DDS — DDS ISSUE# 31] Topic QoS refactor

Ref-86 Topic_qos_refactor and Ref-150
Tracking_of_topic_properties_by_reader_writer

The reason for the DDS specification to add QoS to the Topic was to
allow annotating the information-model with Topic QoS settings. That
way it is possible for the individual applications and their designers
to be relieved from many details and configure the writers/readers
from this information model.

However the definition of the Topic QoS does not match this.

Section 2.1.2.4.3.2 describes a model where if the QoS is 'not set' on
a dataWriter or a DataReader, then the one of the Topic is used
instead. Furthermore it says that in this situation the QoS of the
Entity will "track" that of the Topic.

These statements are not consistent with the PIM or PSM as there is no
way provided to 'not set' a QoS. All the API's force complete setting
of the QoS.

Furthermore the 'tracking' behavior would be very hard to implement
and it would also be very confusing for an user that has a QoS of an
entity magically change when the QoS of the topic changes. Lastly it
would also be hard to handle the case where as a result of the
'tracking' the QoS of an entity becomes incompatible with that of
other entities already associated with it.

Given all this it would be desirable to make the use of Topic QoS
consistent with the API's and also simpler to implement and understand

**PROPOSAL**

Modify section 2.1.2.4.3.2.

Remove all references to behaviors regarding what happens if some QoS
is "not set" rather say that QoS is always explicitly set although it
can be set from defaults in the factories.

Also remove the sentence regarding how the entity QoS can "track" the
Topic qos.

Explain that the pattern to create an entity with "default" QoS is to
get the default qos from the factory, and also get it from the Topic,
and then modify the desired policies before creating the entity.

To assist this common pattern we recommend adding the following
utility operations:

Publisher::initialize_from_topic_qos(inout DataReaderQos qos, in Topic
a_topic, in long mask)

Subscriber::initialize_from_topic_qos(inout DataWriterQos qos, in
Topic a_topic, in long mask)

Specify that all QoS on Topic is immutable.

see below