In an Internet election system, each voter is sent a PIN in the mail. When that voter wants to vote at the election web site, she enters her PIN and her vote, and then a voting transaction is executed. In the voting transaction, first the PIN is checked to verify that it is valid and has not been used already, and then the vote tally for the appropriate candidate is incremented. Two tables are used: One contains the valid PINs together with an indication of whether or not each PIN has been used and the other contains the names of the candidates and the vote tally for each. Discuss the issues involved in selecting an appropriate isolation level for the voting transaction. Discuss the issues involved in selecting appropriate isolation levels if a new(read-only) transaction is introduced

that outputs the entire vote tally table.

What will be an ideal response?

The only SQL statements in the voting transaction will be an UPDATE statement that
checks and updates the PIN table and an UPDATE statement that updates the tally
table. Since write locks are always long duration at all isolation levels, schedules of
voting transactions will be serializable at any isolation level.
A transaction that outputs the tally table should run at READ COMMITTED and
use an insensitive cursor in order to output a consistent snapshot of the table.