Recently we migrated one of our application from one VPS server to another. Initially, everything was fine. After a day or so here comes a strange error.

Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached.

The connection string doesn't have any custom value set for Max Pool Size. So the maximum pool size for the application was the default, i.e 100. To check whether the DB have this much open connection, we run the following query in the SQL Management Studio,

    DB_NAME(dbid) as DBName, 
    COUNT(dbid) as NumberOfConnections,
    loginame as LoginName
    dbid > 0
    dbid, loginame

This will list all the DB's and its number of open connections grouped by login name. For us, the number of open connection for the DB was ~100 that's why the error. So we updated the connection string to have Max Pool Size=200. And the application worked fine. Sample connection string with Max Pool Size given below,

<add connectionString="Server=YOUR_SERVER;Database=YOUR_DB_NAME;User Id=YOUR_DB_USER_ID;Password=YOUR_DB_USER_PASSWORD;Max Pool Size=200;" name="Default" providerName="System.Data.SqlClient" />

Few things to remember in this kind of situations are,

  • In your code, make sure all the connections are closed when finished usage.
  • Don't set Max Pool Size to a very large value unless it is necessary.

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  • Abhith


    Abhith Rajan is a full-stack developer, consultant, Sysberries employee, and biker. Who passionate about C# and JavaScript. Works on his blog, his personal finance web app and its react native mobile app in his free time.

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