Question

I have a problem with subscribing the data (using the java platform). When a subscriber subscribes to a topic, that subscribed data must be removed from the DDS. But in my case whenever I subscribe to the data the same data is subscribed many times. The data is not removed from the DDS. I tried with QoS but I don’t know how to use it.

Please suggest how I can remove the read data from the DDS.

Answer 1

This behavior is not caused by your QoS settings, but by your method of accessing the DataReader. When you retrieve your data, you are probably calling something like the following read() in a loop:

FooReader.read(
    dataSeq, infoSeq, 10,
    ANY_SAMPLE_STATE.value,
    ANY_VIEW_STATE.value,
    ANY_INSTANCE_STATE.value);

The read() method invoked like this will return all currently available samples in your FooReader. After the read(), those samples still remain available in the FooReader, that is how the read() method behaves. Think of a read as a “peek”. The next time that you invoke the read() method in this way, you will see all samples that you saw before, unless they have been overwritten by a new update from a DataWriter.

To resolve your issue, you could replace the read() with a take(), like this:

FooReader.take(
    dataSeq, infoSeq, 10,
    ANY_SAMPLE_STATE.value,
    ANY_VIEW_STATE.value,
    ANY_INSTANCE_STATE.value);

The take() method is different from the read() method in that it does a destructive read; it not only reads the data but also removes it from FooReader. That way, you will never receive the same sample twice. In fact, if you consistently use take() as opposed to read(), you will never be able to see any sample twice.

Another way to resolve your issue is to stick with read(), but adjust the requested SAMPLE_STATE, from ANY to NOT_READ, like this:

FooReader.read(
    dataSeq, infoSeq, 10,
    NOT_READ_SAMPLE_STATE.value,
    ANY_VIEW_STATE.value,
    ANY_INSTANCE_STATE.value);

That way, you will only read samples that you have not read previously. The difference with take() in this case is that the data does remain available in your FooReader, which might be useful if you want to re-read it at a later stage (in which case you need to use the ANY sample state as opposed to NOT_READ to obtain previously read samples).