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We have an AWS RDS PostgreSQL (v14.10) instance running on a db.t3.micro instance. Randomly throughout the day (some days more than others), our .NET Core application starts failing with the following exceptions:

An exception occurred while iterating over the results of a query for context type 'App.Server.Database.DbContext'.
System.InvalidOperationException: An exception has been raised that is likely due to a transient failure.
---> Npgsql.PostgresException (0x80004005): 53300: sorry, too many clients already

System.InvalidOperationException: An exception has been raised that is likely due to a transient failure.
---> Npgsql.NpgsqlException (0x80004005): Exception while reading from stream
---> System.TimeoutException: Timeout during reading attempt

We haven't been able to track down the root cause yet, thinking we have an application side connection leak somewhere, but that hasn't yielded any results.

With the t3.micro instance type, our max_connections is set to 81 (using this query select * from pg_settings where name='max_connections';), but we never see the DatabaseConnection metric in RDS monitoring get above 25-30. See the following screenshot of our metrics. We started throwing these errors just after 12:30 (see red arrow on right)

enter image description here

Has anyone see this issue before, or is there something else I should be looking into? Can anyone explain why the DatabaseConnections metric shows a count much lower than our max?

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  • The connection count could easily spike from 25 to over 81 within less than one monitoring interval. Turn on log_connections and log_disconnections so you can see what is actually happening.
    – jjanes
    Commented Aug 14 at 20:43

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