Handling nested transactions

You can manage nested transactions in different parts of a program.

A program can become very complex if it contains a lot of nested functions calls, doing SQL processing within transactions. You may want to centralize transaction control commands in wrapper functions. The fgldbutl.4gl library contains special functions to manage the beginning and the end of a transaction with an internal counter, in order to implement nested function calls inside a unique transaction.

MAIN
  IF a() <> 0 THEN
       ERROR "..."
  END IF
  IF b() <> 0 THEN
       ERROR "..."
  END IF
END MAIN

FUNCTION a()
  DEFINE s INTEGER
  LET s = db_start_transaction()
  UPDATE ...
  LET s = SQLCA.SQLCODE
  IF s = 0 THEN
     LET s = b()
  END IF
  LET s = db_finish_transaction((s==0))
  RETURN s
END FUNCTION

FUNCTION b()
   DEFINE s INTEGER
   LET s = db_start_transaction()
   UPDATE ...
   LET s = SQLCA.SQLCODE
   LET s = db_finish_transaction((s==0))
   RETURN s 
END FUNCTION

In this example, you see in the MAIN block that both functions a() and b() can be called separately. However, the transaction SQL commands will be used only if needed: When function a() is called, it starts the transaction, then calls b(), which does not start the transaction since it was already started by a(). When function b() is called directly, it starts the transaction.

The function db_finish_transaction() is called with the expression (s==0), which is evaluated before the call. This allows you to write in one line the equivalent of the following IF statement:

IF s==0 THEN
   LET s = db_finish_transaction(1)
ELSE
   LET s = db_finish_transaction(0)
END IF