Extensions to the transaction package.
nti.transactions.loop.TransactionsLoop is a retryable transaction manager. It is conceptually similar to the attempts context manager provided by the transaction package itself, but much more powerful and extensible via subclasses. Features include:
- Configurable commit vetos.
- Extensible tests for which exceptions should be retried.
- The ability to abort the transaction and bypass a potentially expensive commit when there are expected to be no side-effects.
- Sleeping between retries.
- Extensive logging and timing.
The TransactionLoop can be used as-is, or it can be subclassed for customization. For use in a Pyramid tween, for example, a minimal subclass might look like this (see nti.transactions.pyramid_tween for a full-featured tween):
>>> class PyramidTransactionLoop(TransactionLoop):
... def prep_for_retry(self, number, request):
... request.make_body_seekable()
... def describe_transaction(self, request):
... return request.urlA few data managers are provided for convenience.
The first data manager is used to put an object in a queue (something with the full and put_nowait methods) when a transaction succeeds. If the queue is full, then the transaction will not be allowed to commit:
>>> from nti.transactions.queue import put_nowait
>>> put_nowait(queue, object)This is a special case of the ObjectDataManager, which will call one method with any arguments when a transaction commits. It can be configured to vote on whether the transaction should be allowed to commit. or not. This is useful for, say, putting an item in a Redis queue when the transaction is successful. It can be constructed directly, but the do function is a shorthand way of joining one to the current transaction:
>>> from nti.transactions.manager import do
>>> do(print, args=("Committed"))Caution
See the documentation of this object for numerous warnings about side-effects and its interaction with the transaction machinery. Use it with care!