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| A '''precedence graph''', also named '''conflict graph''' and '''[[serializability]] graph''', is used in the context of [[concurrency control]] in [[database]]s.
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| The precedence graph for a schedule S contains:
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| * A node for each committed transaction in S
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| * An arc from T<sub>i</sub> to T<sub>j</sub> if an action of T<sub>i</sub> precedes and [[Schedule (computer science)#Conflicting actions|conflicts]] with one of T<sub>j</sub>'s actions.
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| ==Precedence graph example==
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| [[Image:Precedence graph.svg|200px|right]]
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| :<math>D = \begin{bmatrix}
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| T1 & T2 & T3 \\
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| R(A) & & \\
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| & W(A) & \\
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| & Com. & \\
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| W(A) & & \\
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| Com. & & \\
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| & & W(A)\\
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| & & Com.\\
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| \end{bmatrix}</math>
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| or
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| :'''<math>D = R1(A)</math> <math>W2(A)</math> <math> Com.2</math> <math> W1(A)</math> <math> Com.1</math> <math> W3(A)</math> <math> Com.3 </math>'''
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| A precedence graph of the schedule D, with 3 transactions. As there is a cycle (of length 2; with two edges) through the committed transactions T1 and T2, this [[Schedule (computer science)|schedule]] (history) is ''not'' [[Serializability#View and conflict serializability|Conflict serializable]].
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| ==Testing Serializability with Precedence Graph==
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| The drawing sequence for the precedence graph:-
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| # For each transaction T<sub>i</sub> participating in schedule S, create a node labelled T<sub>i</sub> in the precedence graph. So the precedence graph contains T<sub>1</sub>, T<sub>2</sub>, T<sub>3</sub>
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| # For each case in S where T<sub>i</sub> executes a write_item(X) then T<sub>j</sub> executes a read_item(X), create an edge (T<sub>i</sub> --> T<sub>j</sub>) in the precedence graph. This occurs nowhere in the above example, as there is no read after write.
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| # For each case in S where T<sub>i</sub> executes a read_item(X) then T<sub>j</sub> executes a write_item(X), create an edge (T<sub>i</sub> --> T<sub>j</sub>) in the precedence graph. This will bring to front a directed graph from T<sub>1</sub> to T<sub>2</sub>.
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| # For each case in S where T<sub>i</sub> executes a write_item(X) then T<sub>j</sub> executes a write_item(X), create an edge (T<sub>i</sub> --> T<sub>j</sub>) in the precedence graph. It creates a directed graph from T<sub>2</sub> to T<sub>1</sub>, T<sub>1</sub> to T<sub>3</sub>, and T<sub>2</sub> to T<sub>3</sub>.
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| # The schedule S is serializable if the precedence graph has no cycles. As T<sub>1</sub> and T<sub>2</sub> constitute a bicycle, then we cannot declare S as serializable or not and serializability has to be checked using other methods.
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| ==External links==
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| * [http://portal.acm.org/citation.cfm?id=1202608&dl=GUIDE&coll=GUIDE&CFID=9802819&CFTOKEN=82728908 The Fundamentals of Database Systems, 5th Edition] the use of precedence graphs is discussed in chapter 17, as they relate to tests for [[serializability#View and conflict serializability|conflict serializability]].
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| * Abraham Silberschatz, Henry Korth, and S. Sudarshan. 2005. Database Systems Concepts (5 ed.), PP. 628–630. McGraw-Hill, Inc., New York, NY, USA.
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| [[Category:Database management systems]]
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| [[el:Γράφος Σειριοποιησιμότητας]]
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| [[ru:Граф предшествования]]
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Friends contact him Royal. Her buddies say it's not great for her but what she loves performing is flower arranging and she is attempting to make it a occupation. Delaware has usually been my living place and will never transfer. Bookkeeping is how he supports his family and his wage has been really satisfying.
Feel free to visit my blog; shipzula.com