Writing a Resource Manager

Simple Resource Manager

>>> from transaction.tests.examples import ResourceManager

This transaction.tests.examples.ResourceManager class provides a trivial resource-manager implementation and doc strings to illustrate the protocol and to provide a tool for writing tests.

Our sample resource manager has state that is updated through an inc method and through transaction operations.

When we create a sample resource manager:

>>> rm = ResourceManager()

It has two pieces state, state and delta, both initialized to 0:

>>> rm.state
0
>>> rm.delta
0

state is meant to model committed state, while delta represents tentative changes within a transaction. We change the state by calling inc:

>>> rm.inc()

which updates delta:

>>> rm.delta
1

but state isn’t changed until we commit the transaction:

>>> rm.state
0

To commit the changes, we use 2-phase commit. We execute the first stage by calling prepare. We need to pass a transation. Our sample resource managers don’t really use the transactions for much, so we’ll be lazy and use strings for transactions. The sample resource manager updates the state when we call tpc_vote:

>>> t1 = '1'
>>> rm.tpc_begin(t1)
>>> rm.state, rm.delta
(0, 1)

>>> rm.tpc_vote(t1)
>>> rm.state, rm.delta
(1, 1)

Now if we call tpc_finish:

>>> rm.tpc_finish(t1)

Our changes are “permanent”. The state reflects the changes and the delta has been reset to 0.

>>> rm.state, rm.delta
(1, 0)

The tpc_begin() Method

Called by the transaction manager to ask the RM to prepare to commit data.

>>> rm = ResourceManager()
>>> rm.inc()
>>> t1 = '1'
>>> rm.tpc_begin(t1)
>>> rm.tpc_vote(t1)
>>> rm.tpc_finish(t1)
>>> rm.state
1
>>> rm.inc()
>>> t2 = '2'
>>> rm.tpc_begin(t2)
>>> rm.tpc_vote(t2)
>>> rm.tpc_abort(t2)
>>> rm.state
1

It is an error to call tpc_begin more than once without completing two-phase commit:

>>> rm.tpc_begin(t1)

>>> rm.tpc_begin(t1)
Traceback (most recent call last):
...
ValueError: txn in state 'tpc_begin' but expected one of (None,)
>>> rm.tpc_abort(t1)

If there was a preceeding savepoint, the transaction must match:

>>> rollback = rm.savepoint(t1)
>>> rm.tpc_begin(t2)
Traceback (most recent call last):
,,,
TypeError: ('Transaction missmatch', '2', '1')

>>> rm.tpc_begin(t1)

The tpc_vote() Method

Verify that a data manager can commit the transaction.

This is the last chance for a data manager to vote ‘no’. A data manager votes ‘no’ by raising an exception.

Passed transaction, which is the ITransaction instance associated with the transaction being committed.

The tpc_finish() Method

Complete two-phase commit

>>> rm = ResourceManager()
>>> rm.state
0
>>> rm.inc()

We start two-phase commit by calling prepare:

>>> t1 = '1'
>>> rm.tpc_begin(t1)
>>> rm.tpc_vote(t1)

We complete it by calling tpc_finish:

>>> rm.tpc_finish(t1)
>>> rm.state
1

It is an error ro call tpc_finish without calling tpc_vote:

>>> rm.inc()
>>> t2 = '2'
>>> rm.tpc_begin(t2)
>>> rm.tpc_finish(t2)
Traceback (most recent call last):
...
ValueError: txn in state 'tpc_begin' but expected one of ('tpc_vote',)

>>> rm.tpc_abort(t2)  # clean slate

>>> rm.tpc_begin(t2)
>>> rm.tpc_vote(t2)
>>> rm.tpc_finish(t2)

Of course, the transactions given to tpc_begin and tpc_finish must be the same:

>>> rm.inc()
>>> t3 = '3'
>>> rm.tpc_begin(t3)
>>> rm.tpc_vote(t3)
>>> rm.tpc_finish(t2)
Traceback (most recent call last):
...
TypeError: ('Transaction missmatch', '2', '3')

The tpc_abort() Method

Abort a transaction

The abort method can be called before two-phase commit to throw away work done in the transaction:

>>> rm = ResourceManager()
>>> rm.inc()
>>> rm.state, rm.delta
(0, 1)
>>> t1 = '1'
>>> rm.tpc_abort(t1)
>>> rm.state, rm.delta
(0, 0)

The abort method also throws away work done in savepoints:

>>> rm.inc()
>>> r = rm.savepoint(t1)
>>> rm.inc()
>>> r = rm.savepoint(t1)
>>> rm.state, rm.delta
(0, 2)
>>> rm.tpc_abort(t1)
>>> rm.state, rm.delta
(0, 0)

If savepoints are used, abort must be passed the same transaction:

>>> rm.inc()
>>> r = rm.savepoint(t1)
>>> t2 = '2'
>>> rm.tpc_abort(t2)
Traceback (most recent call last):
...
TypeError: ('Transaction missmatch', '2', '1')

>>> rm.tpc_abort(t1)

The abort method is also used to abort a two-phase commit:

>>> rm.inc()
>>> rm.state, rm.delta
(0, 1)
>>> rm.tpc_begin(t1)
>>> rm.state, rm.delta
(0, 1)
>>> rm.tpc_vote(t1)
>>> rm.state, rm.delta
(1, 1)
>>> rm.tpc_abort(t1)
>>> rm.state, rm.delta
(0, 0)

Of course, the transactions passed to prepare and abort must match:

>>> rm.tpc_begin(t1)
>>> rm.tpc_abort(t2)
Traceback (most recent call last):
...
TypeError: ('Transaction missmatch', '2', '1')

>>> rm.tpc_abort(t1)

This should never fail.

The savepoint() Method

Provide the ability to rollback transaction state

Savepoints provide a way to:

  • Save partial transaction work. For some resource managers, this could allow resources to be used more efficiently.
  • Provide the ability to revert state to a point in a transaction without aborting the entire transaction. In other words, savepoints support partial aborts.

Savepoints don’t use two-phase commit. If there are errors in setting or rolling back to savepoints, the application should abort the containing transaction. This is not the responsibility of the resource manager.

Savepoints are always associated with a transaction. Any work done in a savepoint’s transaction is tentative until the transaction is committed using two-phase commit.

>>> rm = ResourceManager()
>>> rm.inc()
>>> t1 = '1'
>>> r = rm.savepoint(t1)
>>> rm.state, rm.delta
(0, 1)
>>> rm.inc()
>>> rm.state, rm.delta
(0, 2)
>>> r.rollback()
>>> rm.state, rm.delta
(0, 1)
>>> rm.tpc_begin(t1)
>>> rm.tpc_vote(t1)
>>> rm.tpc_finish(t1)
>>> rm.state, rm.delta
(1, 0)

Savepoints must have the same transaction:

>>> r1 = rm.savepoint(t1)
>>> rm.state, rm.delta
(1, 0)
>>> rm.inc()
>>> rm.state, rm.delta
(1, 1)
>>> t2 = '2'
>>> r2 = rm.savepoint(t2)
Traceback (most recent call last):
...
TypeError: ('Transaction missmatch', '2', '1')

>>> r2 = rm.savepoint(t1)
>>> rm.inc()
>>> rm.state, rm.delta
(1, 2)

If we rollback to an earlier savepoint, we discard all work done later:

>>> r1.rollback()
>>> rm.state, rm.delta
(1, 0)

and we can no longer rollback to the later savepoint:

>>> r2.rollback()
Traceback (most recent call last):
...
TypeError: ('Attempt to roll back to invalid save point', 3, 2)

We can roll back to a savepoint as often as we like:

>>> r1.rollback()
>>> r1.rollback()
>>> r1.rollback()
>>> rm.state, rm.delta
(1, 0)

>>> rm.inc()
>>> rm.inc()
>>> rm.inc()
>>> rm.state, rm.delta
(1, 3)
>>> r1.rollback()
>>> rm.state, rm.delta
(1, 0)

But we can’t rollback to a savepoint after it has been committed:

>>> rm.tpc_begin(t1)
>>> rm.tpc_vote(t1)
>>> rm.tpc_finish(t1)

>>> r1.rollback()
Traceback (most recent call last):
...
TypeError: Attempt to rollback stale rollback