

Taking application mutexes/sempahores within a Listener callback may lead to unexpected deadlock scenarios.

When a Listener callback is invoked the EA (Exclusive Area) of the Entity 'E' to which the callback applies is taken by the middleware.

If the application takes an application mutex 'M' within a critical section in which the application makes DDS calls affecting 'E', this may lead to following deadlock:

The middleware thread is within the entity EA trying to acquire the mutex 'M'. At the same time, the application thread has acquired 'M' and is blocked trying to acquire the entity EA.

Do not write data with a DataWriter within the on_data_available callback

Avoid writing data with a DataWriter within the DDSDataReaderListener::on_data_available() callback. If the write operation blocks because e.g. the send window is full, this will lead to a deadlock.

Do not call wait_for_acknowledgements within the on_data_available callback

Do not call the DDSDataWriter::wait_for_acknowledgments within the DDSDataReaderListener::on_data_available() callback. This will lead to deadlock.

See also: EXCLUSIVE_AREA; Status Kinds; DDSWaitSet, DDSCondition