Asynchronous WaitSet
Concept
The Asynchronous WaitSet (AsyncWaitSet) is a specialization of the WaitSet that performs the wait asynchronously using one or more separate threads of execution. Additionally, the AsyncWaitSet dispatches the attached and active conditions upon wakeup.
The AsyncWaitSet facilitates multi-threaded event processing. Your application can easily process the statuses generated by the domain entities and even combine them with application-specific events by means of GuardCondition.
To learn more about the AsyncWaitSet, refer to the Connext DDS API online documentation. (Eg. AsyncWaitSet in Modern C++ API).
Example Description
This example shows how to use the AsyncWaitSet to process Connext DDS and application-specific events. There are two applications supplied in this example: Publisher and Subscriber applications.
The interaction between them is basic. The Publisher sends samples (whose type is defined in AwsExample.idl) and the Subscriber reads the received samples and print their content in the standard output.
Both publisher and subscriber applications use an AsyncWaitSet to send and receive data respectively.
Publisher
The publisher application shows how to use an AsyncWaitSet to send data asynchronously in a single separate thread. The application controls when to send samples by means of a GuardCondition.
The publisher sends AwsExample samples periodically, increasing the member number in one unit for each sample sent. The samples are sent in the context of a single-threaded AsyncWaitSet, to which the application attaches the GuardCondition to drive the publication rate.
The application's main thread periodically triggers the GuardCondition so the AsyncWaitSet can dispatch it upon wakeup. The application installs a condition handler on the GuardCondition that simply writes a sample on the DataWriter.
Subscriber
The subscriber application shows how to use an AsyncWaitSet to process received samples using multiple-threads. The application processes the sample reception by means of the StatusCondition of a DataReader.
The subscriber simply reads samples as they are received. The application uses the StatusCondition of the DataReader to enable the DATA_AVAILABLE status and attach it to a multi-threaded AsyncWaitSet.
On new data available, the StatusCondition triggers and the AsyncWaitSet will wake up and use any of its available thread to dispatch the condition. The application installs a condition handler on the StatusCondition that simply reads the samples from the DataReader and prints their content.
To illustrate the concurrency of an AsyncWaitSet, the StatusCondition handler sleeps a random period of time after the read operation. This is purposely intended to emulate long processing time and keep the AsyncWaitSet's dispatching thread busy.
Additionally, the example also shows how a condition can be dispatched concurrently. The StatusCondition handler, right before the aforementioned sleep operation, calls AsyncWaitSet::unblock_condition() to indicate that the condition can be dispatched again if necessary. Then, if more samples arrive, the AsyncWaitSet will use any available thread to dispatch the StatusCondition again, causing a concurrent dispatch.