transaction Developer Notes¶
Transaction objects manage resources for an individual activity. This document contains some notes that will help in understanding how transactions work, and how to use them to accomplish specific objectives.
A transaction commit involves an interaction between the transaction
object and one or more resource managers. The transaction manager
calls the following four methods on each resource manager; it calls
IDataManager.tpc_begin on each resource manager before calling
IDataManager.commit on any of them.
Sometimes, applications want to execute some code when a transaction
is committed. For example, one might want to delay object indexing
until a transaction commits, rather than indexing every time an object
is changed. Or someone might want to check invariants only after a set
of operations. A pre-commit hook is available for such use cases: use
ITransaction.addBeforeCommitHook, passing it a callable and
arguments. The callable will be called with its arguments at the start
of the commit.
Sometimes, applications want to execute code after a transaction
commit attempt succeeds or aborts. For example, one might want to
launch non transactional code after a successful commit. Or still
someone might want to launch asynchronous code after. A post-commit
hook is available for such use cases: use
ITransaction.addAfterCommitHook, passing it a callable and
arguments. The callable will be called with a Boolean value
representing the status of the commit operation as first argument
(true if successfull or false iff aborted) preceding its arguments at
the start of the commit.
When errors occur during two-phase commit, the transaction manager
aborts all joined the data managers. The specific methods it calls depend
on whether the error occurs before or after any call to
joined to that transaction.
If a data manager has not voted, then the data manager will have one
or more uncommitted objects. There are two cases that lead to this
state; either the transaction manager has not called
IDataManager.commit for any joined data managers, or the call that
failed was a
IDataManager.commit for one of the joined data
managers. For each uncommitted data manager, including the object that
failed in its
IDataManager.abort is called.
Once uncommitted objects are aborted,
called on each data manager.
Transaction Manager Lifecycle Notifications (Synchronization)¶
You can register sychronization objects (
synchronizers) with the tranasction manager. The synchronizer must
ISynchronizer.afterCompletion methods. The transaction manager calls
beforeCompletion when it starts a top-level two-phase commit. It
afterCompletion when a top-level transaction is committed or
aborted. The methods are passed the current
ITransaction as their only
Explicit vs implicit transactions¶
By default, transactions are implicitly managed. Calling
on a transaction manager implicitly aborts the previous transaction
abort() implicitly begins a new
one. This behavior can be convenient for interactive use, but invites
- Calling begin() without realizing that there are outstanding changes that will be aborted.
- Interacting with a database without controlling transactions, in which case changes may be unexpectedly discarded.
For applications, including frameworks that control transactions,
transaction managers provide an optional explicit mode. Transaction
managers have an
explicit constructor keyword argument that, if
True puts the transaction manager in explicit mode. In explicit mode:
- It is an error to call
savepoint()without a preceding
begin()call. Doing so will raise a
- It is an error to call
begin()after a previous
begin()without an intervening
abort()call. Doing so will raise an
In explicit mode, bugs like those mentioned above are much easier to avoid because they cause explicit exceptions that can typically be caught in development.
An additional benefit of explicit mode is that it can allow data managers to manage resources more efficiently.
Transaction managers have an explicit attribute that can be queried to determine if explicit mode is enabled.