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Subtransactions in ZODB 3
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ZODB 3 provides limited support for subtransactions. Subtransactions
are nested to *one* level. There are top-level transactions and
subtransactions.  When a transaction is committed, a flag is passed
indicating whether it is a subtransaction or a top-level transaction.
Consider the following exampler commit calls:

- commit()

  A regular top-level transaction is committed.

- commit(1)

  A subtransaction is committed. There is now one subtransaction of
  the current top-level transaction.

- commit(1)

  A subtransaction is committed. There are now two subtransactions of
  the current top-level transaction.

- abort(1)

  A subtransaction is aborted. There are still two subtransactions of
  the current top-level transaction; work done since the last
  commit(1) call is discarded.

- commit()

  We now commit a top-level transaction. The work done in the previous
  two subtransactions *plus* work done since the last abort(1) call
  is saved.

- commit(1)

  A subtransaction is committed. There is now one subtransaction of
  the current top-level transaction.

- commit(1)

  A subtransaction is committed. There are now two subtransactions of
  the current top-level transaction.

- abort()

  We now abort a top-level transaction. We discard the work done in
  the previous two subtransactions *plus* work done since the last
  commit(1) call.
