Consider the recovery aspects of the nested transactions. Assume that a withdraw transaction will abort if the account will be overdrawn and that in this case the parent transaction will also abort. Describe serially equivalent interleavings of T1, T2, U1 and U2 with the following properties:

Question

(i) that is strict;

(ii) that is not strict.

To what extent does the criterion of strictness reduce the potential concurrency gain of nested transactions?

Answer 1

If a child transaction’s abort can cause the parent to abort, with the effect that the other children abort, then strict executions must delay reads and writes until all the relations (siblings and ancestors) of transactions that have previously written the same objects are either committed or aborted. Our deposit and withdraw operations read and then write the balances.

i) For strict executions serially equivalent to T1; T2; U1; U2 we note that T2 has written B. We then delay U2’s deposit until after the commit of T2 and its sibling T1. The following is an example of such an interleaving:





ii) Discusses all possible serially equivalent executions. They are non-strict if they do not obey the constraints discussed in part (i).



The criterion of strictness does not in any way reduce the possible concurrency between siblings (e.g. T1 and T2). It does make unrelated transactions wait for entire families to commit instead of single members with which it is in conflict over access to a data item.