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This wait type is when a thread is waiting for access to a critical section of code (one which only one thread can be executing) as part of reading or updating the list of availability group event subscribers.
(Books Online description: “The publisher for an availability replica event (such as a state change or configuration change) is waiting for exclusive read/write access to the list of event subscribers. Internal use only.”)
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Added in SQL Server version:
Removed in SQL Server version:
Extended Events wait_type value:
Maps to the HADR_ARPROXY_NOTIFICATION_SUBSCRIBER_LIST map_value in sys.dm_xe_map_values (thanks to Jonathan’s post here).
The map_key value in sys.dm_xe_map_values is 837 in 2012 and 856 in 2014 RTM. After 2014 RTM, you must check the DMV to get the latest value as some map_key values have changed in later builds.
I have not seen this wait type be a noticeable contention point.
Known occurrences in SQL Server (list number matches call stack list):
- Waiting for access to the event subscriber list (in this case, while doing routine sending of log blocks from the principal replica to secondary replicas)
- Waiting for access to the event subscriber list (in this case, while making some kind of change to the availability group, I think)
And other similar call stacks.
Abbreviated call stacks (list number matches known occurrences list):