DDS15 — Add restrictions and guidelines that are required when writing GROUP coherent sets

RTPS 2.4 added support for GROUP ordered access and coherent access. As part of this effort, a number of restrictions and guidelines were identified that should be added to DDS in order to properly support these features.

The restrictions include:

• Writers shall not be created and/or deleted from the time a GROUP Publisher called begin_coherent_changes() to end_coherent_changes()
• Readers belonging to a subscriber with PRESENTATION access_scope=GROUP must all have the same value for DESTINATION_ORDER, RELIABILITY?, DURABILITY, OWNERSHIP
• Writers belonging to a Publisher with PRESENTATION access_scope=GROUP should have the same value for OWNERSHIP, DURABILITY, RELIABILITY?, DESTINATION_ORDER?
  • With OWNERSHIP: kind (i.e. all SHARED or all EXCLUSIVE) and in the case they are exclusive all Writers must have value of the the same OWNERSHIP_STRENGTH.

• All samples in the coherent set should have the same reception_timestamp and source_timestamp:
  • If access_scope = GROUP the reception and source timestamp are equal to that of the EndCoherentChanges sample.
  • If access_scope < GROUP the reception and source timestamp correspond to the ones from the last sample in the coherent set which may or may not be the EndCoherentChanges sample.

Guidelines:

• Adding a note for users that it may often be required to change the HISTORY kind/depth from KEEP_LAST,1. This is because a publisher that is quickly publishing coherent sets which update the same instances could very easily invalidate a coherent set that has not been fully delivered to the subscribers. Replacing a sample in history that is part of a coherent set invalidates that coherent set. It may be worth adding a callback that is called when a writer removes a coherent set from its queue
  • In general, writers that are part of a GROUP Publisher should be configured homogeneously.