Database Reference
In-Depth Information
The starvation of the high security class level transactions can be
prevented by disallowing them to be reexecuted to read the new ver-
sions generated by the low security class level transactions to save the
consistency of the data when existing in the invisible area.
A serializable schedule can be presented for the multilevel transac-
tions by making the scheduler a one-copy serializable and one-copy
serial. The high classification level transaction will resume reading the
data that have been stored in the database.
The proof of the one-copy serializability of the enhanced secure
multiversion concurrency control model will be described as follows:
• Definition 1: a multiversion history H on T and a datum
X. A version order for X is a total order for all versions of X
in H. We define the version order by < and write X i < X j if
the version X i precedes X j in the version order. H = {Σ T , < T }
where:
1. Σ T = U i Σ i for i = 1,……, n.
2. < T = U i < i for i = 1,……, n.
3. For any conflicting operations O i , O j in Σ T , either O i < T O j |
O j < T O i .
• Definition 2: H is a multiversion history on T and there is
some version order e for each item X. A multiversion serializa-
tion graph MVSG(H) for a multiversion history H over T is a
directed graph such that
• Nodes of MVSG(H) are transactions in T.
• There is a (directed) edge, T i T j , i j , in MVSG(H)
whenever R j [ X i ] ∈ H.
• MVSG(H) also contains version order edges: For each
R k [ X j ] and W i [ X i ] in H , there is an edge T i T j if
X i  <  X j ; otherwise, there is an edge T K T i .
The retrieval anomaly problem can be prevented by assigning an
infinite write timestamp to the new data versions that are created by
the low transaction so that they are invisible to a high transaction.
The fast version selection can be performed by giving the read
operation the recent versions of the data that are not assigned to infi-
nite write timestamp because the data versions with infinite write
timestamp are visible to the transaction when the transaction resumes
at the end of its invisible area.
Search WWH ::




Custom Search