Meeting Room Random Free/Busy Error MapiExceptionRpcServerTooBusy

The other day I was faced with an issue where one or more meeting rooms would randomly not show their free/busy information depending on how many meeting room I was querying at the same time in Outlook. If using Outlook on the Web (aka OWA), a few rooms would not have free/busy information available.

After some digging, I found the following two events logged in the Exchange server (the output has been truncated):

Log Name:      Application
Source:        MSExchange Availability
Event ID:      4009
Task Category: Availability Service
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      EXAIO
Description:
Process Microsoft.Exchange.InfoWorker.Common.Delayed`1[System.String]: Unable to open connection for mailbox <IT - 01B - Small CF>SMTP:[email protected] Exception returned is: Microsoft.Exchange.Data.Storage.StorageTransientException: Cannot query rows in a table. ---> Microsoft.Mapi.MapiExceptionRpcServerTooBusy: MapiExceptionRpcServerTooBusy: Unable to query table rows.

 

Log Name:      Application
Source:        MSExchange Availability
Event ID:      4009
Task Category: Availability Service
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      EXAIO
Description:
Process Microsoft.Exchange.InfoWorker.Common.Delayed`1[System.String]: Unable to open connection for mailbox <HR - 04B - Small CF>SMTP:[email protected] Exception returned is: Microsoft.Exchange.Data.Storage.StorageTransientException: Cannot open mailbox /o=ADIAEXCHANGE/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Configuration/cn=Servers/cn=EXAIO/cn=Microsoft System Attendant. ---> Microsoft.Mapi.MapiExceptionRpcServerTooBusy: MapiExceptionRpcServerTooBusy: Unable to make connection to the server.

The problem was that the RPC Pool limit was being reached. For each database we can have up to 32 concurrent RPC connections from the same client process. In my case, most meeting rooms were distributed across mainly two databases:

Count Name
----- ----
   83 MDB01
   51 MDB02
    5 MDB03
    3 MDB04
    7 MDB05
    5 MDB06
    4 MDB07
    1 MDB08
    1 MDB09

As you probably guessed, the resolution is to simply split meeting rooms across all databases (when possible). Once I distributed them so there is only 17/18 meeting rooms per database, this error no longer happened.

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