3.1.4. Conditions and Wait-sets

Conditions (in conjunction with wait-sets) provide an alternative mechanism to allow the middleware to notify communication status changes (including arrival of data) to the application.

This mechanism is wait-based. Its general use pattern is as follows:

  • The application indicates which relevant information it wants to get, by means of Condition objects (GuardCondition, StatusCondition, or ReadCondition) and attaching them to a wait-set.

  • It then waits on that wait-set until the trigger value of one or several Condition objects become true.

  • It then uses the result of the wait (i.e., the list of Condition objects with trigger_value == true) to actually get the information by calling:

    • get_status_changes and then get_<communication_status> on the relevant Entity, when the condition is a StatusCondition and the status changes refer to plain communication status.

    • get_status_changes and then get_datareaders on the relevant Subscriber, when the condition is a StatusCondition and the status changes refer to DataOnReaders.

    • get_status_changes and then read/take on the relevant DataReader, when the condition is a StatusCondition and the status changes refer to DataAvailable.

    • Directly read_w_condition/take_w_condition on the DataReader with the Condition as a parameter, when it is a ReadCondition

The first step is usually done in an initialization phase, while the others are put in the application main loop.

class ApplicationJob
{
    WaitSet wait_set_;
    GuardCondition terminate_condition_;
    std::thread thread_;

    void main_loop()
    {
        // Main loop is repeated until the terminate condition is triggered
        while (false == terminate_condition_.get_trigger_value())
        {
            // Wait for any of the conditions to be triggered
            ReturnCode_t ret_code;
            ConditionSeq triggered_conditions;
            ret_code = wait_set_.wait(triggered_conditions, eprosima::fastrtps::c_TimeInfinite);
            if (ReturnCode_t::RETCODE_OK != ret_code)
            {
                // ... handle error
                continue;
            }

            // Process triggered conditions
            for (Condition* cond : triggered_conditions)
            {
                StatusCondition* status_cond = dynamic_cast<StatusCondition*>(cond);
                if (nullptr != status_cond)
                {
                    Entity* entity = status_cond->get_entity();
                    StatusMask changed_statuses = entity->get_status_changes();

                    // Process status. Liveliness changed and data available are depicted as an example
                    if (changed_statuses.is_active(StatusMask::liveliness_changed()))
                    {
                        std::cout << "Liveliness changed reported for entity " << entity->get_instance_handle() << std::endl;
                    }

                    if (changed_statuses.is_active(StatusMask::data_available()))
                    {
                        std::cout << "Data avilable on reader " << entity->get_instance_handle() << std::endl;

                        FooSeq data_seq;
                        SampleInfoSeq info_seq;
                        DataReader* reader = static_cast<DataReader*>(entity);

                        // Process all the samples until no one is returned
                        while (ReturnCode_t::RETCODE_OK == reader->take(data_seq, info_seq,
                                                    LENGTH_UNLIMITED, ANY_SAMPLE_STATE,
                                                    ANY_VIEW_STATE, ANY_INSTANCE_STATE))
                        {
                            // Both info_seq.length() and data_seq.length() will have the number of samples returned
                            for (FooSeq::size_type n = 0; n < info_seq.length(); ++n)
                            {
                                // Only samples for which valid_data is true should be accessed
                                if (info_seq[n].valid_data)
                                {
                                    // Process sample on data_seq[n]
                                }
                            }

                            // must return the loaned sequences when done processing
                            reader->return_loan(data_seq, info_seq);
                        }
                    }
                }
            }
        }
    }

public:

    ApplicationJob(
            const std::vector<DataReader*>& readers,
            const std::vector<DataWriter*>& writers)
    {
        // Add a GuardCondition, so we can signal the processing thread to stop
        wait_set_.attach_condition(terminate_condition_);

        // Add the status condition of every reader and writer
        for (DataReader* reader : readers)
        {
            wait_set_.attach_condition(reader->get_statuscondition());
        }
        for (DataWriter* writer : writers)
        {
            wait_set_.attach_condition(writer->get_statuscondition());
        }
        
        thread_ = std::thread(&ApplicationJob::main_loop, this);            
    }

    ~ApplicationJob()
    {
        // Signal the GuardCondition to force the WaitSet to wake up
        terminate_condition_.set_trigger_value(true);
        // Wait for the thread to finish
        thread_.join();
    }
};

// Application initialization
ReturnCode_t ret_code;
std::vector<DataReader*> application_readers;
std::vector<DataWriter*> application_writers;

// Create the participant, topics, readers, and writers.
ret_code = create_dds_application(application_readers, application_writers);
if (ReturnCode_t::RETCODE_OK != ret_code)
{
    // ... handle error
    return;
}

{
    ApplicationJob main_loop_thread(application_readers, application_writers);

    // ... wait for application termination signaling (signal handler, user input, etc)

    // ... Destructor of ApplicationJob takes care of stopping the processing thread
}

// Destroy readers, writers, topics, and participant
destroy_dds_application();

Calling the wait operation on the wait-set will block the calling thread if the trigger value of all the conditions attached to it are false. The thread will wake up, and the wait operation will return OK, whenever the trigger value of any of the attached conditions becomes true.

3.1.4.1. GuardCondition

A condition for which the trigger value is completely controlled by the application via its set_trigger_value operation.

3.1.4.2. StatusCondition

A condition that triggers whenever there are changes on the communication statuses of an Entity.

The sensitivity of the StatusCondition to a particular communication status is controlled by the list of enabled_statuses set on the condition by means of the set_enabled_statuses operation.

3.1.4.3. ReadCondition

A condition that triggers whenever the DataReader that created it contains at least a sample with SampleState, ViewState, and InstanceState matching those of the ReadCondition.

The fact that the trigger value of a ReadCondition is dependent on the presence of samples on the associated DataReader implies that a single take operation can potentially change the trigger value of several ReadCondition conditions. For example, if all samples are taken, any ReadCondition associated with the DataReader that were triggered before, will see their trigger value changed to false. Note that this does not guarantee that WaitSet objects that were separately attached to those conditions will not be woken up. Once we have trigger_value == true on a condition, it may wake up the attached wait-set. The condition transitioning to trigger_value == false does not necessarily ‘unwakeup’ the wait-set, as ‘unwakening’ may not be possible in general. The consequence is that an application blocked on a wait-set may return from the wait with a list of conditions, some of which are no longer triggered. This is unavoidable if multiple threads are concurrently waiting on separate wait-set objects and taking data associated with the same DataReader entity.

To elaborate further, consider the following example: A ReadCondition that has a sample_state_mask = {NOT_READ} will have trigger_value == true whenever a new sample arrives and will transition to false as soon as all the newly-arrived samples are either read (so their status changes to READ) or taken (so they are no longer managed by the DataReader). However, if the same ReadCondition had a sample_state_mask = {READ, NOT_READ}, then the trigger_value would only become false once all the newly-arrived samples are taken (it is not sufficient to read them as that would only change the SampleState to READ which overlaps the mask on the ReadCondition).