A precedence graph of the schedule D, with 3 transactions.
2.
:In the "'precedence graph "'transactions are nodes and precedence relations are directed edges.
3.
A deadlock is a reflection of a potential cycle in the " precedence graph ", that would occur without the blocking.
4.
A deadlock is a reflection of a potential cycle in the precedence graph, that would occur without the blocking when conflicts are materialized.
5.
This means that a cycle consisting of committed transactions only is generated in the ( general ) precedence graph, if and only if conflict-serializability is violated.
6.
A cycle of committed transactions ( with materialized conflicts ) in the " precedence graph " ( conflict graph ) represents a serializability violation, and should be avoided for maintaining serializability.
7.
Schedule compliance with conflict serializability can be tested with the precedence graph ( " serializability graph ", " serialization graph ", " conflict graph " ) for committed transactions of the schedule.
8.
Cycles of committed transactions can be prevented by aborting an " undecided " ( neither committed, nor aborted ) transaction on each cycle in the precedence graph of all the transactions, which can otherwise turn into a cycle of committed transactions ( and a committed transaction cannot be aborted ).
9.
As a result, access by another transaction may be blocked, typically upon a conflict ( the lock delays or completely prevents the conflict from being materialized and be reflected in the precedence graph by blocking the conflicting operation ), depending on lock type and the other transaction's access operation type.
10.
Another definition for conflict-serializability is that a schedule is conflict-serializable if and only if its precedence graph / serializability graph, when only committed transactions are considered, is acyclic ( if the graph is defined to include also uncommitted transactions, then cycles involving uncommitted transactions may occur without conflict serializability violation ).
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