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

Data Fields
void *	listener_data
	A place for listener implementors to keep a pointer to data that may be needed by their listener.

Detailed Description

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

Entity:: DDS_Entity

QoS:: QoS Policies

Status:: Status Kinds

All the supported kinds of concrete DDS_Listener interfaces (one per concrete DDS_Entity type) derive from this root and add functions 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 DDS_Listener instance may be shared among multiple entities if you so desire. Consequently, the provided parameter contains a reference to the concerned DDS_Entity.

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 DDS_Entity 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 DDS_Entity 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 DDS_DomainParticipant) 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 DDS_SubscriberListener::on_data_on_readers with a parameter of the related DDS_Subscriber;

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

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 DDS_DataReader objects by calling DDS_Subscriber_get_datareaders on the related DDS_Subscriber and then get the data by calling FooDataReader_read or FooDataReader_take on the returned DDS_DataReader objects), or it wants to treat each DDS_DataReader independently and it may choose the second option (and then get the data by calling FooDataReader_read or FooDataReader_take on the related DDS_DataReader).

Note that if DDS_SubscriberListener::on_data_on_readers is called, RTI Connext will not try to call DDS_DataReaderListener::on_data_available. However, an application can force a call to the DDS_DataReader objects that have data by calling DDS_Subscriber_notify_datareaders.

Operations Allowed in Listener Callbacks

The operations that are allowed in DDS_Listener callbacks depend on the DDS_ExclusiveAreaQosPolicy QoS policy of the DDS_Entity to which the DDS_Listener is attached – or in the case of a DDS_DataWriter of DDS_DataReader listener, on the DDS_ExclusiveAreaQosPolicy QoS of the parent DDS_Publisher or DDS_Subscriber. For instance, the DDS_ExclusiveAreaQosPolicy settings of a DDS_Subscriber will determine which operations are allowed within the callbacks of the listeners associated with all the DataReaders created through that DDS_Subscriber.

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 DDS_Listener is attached
Within a DDS_Topic listener callback, any operations on any subscribers, readers, publishers or writers

An attempt to call a disallowed function 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 DDS_DataReader or DDS_Subscriber listener callback, invoking any operation on any other DDS_Subscriber or on any DDS_DataReader belonging to another DDS_Subscriber.
Within a DDS_DataReader or DDS_Subscriber listener callback, invoking any operation on any DDS_Publisher (or on any DDS_DataWriter belonging to such a DDS_Publisher) that has DDS_ExclusiveAreaQosPolicy::use_shared_exclusive_area set to DDS_BOOLEAN_TRUE.
Within a DDS_DataWriter of DDS_Publisher listener callback, invoking any operation on another Publisher or on a DDS_DataWriter belonging to another DDS_Publisher.
Within a DDS_DataWriter of DDS_Publisher listener callback, invoking any operation on any DDS_Subscriber or DDS_DataReader.

An attempt to call a disallowed function 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 DDS_Publisher or DDS_Subscriber (or on the DDS_Publisher or DDS_Subscriber of the DDS_DataWriter or DDS_DataReader) 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 DDS_WaitSet.

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 DDS_WaitSet.

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 DDS_DataReaderListener::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 DDS_DataWriter_wait_for_acknowledgments within the DDS_DataReaderListener::on_data_available() callback. This will lead to deadlock.

See Also: EXCLUSIVE_AREA; Status Kinds; DDS_WaitSet, DDS_Condition

Field Documentation

void* DDS_Listener::listener_data