Concurrency Control in Multilevel Relational Databases - Multilevel Security for Relational Databases - page 134

serializable executions of the data manipulation language operations.
The most common concurrency control models are two-phase-locking,
timestamp-ordering, and optimistic concurrency control models.
In the two-phase-locking (2PL) model, the data manipulation lan-
guage operation should need to have a write/read lock before it writes
or reads a data item [86].
In the timestamp-ordering model, a unique timestamp is assigned
to every data manipulation language operation and implements a read
timestamp and a write timestamp for each data item [87]. When a data
manipulation language operation is issued to read or write on the data
item, this operation is allowed only if the read or the write timestamp
of the data item is lower than the timestamp of the data manipulation
language operation; otherwise, the operation will be rejected.
In the traditional optimistic model, data manipulation language
operations are allowed to read and write on the data item without any
restriction.
Concurrency control is important for the multilevel relational data-
base because the covert channel problem can be found through the
overlap of the multilevel security operations [88]. In the multilevel
relational database, the concurrency control model should ensure
that the covert channel does not exist during the executions of the
operations at different levels of security. The covert channel problem
happens when a low classification level data manipulation language
operation is delayed or aborted by another high classification level
operation due to the need to access shared data items. So, by delaying
low classification level operations, high classification level informa-
tion can be indirectly known to the lower security level.
In the multilevel relational database, the following conflicts may
occur:
• Read-down conflict among different classification levels
• Read-write conflict at the same classification levels
• Write-write conflict at the same classification levels
Read-down conflict needs to be treated differently from the conflict
in relational multilevel security database systems because a relational
multilevel security database operation can read data in its classifica-
tion level and the classification level lower than it but can write data
only in its classification level.