@Ashok Kumar T 
you prompt reply would be really appreciated.

@Ashok Kumar T  Thanks for sharing the details. could you please share the further granular
level mapping that maps individual source with Target such as Properties in the Medication class to HS.SDA3.Medication (property level mapping in the DTL. I completely understood you mapping at source level but i need to do each source level mapping in the following DTLs.
source.diagnosis to HS.SDA3.Diagnosis
source.Immunization to HS.SDA3.Vaccination 
source.Medication to HS.SDA3.Medication

@Eduard Lebedyuk  do you think this would the cause of this error? What i am thinking is the DB connection open and Close is not in sync. It tries to open the DB connection while it is not closed properly or not even closed.
Is there a way i can check the connection is open or not before initiating/opening it again or establishing the connection with an external DB?.