>[...] Right before this event started, one DNS Enactor experienced unusually high delays needing to retry its update on several of the DNS endpoints. As it was slowly working through the endpoints, several other things were also happening. First, the DNS Planner continued to run and produced many newer generations of plans. Second, one of the other DNS Enactors then began applying one of the newer plans and rapidly progressed through all of the endpoints. The timing of these events triggered the latent race condition. When the second Enactor (applying the newest plan) completed its endpoint updates, it then invoked the plan clean-up process, which identifies plans that are significantly older than the one it just applied and deletes them. At the same time that this clean-up process was invoked, the first Enactor (which had been unusually delayed) applied its much older plan to the regional DDB endpoint, overwriting the newer plan. The check that was made at the start of the plan application process, which ensures that the plan is newer than the previously applied plan, was stale by this time due to the unusually high delays in Enactor processing. [...]
It outlines some of the mechanics but some might think it still isn't a "Root Cause Analysis" because there's no satisfying explanation of _why_ there were "unusually high delays in Enactor processing". Hardware problem?!? Human error misconfiguration causing unintended delays in Enactor behavior?!? Either the previous sequence of events leading up to that is considered unimportant, or Amazon is still investigating what made Enactor behave in an unpredictable way.