DDSIRTP26 — DDSI-RTPS should specify explicitly how to pick between two "equal" Writers.

There are a number of scenarios in which the DataReader should pick between samples belonging to the same instance that are considered "equal". These scenarios are:

• Samples originate from two different EXCLUSIVE_OWNERSHIP Writers that have equal ownership_strengh.
• Samples originate from two different SHARED_OWNERSHIP, BY_SOURCE_TIMESTAMP Writers, and have equal source_timestamps.

In these scenarios the Reader should pick one Writer over the other, but the DDS Core spec doesn't say how this decision should be made: it just mentions that whatever you pick, it should be picked consistently throughout the system,

However, in case of interoperability between different vendors, there is no way to guarantee a consistent pick between different vendors as long as the algorithm used to make the pick isn't clearly specified.

Typically vendors use the GUID of the Writers to make a consistent pick throughout the system, but the order between different GUIDs is not described anywhere and different vendors could make opposite choices, resulting in the fact that eventual consistency in your system might never be reached.

So although it makes sense to use Writer GUIDs to enforce consistent picks, the algorithm to follow cannot be described in the DDS core specification since it doesn't specify the semantics of a a GUID. That means that the only logical place to provide such a mechanism is in the DDSI-RTPS specification.