Consultor Eletrônico

Status: Unverified

GOAL:

How to Implement an Optimistic Locking Strategy?

GOAL:

KB21500

FACT(s) (Environment):

Progress 8.x

FACT(s) (Environment):

Progress 9.x

FIX:

When developing a 4GL application for deployment in a multi-user
environment, you will want to design a record locking strategy that
avoids keeping records locked unnecessarily for long periods of time.
The Optimistic Locking Strategy is recommended to accomplish this
objective. Optimistic Locking is a strategy whereby records are
initially accessed using NO-LOCK, with an EXCLUSIVE-LOCK used
afterwards for a very short period of time.

This example runs 2 sessions against the Sports2000 Database.
In the first session, procedure 1 accesses a record with
EXCLUSIVE-LOCK. The second session will perform the same process
using Optimistic Locking Strategy instead.


/* procedure 1 */
PROMPT-FOR customer.custnum.

FIND customer USING INPUT customer.custnum
EXCLUSIVE-LOCK NO-ERROR NO-WAIT.
IF AVAILABLE customer THEN DO:
DISPLAY Customer.custnum customer.NAME WITH 1 COLUMN.
UPDATE customer.NAME.
END.
PAUSE.

/* procedure 2 -- Optimistic Locking Example */

PROMPT-FOR customer.custnum.
FIND customer USING INPUT customer.custnum NO-LOCK NO-ERROR NO-WAIT.
IF AVAILABLE customer THEN
DISPLAY Customer.custnum customer.NAME WITH 1 COLUMN.
PAUSE.
MESSAGE "Do you wish to change this record?"
VIEW-AS ALERT-BOX QUESTION BUTTONS YES-NO UPDATE upd AS LOGICAL.

IF upd THEN DO:
FIND CURRENT customer no-LOCK NO-ERROR NO-WAIT.

IF AVAILABLE customer THEN DO:
IF CURRENT-CHANGED customer THEN DO:
DISPLAY Customer.custnum customer.NAME WITH 1 COLUMN.
Assign customer.NAME.
END.
ELSE DO:
Assign customer.NAME.
END.
END.
ELSE IF NOT AVAILABLE customer THEN DO:
IF LOCKED customer THEN DO:
MESSAGE "This Record is Locked by another user"
VIEW-AS ALERT-BOX.
END.
ELSE DO:
MESSAGE "This record has been deleted" VIEW-AS ALERT-BOX.
END.
END.
END.