Let objects x and y be stored at site 1, and objects z and w be stored at site 2

Question

Let objects x and y be stored at site 1, and objects z and w be stored at site 2. Determine, for each of the following executions, whether the execution is serializable or not. If it is serializable, determine all possible total orders of transactions. If it is not serializable, prove that there is no total order possible.

Execution 1:

S1: Ri(x) Rj(x) Wj(y) Wi(x)

S2: Ri(w) Rj(z) Wj(w) Wi(w)

Execution 2:

S1: Ri(x) Rj(x) Wj(y) Wi(y)

S2: Wi(z)

Execution 3:

S1: Ri(x) Rj(x) Wi(x) Wj(y)

S2: Ri(z) Rj(z) Wj(z) Wi(w)

Execution 4:

S1: Ri(y) Rj(x) Wj(x)

S2: Wi(z) Ri(w) Rj(w) Wi(w)

Explanation / Answer

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I am giving you the concept. I am sure you can easily work out the solution. Even if you can't , you can ask me then.

A transaction is serializable if it is equivalent to a serial schedule.

So in each of the parts, just check whether 2 instructions can be swapped(they can be swapped iff none of them are writes, that is, both are read operations).

If by this series of swapping, you can get a serial schedule then the transaction is serializable.

Cheers!

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