iX V6.5 (30391-90011)

Chapter 7 315

Logging and Recovery

Logical Transactions

Figure 7-1. illustrates the concept of a logical transaction using a static and a dynamic

transaction. Note that a transaction block is also illustrated. A transaction block

consists of all transactions between a call to DBOPEN and a call to DBCLOSE. For further

information about transaction blocks, refer to "FILE Command" later in this chapter.

Figure 7-1. Transactions and Transaction Blocks

Locking Requirements for Logical Transactions

DBRECOV requires that all multiple-intrinsic database transactions execute independently

of all other transactions. Transaction independence within the database can be ensured in

a user program by locking data before a transaction and then releasing locks after a DBEND

or DBXEND is called, thus eliminating the possibility of another user modifying the same

data at the same time. The following example may clarify the need for locking data to be

modiﬁed.

Suppose transaction A consists of adding two records to the database that are later

modiﬁed by transaction B. Transaction B is dependent upon transaction A, because the

records must exist before they can be modiﬁed. Recall that a transaction is deﬁned as a

sequence of one or more modiﬁcations that transfer the database from one consistent state

to another. A database may be in an inconsistent state during a transaction. Therefore, if

transactions A and B are executing concurrently without locking, transaction B may be

viewing the database in an inconsistent state and consequently could be generating

invalid results. However, if transaction A locks the data and completes properly, this

problem is avoided because transaction B cannot access the data until transaction A has

released its locks.

A second problem due to inadequate locking affects suppression of transactions by the

recovery system (see Figure 7-2.) Suppose transaction A intends to add six records to the

database, and after adding three records, transaction B is executed by another process.

Transaction B concurrently modiﬁes one of the records added by transaction A and then

completes. Suppose that at this time, the system fails and recovery is executed. Because