TurboIMAGE/XL Database Management System Reference Manual MPE/iX V6.5 (30391-90011)

Chapter 4 111
Using the Database
Updating Data
Updating Data
TurboIMAGE/XL allows you to change the values of data items if the user class number
with which you opened the database grants this capability to you. These items cannot be
master data set key items or detail data set search or sort items, unless you take
advantage of the critical item update (CIUPDATE) option. Depending on the CIUPDATE
option settings for the database and the current process, you could change the values of
detail data set search and sort items if the database access mode is 1, 3, or 4. This option is
described later in this section.
Before you call the DBUPDATE library procedure to change the item values, you must call
DBGET to locate the entry you intend to update. This sets the current record address for the
data set. The DBUPDATE library procedure uses the current record address to locate the
data items whose values are to be changed.
A lock can be established before the call to DBGET to guard against accidental modification
of the record by another user. This is recommended in any shared access mode (as
discussed below).
When the program calls DBUPDATE, it specifies the data set name, a list of data items to be
changed, and the name of a buffer containing values for the items. For example, if a
program changes the street address of a customer in the CUSTOMER data set of the
ORDERS database, the program can first locate the entry to be changed by calling DBGET
in calculated access mode with the customer's account number and then calling the
DBUPDATE procedure to change the value of the STREET-ADDRESS data item in that
entry.
Access Modes and User Class Number
To update data items, the database must be opened in access mode 1, 2, 3, or 4. If it is
opened in access mode 1, the data entry, data set, or database must be locked while the
update is occurring. If the CIUPDATE option settings for the database and the current
process permit updates of detail data set search and sort items, the database must be
opened in access mode 1, 3, or 4 to take advantage of this option.
TurboIMAGE/XL guarantees that all updates to a data entry will be carried out even if
they are requested by different users concurrently and locking is not used. To ensure this,
TurboIMAGE/XL always completes the processing of one DBUPDATE request before it
begins processing under another. However, data consistency problems can still occur if an
update is based on data values that are not current. For example, while withdrawing 10
items from the stock, two users may read the same data entry from the INVENTORY data
set. If the current value of ONHANDQTY is 30 and they each subtract 10 from it and then
update the entry, both updates will operate successfully but the new value will be 20
rather than 10. To prevent such errors, a lock covering the data entry can be put in effect
before it is read and released after it is updated.
TurboIMAGE/XL attempts to enforce this locking technique for users in database access
mode 1 by checking to see if an appropriate lock is in effect before executing an update.
However, to have its proper effect, the lock should be made before the call to DBGET.