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Subtransactions in ZODB 3
=========================
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.
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