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

94 Chapter4
Using the Database
Opening the Database
Database Control Blocks
TurboIMAGE/XL executes using data stored in different types of control blocks stored in
privileged mapped files or virtual data objects: the Database System Control Block (DBS),
the Database Globals Control Block (DBG), the Database Buffer Area Control Block
(DBB), the Database User Local Control Block (DBU), the Remote Database Control Block
(DBR), Database User Local Index Control Block (DBUX), the Lock Table (TURBOLKT),
Global Dynamic Multi-database Transaction Table (TURBOGTX), DBQUIESCE and DBOPEN
Table (QOPEN), and DBQUIESCE Lock Table (QLOCK). These are described below:
• The Database System Control Block (DBS) is created by DBOPEN if it does not already
exist. The DBS is used as a system-wide table to locate the current Database Globals
Control Block (DBG) for any opened database. Each system has only one DBS, created
as a permanent file called TURBODBS in the PUB group and the SYS account on that
system.
• The Database Globals Control Block (DBG) is created for a particular database when
the first user's process calls the DBOPEN procedure to open the database. The DBG
contains global information required by TurboIMAGE/XL intrinsics during run-time,
including a pointer to the Database Buffer Area Control Block (DBB). Each open
database has exactly one DBG regardless of the number of concurrent access paths to
the database. All TurboIMAGE/XL procedures on a particular database (except
DBERROR and DBEXPLAIN) reference the DBG. In addition, the DBG contains the lock
table which holds user-level locking information. The DBG is purged when the last
user's process closes the database (DBCLOSE).
• The Database Buffer Area Control Block (DBB) is created for a particular database
when the first user's process calls the DBOPEN procedure to open the database. The DBB
contains a set of buffer headers which point to data in memory from any of the data
sets, and contains a pointer to the DBG. Global information regarding logging and
recovery is also contained within the DBB. The DBB is used to retrieve, log, and update
data located in the data set files on disk. The DBB is purged when the last user closes
the database (DBCLOSE).
• One Database User Local Control Block (DBU) is created each time a user's process
successfully calls DBOPEN. Each DBU contains information about the user's individual
access to the database and contains pointers to the DBS, DBG, and DBB. The privileged
mapped file containing the DBU is associated with this DBOPEN. The DBU is purged
when the corresponding DBCLOSE closes the database. A process can open a maximum
of 127 databases (or one database 63 times), depending upon the system resources;
therefore, a maximum of 127 DBUs can be created. It is recommended that a process
close a database once it is no longer needed for that process.
• One Remote Database Control Block (DBR) is created on the local system each time a
user's process successfully opens a remote database. The DBR contains database set
and item information as well as the work areas necessary to set up communications to
the remote computer.
• The Database User Local Index Control Block (DBUX) is created the first time the
user's process calls DBOPEN. One DBUX exists for each user's process. Its purpose is to
keep track of the addresses of all the DBUs and/or DBRs for that process. Because a
maximum of 127 entries are allowed in the DBUX, each process is allowed 127 DBOPENs