When the time is right, old Nexus instances need to hand off to the new ones.

• "The time is right" is probably a planner step that looks at inventory and determines that this is the next step in the update.
• When we've made this determination, old Nexus instances quiesce themselves and check on whether all other old Nexus instances are quiesced.
• Once an old Nexus instance determines that all of them are quiesced in this way, it invokes the db-handoff API on the new Nexus instances
  • EDIT: in the comment below, this handoff may actually be implicit, by reading the DB? It may not need to be an explicit handoff API