<<interface>> Abstract base class for all Listener interfaces. More...
Detailed Description
<<interface>> Abstract base class for all Listener interfaces.
- QoS:
- QoS Policies
- Status:
- Status Kinds
All the supported kinds of concrete com.rti.dds.infrastructure.Listener interfaces (one per concrete com.rti.dds.infrastructure.Entity 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 com.rti.dds.infrastructure.Listener instance may be shared among multiple entities if you so desire. Consequently, the provided parameter contains a reference to the concerned com.rti.dds.infrastructure.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 com.rti.dds.infrastructure.Entity as well as those that embed it, as expressed in the following figure:
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 com.rti.dds.infrastructure.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 com.rti.dds.domain.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 com.rti.dds.subscription.SubscriberListener.on_data_on_readers with a parameter of the related com.rti.dds.subscription.Subscriber;
- If this does not succeed (there is no listener or the operation is not enabled), RTI Connext tries to trigger com.rti.dds.subscription.DataReaderListener.on_data_available on all the related com.rti.dds.subscription.DataReaderListener objects, with a parameter of the related com.rti.dds.subscription.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 com.rti.dds.subscription.DataReader objects by calling com.rti.dds.subscription.Subscriber.get_datareaders on the related com.rti.dds.subscription.Subscriber and then get the data by calling com.rti.ndds.example.FooDataReader.read or com.rti.ndds.example.FooDataReader.take on the returned com.rti.dds.subscription.DataReader objects), or it wants to treat each com.rti.dds.subscription.DataReader independently and it may choose the second option (and then get the data by calling com.rti.ndds.example.FooDataReader.read or com.rti.ndds.example.FooDataReader.take on the related com.rti.dds.subscription.DataReader).
Note that if com.rti.dds.subscription.SubscriberListener.on_data_on_readers is called, RTI Connext will not try to call com.rti.dds.subscription.DataReaderListener.on_data_available. However, an application can force a call to the com.rti.dds.subscription.DataReader objects that have data by calling com.rti.dds.subscription.Subscriber.notify_datareaders.
Operations Allowed in Listener Callbacks
The operations that are allowed in com.rti.dds.infrastructure.Listener callbacks depend on the com.rti.dds.infrastructure.ExclusiveAreaQosPolicy QoS policy of the com.rti.dds.infrastructure.Entity to which the com.rti.dds.infrastructure.Listener is attached -- or in the case of a com.rti.dds.publication.DataWriter of com.rti.dds.subscription.DataReader listener, on the com.rti.dds.infrastructure.ExclusiveAreaQosPolicy QoS of the parent com.rti.dds.publication.Publisher or com.rti.dds.subscription.Subscriber. For instance, the com.rti.dds.infrastructure.ExclusiveAreaQosPolicy settings of a com.rti.dds.subscription.Subscriber will determine which operations are allowed within the callbacks of the listeners associated with all the DataReaders created through that com.rti.dds.subscription.Subscriber.
Note: these restrictions do not apply to builtin topic listener callbacks.
Regardless of whether com.rti.dds.infrastructure.ExclusiveAreaQosPolicy.use_shared_exclusive_area is set to com.rti.dds.infrastructure.true or com.rti.dds.infrastructure.false, the following operations are not allowed:
- Within any listener callback, deleting the entity to which the com.rti.dds.infrastructure.Listener is attached
- Within a com.rti.dds.topic.Topic 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 com.rti.dds.infrastructure.RETCODE_ILLEGAL_OPERATION.
If com.rti.dds.infrastructure.ExclusiveAreaQosPolicy.use_shared_exclusive_area is set to com.rti.dds.infrastructure.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 com.rti.dds.subscription.DataReader or com.rti.dds.subscription.Subscriber listener callback, invoking any operation on any other com.rti.dds.subscription.Subscriber or on any com.rti.dds.subscription.DataReader belonging to another com.rti.dds.subscription.Subscriber.
- Within a com.rti.dds.subscription.DataReader or com.rti.dds.subscription.Subscriber listener callback, invoking any operation on any com.rti.dds.publication.Publisher (or on any com.rti.dds.publication.DataWriter belonging to such a com.rti.dds.publication.Publisher) that has com.rti.dds.infrastructure.ExclusiveAreaQosPolicy.use_shared_exclusive_area set to com.rti.dds.infrastructure.true.
- Within a com.rti.dds.publication.DataWriter of com.rti.dds.publication.Publisher listener callback, invoking any operation on another Publisher or on a com.rti.dds.publication.DataWriter belonging to another com.rti.dds.publication.Publisher.
- Within a com.rti.dds.publication.DataWriter of com.rti.dds.publication.Publisher listener callback, invoking any operation on any com.rti.dds.subscription.Subscriber or com.rti.dds.subscription.DataReader.
An attempt to call a disallowed method from within a callback will result in com.rti.dds.infrastructure.RETCODE_ILLEGAL_OPERATION.
The above limitations can be lifted by setting com.rti.dds.infrastructure.ExclusiveAreaQosPolicy.use_shared_exclusive_area to com.rti.dds.infrastructure.true on the com.rti.dds.publication.Publisher or com.rti.dds.subscription.Subscriber (or on the com.rti.dds.publication.Publisher or com.rti.dds.subscription.Subscriber of the com.rti.dds.publication.DataWriter or com.rti.dds.subscription.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 com.rti.dds.infrastructure.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 com.rti.dds.infrastructure.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 com.rti.dds.subscription.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 com.rti.dds.publication.DataWriter.wait_for_acknowledgments within the com.rti.dds.subscription.DataReaderListener::on_data_available() callback. This will lead to deadlock.