TurboIMAGE/XL Database Management System Reference Manual (30391-90012)

98 Chapter4
Using the Database
Opening the Database
option, which can permit you to update the values of detail data set search and sort
items, is not available in this mode. Therefore, the access mode 2 user is able to read all
data entries and update some data entries, but is not permitted to add or delete data
entries in any data set.
The programmer must be aware of the possibility that other access mode 2 users are
simultaneously updating data entries. In many applications, it may be possible to
arrange for each user's process to update unique data entries or data items so that the
database will correctly reflect all changes, even data items in the same entry updated
by different processes. On the other hand, if two or more processes update the same
data items of the same entry, the database will reflect only the latest values. Locking
can be used, if desired, to coordinate update sequences to an entry or to coordinate with
access mode 6 readers.
• Access Mode 3. The database is opened for exclusive modify access. If any other users
are accessing the database, it cannot be opened in this mode. All TurboIMAGE/XL
procedures are available to the access mode 3 user. The critical item update (CIUPDATE)
option, which can permit you to update the values of detail data set search and sort
items, is available in this mode. No other concurrent process is permitted to gain any
type of access to the database.
• Access Mode 4. The database is opened for semi-exclusive modify access. Only one
access mode 4 user can access the database, and all other current users must be in
access mode 6 (read only). The access mode 4 user is permitted to call any
TurboIMAGE/XL procedure and has complete control over database content. The
critical item update (CIUPDATE) option, which can permit you to update the values of
detail data set search and sort items, is available in this mode. Other read-only users
are permitted concurrent access to the database in mode 4, but not in mode 3. Locking
can be used to coordinate with access mode 6 readers.
• Access Mode 5. The database is opened for shared read access. All other concurrent
users must be in access mode 1 or 5. Access mode 5 operates the same as access mode 1,
except that the DBUPDATE, DBPUT, and DBDELETE procedures, which alter the
database, are disabled for the access mode 5 user. Locking can be used, if desired, to
ensure that data is not being modified while you are reading it.
Access mode 5 is appropriate for inquiry-type applications if they can tolerate the
possibility of database modifications taking place simultaneously with access mode 1
users.
• Access Mode 6. The database is opened for shared read access. Concurrent users must
be in access mode 2, 4, 6, or 8. Access mode 6 can also be used while the database is
being stored with the TurboIMAGE/XL utility program DBSTORE. Some of these modes
are incompatible with each other as shown in the preceding discussion of concurrent
access modes. All TurboIMAGE/XL procedures that alter the database are disabled.
Locking can be used to synchronize with users who are concurrently updating.
Access mode 6 is appropriate for inquiry-type applications if they can tolerate the
possibility of database modifications taking place simultaneously with access mode 2
and 4 users.