DataReader Use Cases

Working with data readers.

Working with data readers.

Setting up a data reader

• Set up subscriber
• Set up a topic
• Create a data reader, FooDataReader, of user data type Foo:

  struct DDS_DataReaderQos reader_qos = DDS_DataReaderQos_INITIALIZER;
  DDS_DataReader* reader;
  struct DDS_DataReaderListener reader_listener =
      DDS_DataReaderListener_INITIALIZER;
  DDS_ReturnCode_t retcode;
   
  /* MyReaderListener_* functions are user defined to match
     DDS_ReaderListener functions */
  reader_listener.on_requested_deadline_missed = 
      MyReaderListener_RequestedDeadlineMissed;
  reader_listener.on_requested_incompatible_qos = 
      MyReaderListener_RequestedIncompatibleQos;
  reader_listener.on_sample_rejected = MyReaderListener_SampleRejected;
  reader_listener.on_liveliness_changed =
      MyReaderListener_LivelinessChanged;
  reader_listener.on_data_available = MyReaderListener_DataAvailable;
  reader_listener.on_subscription_match =
      MyReaderListener_SubscriptionMatch;
  reader_listener.on_sample_lost = MyReaderListener_SampleLost;
   
  retcode = DDS_Subscriber_get_default_datareader_qos(
              subscriber, &reader_qos);
   
  if (retcode != DDS_RETCODE_OK) {
      printf("***Error: failed to get default datareader qos\n");
  }
   
  reader = DDS_Subscriber_create_datareader(subscriber, 
              DDS_Topic_as_topicdescription(topic), 
              &reader_qos, 
              &reader_listener /* or NULL */,
        DDS_STATUS_MASK_ALL);
   
  if (reader == NULL) {
      printf("***Error: failed to create reader\n");
  }

Managing instances

• Given a data reader

  FooDataReader* reader = ...;

• Getting an instance "key" value of user data type Foo

  struct Foo* data = ...;     /* user data of type Foo */
   
  retcode = FooDataReader_get_key_value(reader, data, &instance_handle);
   
  if (retcode != DDS_RETCODE_OK) {
      /* ... check for cause of failure */
  }    

Set up reader to access received data

• Set up data reader
• Set up to handle the DDS_DATA_AVAILABLE_STATUS status, in one or both of the following two ways.
• Enable DDS_DATA_AVAILABLE_STATUS for the DDS_DataReaderListener associated with the data reader
  • The processing to handle the status change is done in the DDS_DataReaderListener_on_data_available() method of the attached listener.
  • Typical processing will access the received data.
• Enable DDS_DATA_AVAILABLE_STATUS for the DDSStatusCondition associated with the data reader
  
  • The processing to handle the status change is done when the data reader's attached status condition is triggered and the DDS_DATA_AVAILABLE_STATUS status on the data reader is changed.
  • Typical processing will access the received data.

Access received data via a reader

• Ensure reader is set up to access received data
• Access the received data, by either:
  • Taking the received data in the reader, OR
  • Reading the received data in the reader

Taking data

• Ensure reader is set up to access received data
• Take samples of user data type T. The samples are removed from the Service. The caller is responsible for deallocating the buffers.

  DDS_SampleStateMask   sample_state_mask = DDS_ANY_SAMPLE_STATE;
  DDS_ViewStateMask     view_state_mask = DDS_ANY_ VIEW_STATE;
  DDS_InstanceStateMask instance_state_mask = DDS_ANY_INSTANCE_STATE;
  struct FooSeq data_seq = FooSeq_INITIALIZER;
  struct DDS_SampleInfoSeq info_seq = DDS_SampleInfoSeq_INITIALIZER;
   
  DDS_ReturnCode_t retcode;
   
  retcode = FooDataReader_take(reader, &data_seq, &info_seq, 
                               max_samples,
                               sample_state_mask, 
                               view_state_mask, 
                               instance_state_mask); 
   
  if (retcode == DDS_RETCODE_NO_DATA) {
      return;
  } else { if (retcode != DDS_RETCODE_OK) {
      printf("***Error: failed to access data from the reader\n");
  }

• Use the received data

  struct Foo* data = NULL;
  struct DDS_SampleInfo* info = NULL;
  int i;
   
  for(i = 0; i <  FooSeq_get_length(&data_seq); ++i) {
      info = DDS_SampleInfoSeq_get_reference(&info_seq, i);
      data = FooSeq_get_reference(&data_seq, i);
      /* Note that depending on the info->sample_state
        it is possible that data will be NULL
       */
      /* ... */
  }

• Return the data samples and the information buffers back to the middleware. IMPORTANT: Once this call returns, you must not retain any pointers to any part of any sample or sample info object.

  DDS_ReturnCode_t retcode;
  retcode = FooDataReader_return_loan(reader, &data_seq, &info_seq);
   
  if (retcode != DDS_RETCODE_OK) {
      printf("***Error: failed to return loan\n");
  }

Reading data

• Ensure reader is set up to access received data
• Read samples of user data type Foo. The samples are not removed from the Service. It remains responsible for deallocating the buffers.

  DDS_SampleStateMask   sample_state_mask = DDS_ANY_SAMPLE_STATE;
  DDS_ViewStateMask     view_state_mask = DDS_ANY_ VIEW_STATE;
  DDS_InstanceStateMask instance_state_mask = DDS_ANY_INSTANCE_STATE;
  struct FooSeq data_seq = FooSeq_INITIALIZER;
  struct DDS_SampleInfoSeq info_seq = DDS_SampleInfoSeq_INITIALIZER;
   
  DDS_ReturnCode_t retcode;
   
  retcode = FooDataReader_read(reader, &data_seq, &info_seq, 
                               max_samples,
                               sample_state_mask, 
                               view_state_mask, 
                               instance_state_mask); 
   
  if (retcode == DDS_RETCODE_NO_DATA) {
      return;
  } else if (retcode != DDS_RETCODE_OK) {
      printf("***Error: failed to access data from the reader\n");
  }

• Use the received data

  struct Foo* data = NULL;
  struct DDS_SampleInfo* info = NULL;
  int i;
   
  for(i = 0; i <  FooSeq_get_length(&data_seq); ++i) {
      info = DDS_SampleInfoSeq_get_reference(&info_seq, i);
      data = FooSeq_get_reference(&data_seq, i);
      /* Note that depending on the info->sample_state
        it is possible that data will be NULL
       */
      /* ... */
  }

• Return the data samples and the information buffers back to the middleware

  retcode = FooDataReader_return_loan(reader, &data_seq, &info_seq);
   
  if (retcode != DDS_RETCODE_OK) {
      printf("***Error: failed to return loan\n");
  }

Tearing down a data reader

• Delete DDS_DataReader:

  DDS_ReturnCode_t retcode;
   
  retcode = DDS_Subscriber_delete_datareader(subscriber, reader);
   
  if (retcode != DDS_RETCODE_OK) {
      printf("***Error: failed to delete reader\n");
  }