OpenEdge Data Management: DataServer for Microsoft SQL Server

Chapter 2: Initial Programming Considerations
Table 18 shows the possible –Dsrv TXN_ISOLATION,n values with the respective
meaning.
Table 18: TXN_ISOLATION values in the –Dsrv parameter
Share locks
The default isolation level for the MS SQL Server DataServer is read uncommitted. At
this level, a SHARE-LOCK and NO-LOCK are identical from the perspective of the
DataServer and the MS SQL Server data source. The higher isolation levels will
determine what kind of share locks will take effect. In MS SQL Server, a repeatable
read and serializable isolation level are synonymous.
The MS SQL Server DataServer ignores ABL SHARE–LOCK option when used in ABL
statements. Instead, share locks are governed by the data source and the available
ODBC isolation levels. If you wish to change the share lock behavior, you may be able
to do so by changing the isolation level at connection time using the -Dsrv parameter.
When you read records with an ABL statement, regardless of whether you include the
SHARE–LOCK option, the MS SQL Server data source typically performs as follows:
• It puts some form of share lock on the record, page, or table if the ODBC isolation
level is anything other than read uncommitted. This occurs regardless of whether
the share lock is specified in an ABL statement.
• After the data source reads the record, it releases the share lock if the isolation
level is read uncommitted or read committed. It may hold share locks until the
completion of a transaction if the isolation level is repeatable read or serializable.
If you hold a record with a share lock, other users can usually access that record and
apply a share lock, but this is dependent on the isolation level they have selected. Refer
to the transaction and locking references in the Microsoft documentation that
addresses ODBC programming or data source reference manuals for more
information.
Exclusive locks
Value Meaning
1 Read uncommitted (default)
2 Read committed
4 Repeatable read
8 Serializable
When you update, delete, or create a record, MS SQL Server puts an exclusive lock
on the record; however, the data source does not apply the exclusive lock to a record
until all share locks on it are released. Therefore, you cannot perform an update on a
record until other users release it. If a record has an exclusive lock on it, no other user
can access it until it is released at the end of a transaction. In a OpenEdge transaction
block, the data source always holds an exclusive lock until the end of a transaction’s
scope if the data source driver supports commitment control boundaries and the ODBC
AUTOCOMMIT feature is not turned on.
94 OpenEdge Data Management: DataServer for Microsoft SQL Server