Issue
Some time when user try to start a new Xevent session gets error msg 25740.
‘Unable to start event session ‘EventSession’ because system is busy. Please try again later.’
Investigation
That’s due to the limitation on number of the Xevent session that can be started on the same time in Azure SQL single database or Elastic pool cap on number of started XEvent sessions.
There is a cap on number of started XEvent sessions in Azure SQL Database:
- In a single Azure SQL Database, the limit is 100.
- In an elastic pool, the limit is 100 database-scoped sessions per pool.
In dense elastic pools, starting a new extended event session may fail due to memory constraints even when the total number of started sessions is below 100.
Mitigation and workaround
User can check sessions running on database using this T-SQL sys.dm_xe_database_sessions, so in order to mitigate this error user will need to confirm the active Xevent sessions and stop some of them or even close them all if not needed to free up a place for new sessions.
On the other hand, the user can always use other alternatives for monitoring the database.