Remove read topic from DDS

data-distribution-service, publish-subscribe

Solution

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).

Problem

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.

Original source