RTI Connext Modern C++ API
Version 7.0.0
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Entity listeners. More...
#include <dds/dds.hpp>
Entity listeners.
Listeners provide a way for RTI Connext to asynchronously alert the application when there are relevant status changes.
Each dedicated listener presents a list of operations that correspond to the relevant communication status changes to which an application may respond.
The same Listener instance may be shared among multiple entities if you so desire. Consequently, the provided parameter contains a reference to the concerned dds::core::Entity.
The general mapping between the plain communication statuses (see Status Kinds) and the listeners' operations is as follows:
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::core::Entity as well as those that embed it, as expressed in the following figure: This behavior allows the application to set a default behavior (e.g., in the listener associated with the dds::domain::DomainParticipant) and to set dedicated behaviors only where needed.
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:
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::sub::DataReader objects by calling dds::sub::find on the related dds::sub::Subscriber and then get the data by calling dds::sub::DataReader::read or dds::sub::DataReader::take on the returned dds::sub::DataReader objects), or it wants to treat each dds::sub::DataReader independently and it may choose the second option (and then get the data by calling dds::sub::DataReader::read or dds::sub::DataReader::take on the related dds::sub::DataReader).
Note that if dds::sub::SubscriberListener::on_data_on_readers is called, RTI Connext will not try to call dds::sub::DataReaderListener::on_data_available. However, an application can force a call to the dds::sub::DataReader objects that have data by calling dds::sub::Subscriber::notify_datareaders.
Listeners can be set in the Entity constructor, or the set_listener()
function (for example, dds::sub::DataReader::set_listener()).
These functions expect a std::shared_ptr
. The Entity keeps a reference to the shared_ptr
so the listener cannot be deleted until the Entity is destroyed or the listener is reset with set_listener(nullptr)
.
entity.set_listener(nullptr)
or entity.close()
explicitly. The operations that are allowed in Listener callbacks depend on the rti::core::policy::ExclusiveArea QoS policy of the dds::core::Entity to which the Listener is attached – or in the case of a dds::pub::DataWriter of dds::sub::DataReader listener, on the rti::core::policy::ExclusiveArea QoS of the parent dds::pub::Publisher or dds::sub::Subscriber. For instance, the rti::core::policy::ExclusiveArea settings of a dds::sub::Subscriber will determine which operations are allowed within the callbacks of the listeners associated with all the DataReaders created through that dds::sub::Subscriber.
Note: these restrictions do not apply to builtin topic listener callbacks.
Regardless of whether rti::core::policy::ExclusiveArea::use_shared_exclusive_area is set to true or false, the following operations are not allowed:
An attempt to call a disallowed method from within a callback will result in dds::core::IllegalOperationError.
If rti::core::policy::ExclusiveArea::use_shared_exclusive_area is set to false, the setting which allows more concurrency among RTI Connext threads, the following are not allowed:
An attempt to call a disallowed method from within a callback will result in dds::core::IllegalOperationError.
The above limitations can be lifted by setting rti::core::policy::ExclusiveArea::use_shared_exclusive_area to true on the dds::pub::Publisher or dds::sub::Subscriber (or on the dds::pub::Publisher or dds::sub::Subscriber of the dds::pub::DataWriter or dds::sub::DataReader) to which the listener is attached. However, the application will pay the cost of reduced concurrency between the affected publishers and subscribers.
Note that all the issues described below are avoided by using dds::core::cond::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:
If the application needs to make a blocking call when data is available, or when another event occurs, the application should use dds::core::cond::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::sub::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::pub::DataWriter::wait_for_acknowledgments within the dds::sub::DataReaderListener::on_data_available() callback. This will lead to deadlock.