Retrieve instance key after unregister

Hello,

Strictly speaking the DataReader is not notified of "unregistrations". It is notified that an Instance has "No Alive DataWriters". If the DataWriter that unregistered the instance was the only DataWriter that was writing that instance (a common case), then the "unregistration" will cause the instance to have no "alive datawriters" and you will get the notification. Note that other situations can also cause this. For example if the DataWriter simply dissapears even if it does not explicitly unregister the instance.

The DataReader receives this notification via a Sample (as if regular data had arrived) except that the DDS_SampleInfo specifies that there is no "data" (i.e. SampleInfo::valid_data is false), and also the SampleInfo::instance_state is set to DDS_NOT_ALIVE_NO_WRITERS_INSTANCE_STATE.  When this happens the SampleInfo::instance_handle holds a reference to the Instance and you can get the associated key calling get DataReader::get_key_value() operation on the DataReader and passing that instance_handle. 

Gerardo

Hello Roderick,

The way the documentation is written is a litlle bit confusing. The get_key_value will still work even if the instance has been unregistered as long as the "unregistration" sample still lives in the DataReader cache, which it does until after " take" it. So if you are reacting to the reception of the notification when you read/take the sample you are holding a valid instance_handle to a sample still in the DataReader cache so it all works.

What would not work--which is what the documentation was trying to say--would be this: Imagine you have the instance_handle stored somewhere in your application code, and then after the instance has been unregistered and the application has called " take" on that sample and returned from the take operation. Then if at a future point in time you were to just call get_key_value() using that instance handle you kept in your application memory the get_key_value() operation would fail.

A side, but perhaps important, note: Depending how you set your DataWriter WRITER_DATA_LIFECYCLE Qos Policy you may get a DDS_NOT_ALIVE_DISPOSED_INSTANCE_STATE instead of a DDS_NOT_ALIVE_NO_WRITERS_INSTANCE_STATE. I have seen this happen if you set the autodispose_unregistered_instances to TRUE and I have also seen this be the default Out-of-the box setting in some version of the product. I consider this a bit quirky so we may change it in future versions but it is something to watch out in your code.

Gerardo