SQL Server Connection Pooling (ADO.NET)
https://msdn.microsoft.com/en-us/library/8xx3tyca(v=vs.110).aspx
http://www.codeproject.com/Articles/17768/ADO-NET-Connection-Pooling-at-a-Glance
http://www.cnblogs.com/delphinet/archive/2010/09/29/1838494.html
http://www.studyofnet.com/news/637.html
http://www.codeproject.com/Articles/46267/Connection-Pooling-in-ASP-NET