Transactions

Not enough as such. They must in addition:

• be composable: either nested or implicit (not in reusable client code)
• provide checkpoints for partial rollbacks

Still their semantics are both...

• too weak:
  • no way to compare two transactions with respect to a resource conflict
• and too strong:
  • not every task may be rolled-back
  • not every task "completes" (e.g. maintain a representation)

Notes: Transactions are usually handled at a very low level: very generic, at coarse granularity.
Operating systems know only processes (or threads...), and can almost only kill them.
Database lock managers know only pages (or objects...), and can only abort transactions.

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