In the literature, the following variations of the read/write page model can be

Question

In the literature, the following variations of the read/write page model can be found:

“No Blind Writes” model: Each transaction writing a data item x must have read x previously.

“Two-Step” model: All read steps of a transaction occur prior to all write steps.

“Action” model: Each step of a transaction represents an atomic execution of a read step followed by a write step of the same data item; thus, distinguishing reads from writes is no longer necessary.

Discuss the interpretation of a transaction along the lines of what has been introduced in this chapter for each of these models. What, if any, are the differences in interpretation to the standard read/write model?

Explanation / Answer

A transaction is a purely syntactic entity whose semantics or interpretations is unknown .if we had more information on semantics or intrepretation is unknown,if we had more information on semantics of the program that launches atransaction and the intended state transformation of the underlying data, this knowledge could be used to interpret a transaction and associate a formal semantics with it .in absences of such information,the best that can be done is a syntactic interpretation of the steps of a transaction that is as general as possible:

Incase pi=r(x),in which i-th step of given transaction is a read step,the current value of x is assigned to a lacal variable vj:

vj:=x

Incase pi-w(x),in which the j-th step of a given transaction is a write step,a possibly new value,compute by respective program,is written into x.Each value written by a transaction t potentially depends on the values of all data items that t has previously read,which is formally expressed as follows

x:=fi(vi ------ vj)

(x is the return value of an arbitarary but unknown function fi) such that :

(ji -----jk)={ji I pj is a read step ^ jr < j}

(All values Vj 1< r < k, that were read prior step to the j-th step of t are used as parameters in function fj)

As an example consider the following transactions

t=r(x) r(y) r(z) w(u) w(x)

Here we have p1=r(x),p2=r(y),p3=r(z),p4=w(u),p5=w(x)

The first three steps assign values (those of data items x,y and z) to variables v1,v2 and v3 respectively.the values of u and x written by the last two steps depend on these variables,in principle as follows:

u=f4(v1,v2,v3)

x=f5(v1,v2,v3)

We mentioned here that the view of a transaction as a sequence of steps is a simplification.

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