OpenEdge Data Management: DataServer for Microsoft SQL Server

Chapter 2: Initial Programming Considerations
This example illustrates how OpenEdge and MS SQL Server data source share locks
differ in scope and duration:
FIND customer WHERE customer.custnum EQ 10 SHARE-LOCK NO-WAIT NO-ERROR.
IF AVAILABLE customer THEN DO:
DISPLAY customer.
PROMPT-FOR customer.
tx:
DO TRANSACTION ON ERROR UNDO tx, RETRY tx:
FIND customer WHERE customer.custnum EQ 10
EXCLUSIVE-LOCK NO-WAIT NO-ERROR.
IF LOCKED customer THEN DO:
MESSAGE "customer locked - retrying".
UNDO tx, RETRY tx.
END.
ELSE
ASSIGN customer.
END.
END.
In this example, the first record is only share-locked within the MS SQL Server data
source if the isolation level setting requires it. (Recall that a SHARE–LOCK specified in an
ABL statement is ignored by the DataServer.) As a result, if the isolation level is read
uncommitted or read committed, the record might be updated by another client before
the second FIND statement executes. This could cause the record that the second FIND
statement fetches to be different from the record fetched by the first FIND statement.
This procedure might cause updated information to be lost because the procedure
applies updates based on the first find of the record, and these updates will overwrite
the values from the second find of the record.
Using the DataServer to access a MS SQL Server database ensures that locks are
upgraded in the data source in the same way as in an OpenEdge database. For
example, the following procedure causes the same behavior whether you access an
OpenEdge database or a MS SQL Server data source:
FIND customer WHERE customer.custnum EQ 10.
DISPLAY customer.
PROMPT-FOR customer.
DO TRANSACTION:
ASSIGN customer.
END.
The record is share-locked when it is fetched. The DataServer upgrades the share lock
to an exclusive lock inside the transaction by locking the record, reading it, and
checking whether the record has changed since it was first fetched. If it has changed,
the lock upgrade fails and you receive an error message.
You might have to wait to access a record under the following circumstances:
• You try to update a record when another user is reading it (it is share-locked). This
also depends on the isolation level.
• You try to read or update a record when another user is updating it (it is
exclusive-locked).
98 OpenEdge Data Management: DataServer for Microsoft SQL Server