Transactions:

A tomicity
C onsistency
I solation
D urability


Concurrency Control:
-------------------------


Transaction:

T1: r1(x) w1(x) 

T2: r2(x) w2(x) 



Concurrent operations:


Schedule:

S0: r1(x) r2(x) w1(x) w2(x)

S1: r1(x) w1(x) r2(x) w2(x)

S2: r2(x) w2(x) r1(x) w1(x) 


What is a good schedule?
--------------------------

A serial schedule (where one transaction executes at a time) is a good
schedule: if I execute one transaction at a time, the resulting
database state is always correct.

A serializable schedule is one that is guaranteed to provide the same
results as a serial schedule.


Conflicting operations
------------------------

Two operations conflict with each other, if they are by two different
transactions and one of them is a write operation

r1(x) w2(x)

w1(x) r2(x)

w1(x) w2(x)


If in two schedules, the conflicting operations occur in the same
order, then their result is guaranteed to be the same: then these two
schedules are equivalent


S0: r1(x) [ r2(x) w1(x) ] w2(x)   --> r2(x) reads values before it is changed

S0': r1(x) [ w1(x) r2(x)] w2(x)   --> r2(x) read the value after T1 writes

S0 and S0' are not guaranteed to produce the same result


A schedule S is serializable (equal to a serial schedule) iff all
conflicting operations in S occur in the same order as a serial
schedule.

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