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Serializable History, Conflict Equivalence, Local/Global History
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Kenneth Chi Wai Ng
Apr 2, 2014, 12:07:34 AM4/2/14
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I want to make sure I understand the content in slide 6-59.
1)
Serializability is defined as "If several transactions are executed concurrently, the results must be the
same as if they were executed serially in some order." Does "some order" mean as long as any serial execution of a permutation of T1, T2, ..., Tn matches a concurrent execution, that that the concurrency execution is serializable?
For example, I am assuming that H1 is considered serializable because it matches the outcome if the transactions were executed in serial fashion as T2, T1, T3 (or T2, T3, T1), but not in any other permutation?
2)
Could it be explained as why H1 is not conflict equivalent to Hs but H3 is? I can tell the difference is that R3(x) is performed before/after C1 but I don't understand why.
3)
What is the difference between local history and global history. Could an example be given? In 6-63, I understand why the two local histories are no serializable (because there is a conflicting operation on T1 then T2 in one, and T2 then T1 in the other), but I don't fully grasp the concept of local history vs global history.
Thanks
1)
Serializability is defined as "If several transactions are executed concurrently, the results must be the
same as if they were executed serially in some order." Does "some order" mean as long as any serial execution of a permutation of T1, T2, ..., Tn matches a concurrent execution, that that the concurrency execution is serializable?
For example, I am assuming that H1 is considered serializable because it matches the outcome if the transactions were executed in serial fashion as T2, T1, T3 (or T2, T3, T1), but not in any other permutation?
2)
Could it be explained as why H1 is not conflict equivalent to Hs but H3 is? I can tell the difference is that R3(x) is performed before/after C1 but I don't understand why.
3)
What is the difference between local history and global history. Could an example be given? In 6-63, I understand why the two local histories are no serializable (because there is a conflicting operation on T1 then T2 in one, and T2 then T1 in the other), but I don't fully grasp the concept of local history vs global history.
Thanks
Kenneth Chi Wai Ng
Apr 2, 2014, 12:11:06 AM4/2/14
to
Also what does it mean by "Global Serialization Order"?
Khuzaima Daudjee
Apr 2, 2014, 10:06:43 AM4/2/14
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On 2014-04-02, 12:11 AM, Kenneth Chi Wai Ng wrote:
> Also what does it mean by "Global Serialization Order"?
The serial order that is conflict equivalent to concurrent
> Also what does it mean by "Global Serialization Order"?
execution of (multiple) transactions at
different sites (possibly on multiple copies of data).
-K
Khuzaima Daudjee
Apr 2, 2014, 10:06:36 AM4/2/14
to
On 2014-04-02, 12:07 AM, Kenneth Chi Wai Ng wrote:
> I want to make sure I understand the content in slide 6-59.
>
> 1)
> Serializability is defined as "If several transactions are executed concurrently, the results must be the
> same as if they were executed serially in some order." Does "some order" mean as long as any serial execution of a permutation of T1, T2, ..., Tn matches a concurrent execution, that that the concurrency execution is serializable?
Some order means a conflict equivalent order.
> I want to make sure I understand the content in slide 6-59.
>
> 1)
> Serializability is defined as "If several transactions are executed concurrently, the results must be the
> same as if they were executed serially in some order." Does "some order" mean as long as any serial execution of a permutation of T1, T2, ..., Tn matches a concurrent execution, that that the concurrency execution is serializable?
> For example, I am assuming that H1 is considered serializable because it matches the outcome if the transactions were executed in serial fashion as T2, T1, T3 (or T2, T3, T1), but not in any other permutation?
>
> 2)
> Could it be explained as why H1 is not conflict equivalent to Hs but H3 is? I can tell the difference is that R3(x) is performed before/after C1 but I don't understand why.
> 3)
> What is the difference between local history and global history. Could an example be given? In 6-63, I understand why the two local histories are no serializable (because there is a conflicting operation on T1 then T2 in one, and T2 then T1 in the other), but I don't fully grasp the concept of local history vs global history.
Global history is history over all involved sites.
-K
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