Transactional Isolation - Transaction Processing - page 122

degree 2 consistency in addition it does not read dirty data of other transactions, and
degree 3 consistency if in addition other transactions do not dirty any data read
by T before T terminates. In our transaction models, degree 1 is the minimum
level required: each transaction commits all its writes only at the end of the
transaction and dirty writes are disallowed. Dirty reads are disallowed at degree 2
and unrepeatable reads at degree 3.
Our definitions for the isolation levels follow from those defined by Berenson
et al. [ 1995 ], who observe that, to ensure that transaction rollback is possible under
all circumstances, the definitions of the isolation levels in the SQL 1992 standard
should be corrected such that dirty writes are disallowed at all levels. Besides the
basic three isolation anomalies, Berenson et al. [ 1995 ] also analyze other isolation
concepts, including snapshot isolation, which is possible to enforce in systems that
do transient versioning (to be discussed in Chap. 12 ). A thorough analysis of the
isolation concepts can also be found in the textbook by Gray and Reuter [ 1993 ].
Much of the classical theory of concurrency control of transactions is based on
the notion of conflict-serializability, using a read-write transaction model in which
the undo actions included in transaction rollback are not modeled explicitly. This
approach was also adopted in many textbooks on transaction processing, such as in
the classic works by Papadimitriou [ 1986 ]andBernsteinetal.[ 1987 ]. Schek et al.
[ 1993 ] and Alonso et al. [ 1994 ] develop the theory using a transaction model in
which also undo actions are represented explicitly. This latter approach has been
adopted in the textbook by Weikum and Vossen [ 2002 ].