The transactions T and U are run with optimistic concurrency control using backward validation and restarting any transactions that fail. Describe the information written to the log file on their behalf. Why is it essential that the commit entries in the log file should be ordered by transaction numbers? How are the write sets of committed transactions represented in the log file?
What will be an ideal response?
Transaction numbers rather than transaction identifiers (or timestamps) go in the recovery file after a transaction has passed its validation. U passes validation because it has no read operations. Suppose U is given the transaction number TU. Transaction T fails validation because its read set {i} overlaps with U's Write set
{i,j}. T is restarted after U. It passes validation with transaction number TT. where TT>TU
It essential that the commit entries in the log file should be ordered by transaction number because transaction numbers reflect the order in which transactions are committed at the server. Recovery takes transactions from newest to oldest by reading the log file backwards.
Write sets are represented in the log file in the prepared entries.
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