AWS RDS - Unable to connect to any of the specified MySQL hosts

Question

I am working on a web application that is using AWS RDS (MySQL). Everything works fine until I send 10/20 concurrent requests. When I start sending more concurrent requests (say 100/200), then RDS does not respond in timely manner and application throws errors -

MySql.Data.MySqlClient.MySqlException (0x80004005): Unable to connect to any of the specified MySQL hosts.
 ---> MySql.Data.MySqlClient.MySqlException (0x80004005): Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.
   at MySql.Data.Common.StreamCreator.GetTcpStream(MySqlConnectionStringBuilder settings)
   at MySql.Data.MySqlClient.NativeDriver.Open()
   at MySql.Data.MySqlClient.NativeDriver.Open()
   at MySql.Data.MySqlClient.Driver.Open()
   at MySql.Data.MySqlClient.Driver.Create(MySqlConnectionStringBuilder settings)
   at MySql.Data.MySqlClient.MySqlPool.CreateNewPooledConnection()
   at MySql.Data.MySqlClient.MySqlPool.GetPooledConnection()
   at MySql.Data.MySqlClient.MySqlPool.TryToGetDriver()
   at MySql.Data.MySqlClient.MySqlPool.GetConnection()
   at MySql.Data.MySqlClient.MySqlConnection.Open()
   at Microsoft.EntityFrameworkCore.Storage.RelationalConnection.OpenDbConnection(Boolean errorsExpected)
   at Microsoft.EntityFrameworkCore.Storage.RelationalConnection.Open(Boolean errorsExpected)
   at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider._TrackEntities[TOut,TIn](IEnumerable`1 results, QueryContext queryContext, IList`1 entityTrackingInfos, IList`1 entityAccessors)+MoveNext()
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)

At the same time, if I go and check the RDS cluster or database monitoring, I do not see any spikes in CPU usage or abnormal connections.

Can you provide any hint as to what should I investigate further to identify the root cause?

[Update 31/10/2022 12:30 PM] In the performance insights of the database, I can see a following graph that interests me:

Hello @Hiren Desai. Did you check the `max_connections` for your DB? https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/CHAP_Limits.html#:~:text=Description-,MariaDB%20and%20MySQL,-max_connections — Nuno Oliveira, Oct 31 '22 at 09:57
Yes, it's way more.. I'm using a R5 large size which has 4K limit. — Hiren Desai, Oct 31 '22 at 10:01
We have to see your code; there seems to be something wrong with your code. — Ermiya Eskandary, Oct 31 '22 at 20:26
Can you be specific which part and may be I can share the appropriate snippet — Hiren Desai, Nov 01 '22 at 10:36

AWS RDS - Unable to connect to any of the specified MySQL hosts

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