home about GMAT Coaching Live Oil and Gold Price Cool Gadgets Review US School Guide Medtech IT and Apps Computer Science SDA Believers Texas Driver License Test Resources How to check for View Serializable and Conflict Serializable November 18th, 2010 | NEU MSCS | Written by Trijito Santoso Tweet 0 34 Like This is a note for myself about how to check whether a schedule is view serializable, conflict serializable, or not. There is various resources in the internet about how to do this, but the examples are a bit scattered, so in this post I just want to make a neat note on how to do it properly with several examples that can cover many possibilities as well. Let’s get started! For the sake of this tutorial, we will use this example: R1(X),R2(Y),R2(Y),W2(X),W3(Y),R1(X) or if we spread it to three different transactions, it will be: T1 | R1(X) R1(X) T2 | R2(Y) R2(Y) W2(X) Search To search, type and hit enter Pages About GMAT Coaching Live Oil and Gold Price Privacy Policy Feedburner Facebook Page Email Share How to check for View Serializable and Conflict Serializable — djitz.com http://djitz.com/neu-mscs/how-to-check-for-view-serializable-and-conflic... 1 of 12 3/24/2012 10:50 AM
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How to check for View Serializableand Conflict SerializableNovember 18th, 2010 | NEU MSCS | Written by Trijito Santoso
Tweet 0
34Like
This is a note for myself about how to check whether a schedule is
view serializable, conflict serializable, or not.
There is various resources in the internet about how to do this, butthe examples are a bit scattered, so in this post I just want to make
a neat note on how to do it properly with several examples that can
cover many possibilities as well.
Let’s get started!
For the sake of this tutorial, we will use this example:
R1(X),R2(Y),R2(Y),W2(X),W3(Y),R1(X)
or if we spread it to three different transactions, it will be:
T1 | R1(X) R1(X)
T2 | R2(Y) R2(Y) W2(X)
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To search, type and hit enter
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T3 | W3(Y)
How to check for Conflict Serializable?To check for conflict serializability takes two steps.
Step #1 : Check for the conflicting actions.
(From Wikipedia)——————————————————
Two or more actions are said to be in conflict if:
1. The actions belong to different transactions.
2. At least one of the actions is a write operation.
3. The actions access the same object (read or write).
The following set of actions is conflicting:
T1:R(X), T2:W(X), T3:W(X)
While the following sets of actions are not:
T1:R(X), T2:R(X), T3:R(X)
T1:R(X), T2:W(Y), T3:R(X)
—————————————————–
For our example, we have a conflict on X (T1 reads it and T2 writes
it).
We also have a conflict on Y (T2 reads it and T3 writes it).
So it has a conflict property but is it serializable?
Step #2 : Check for a cycle in the Precedence Graph.
The easiest way to check for a cyclic transaction, is to paste the
schedule to this web app.
But if you are like me, you won’t be satisfied getting answer from
someone without understanding how exactly they come up with thethe answer (though sometimes I don’t care either..). So let’s find out
how to do exactly just that.
First, draw all the Transactions (Tx):
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Check if there is a Tx that reads an item after a different Tx writes it.
We have T1 that reads X after T2 writes it, so draw arrow from
T2 -> T1
Check if there is a Tx that writes an item after a different Tx reads it.
We have T2 that writes X after T1 reads it, so draw arrow from
T1 -> T2
We also have T3 that writes Y after T2 reads it, so draw arrowfrom T2 -> T3
Check if there is a Tx that writes an item after a different TX writes it.
not in this case
So, the final graph is :
We can see that there is a cycle between T1 and T2, so the graph is
cyclic, and therefore it is not conflict serializable.
Need more example?Graph for R1(X),R2(Y),W3(Z),W2(Y),W2(X),R1(Z),W3(Y),W2(X)
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How to check for View Serializable and Conflict Serializable — djitz.com http://djitz.com/neu-mscs/how-to-check-for-view-serializable-and-conflic...
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Graph for R1(X),R2(Y),R3(Z),W2(Y),W2(X),W1(Z),W3(Y),W2(X)
(same like the previous example)
How to check for View Serializable?To check for view serializability of a schedule, you must create all
possible combinations of the Transactions. So let’s say you have
three transactions, then you need to check for these combinations:
< T1, T2, T3 >< T1, T3, T2 >
< T2, T1, T3 >
< T2, T3, T1 >
< T3, T1, T2 >
< T3, T2, T1 >
Now the schedule is view serializable if:
A Tx reads an initial data in a Schedule, the same Tx also should read
the initial data in one of the transaction combination.
For our example, at least T1 should occur before T2, because
T1 reads initial value X. If T2 occurs before T1, then T1 reads X
value after T2 writes. So remove these combinations:
< T2, T1, T3 >
< T2, T3, T1 >
< T3, T2, T1 >
A Tx reads a data after another Tx has written in a Schedule, the
same Tx also should read the data after another Tx has written it inone of the transaction combination.
In our example, T1 reads X after T2 writes, so it means that T2
should occur before T1. But wait a minute, isn’t it we’ve just
said that T1 should occur before T1 on the previous condition?
That’s what a cycle in the graph also caused. We need T1
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