Transaction demarcation

Should the transaction be demarcated at the service layer or DAO layer? The best or normal practice is to mark the service layer operation as transactional since this will represent the single unit-of-work (or use-case) that we would like to execute in a all-or-nothing manner. Furthermore, the service operation may invoke multiple DAOs which together may need to work in a single transaction and constitute the unit-of-work.

Reference: This article gives a good discussion: http://stackoverflow.com/questions/1079114/spring-transactional-annotation-best-practice