What is java.sql.SQLException: Transient error, please try again

[jinju]

db.DbTransaction getConnection: null
java.sql.SQLException: Transient error, please try again.
	at com.google.appengine.api.rdbms.RdbmsApiProxyClient$ApiProxyBlockingInterface.makeSyncCall(RdbmsApiProxyClient.java:108)
	at com.google.appengine.api.rdbms.RdbmsApiProxyClient$ApiProxyBlockingInterface.openConnection(RdbmsApiProxyClient.java:71)
	at com.google.cloud.sql.jdbc.internal.SqlProtoClient.openConnection(SqlProtoClient.java:58)
	at com.google.cloud.sql.jdbc.Driver.connect(Driver.java:66)
	at com.google.cloud.sql.jdbc.Driver.connect(Driver.java:26)
	at java.sql.DriverManager.getConnection(DriverManager.java:620)
	at java.sql.DriverManager.getConnection(DriverManager.java:222)
	at db.DbTransaction.getConnection(DbTransaction.java:44)

I have used Servlet wise connection pooling in my application . What's the cause of java.sql.SQLException