transaction Documentation

Transaction objects manage resources for an individual activity.

Compatibility issues

The implementation of Transaction objects involves two layers of backwards compatibility, because this version of transaction supports both ZODB 3 and ZODB 4. Zope is evolving towards the ZODB4 interfaces.

Transaction has two methods for a resource manager to call to participate in a transaction – register() and join(). join() takes a resource manager and adds it to the list of resources. register() is for backwards compatibility. It takes a persistent object and registers its _p_jar attribute. TODO: explain adapter

Two-phase commit

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 tpc_begin() on each resource manager before calling commit() on any of them.

  1. tpc_begin(txn)
  2. commit(txn)
  3. tpc_vote(txn)
  4. tpc_finish(txn)

Before-commit hook

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 addBeforeCommitHook(), passing it a callable and arguments. The callable will be called with its arguments at the start of the commit (but not for substransaction commits).

After-commit hook

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 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 (but not for substransaction commits). Commit hooks are not called for transaction.abort().

Error handling

When errors occur during two-phase commit, the transaction manager aborts all the resource managers. The specific methods it calls depend on whether the error occurs before or after the call to tpc_vote() on that transaction manager.

If the resource manager has not voted, then the resource manager will have one or more uncommitted objects. There are two cases that lead to this state; either the transaction manager has not called commit() for any objects on this resource manager or the call that failed was a commit() for one of the objects of this resource manager. For each uncommitted object, including the object that failed in its commit(), call abort().

Once uncommitted objects are aborted, tpc_abort() or abort_sub() is called on each resource manager.

Synchronization

You can register sychronization objects (synchronizers) with the tranasction manager. The synchronizer must implement beforeCompletion() and afterCompletion() methods. The transaction manager calls beforeCompletion() when it starts a top-level two-phase commit. It calls afterCompletion() when a top-level transaction is committed or aborted. The methods are passed the current Transaction as their only argument.

Explicit vs implicit transactions

By default, transactions are implicitly managed. Calling begin() on a transaction manager implicitly aborts the previous transaction and calling commit() or abort() implicitly begins a new one. This behavior can be convenient for interactive use, but invites subtle bugs:

  • 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 get(), commit(), abort(), doom(), isDoomed, or savepoint() without a preceding begin() call. Doing so will raise a NoTransaction exception.
  • It is an error to call begin() after a previous begin() without an intervening commit() or abort() call. Doing so will raise an AlreadyInTransaction exception.

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.

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