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Subject: Re: [boost] Boost library submission (poll for interest)
From: Stefan Strasser (strasser_at_[hidden])
Date: 2010-01-17 19:21:14

Am Monday 18 January 2010 00:25:54 schrieb vicente.botet:
> I don't think the cost of dynamic polymorphism will be high respect to the
> cost of a transaction system, and even less if it concerns persistent

I do, it's a virtual call for each access to a persistent object.
note that if only a base class of the transaction manager is known to
the "transactional entity" it can also only return a base class of the
resource manager, so it also severly limits the interface of the resource
managers, since there are no virtual template functions.
but since we agree on the result there is no need to discuss this in detail I

what do you think about the basic_* and alias approach? is that feasible for
your library?

> > 4.
> > please have a look at the following parts of the documentation:
> >
> >
> >ml/persistent/configuring.html#persistent.configuring.defaultconf
> >
> > TransactionManager concept:
> >
> >ml/boost/persistent/TransactionManager.html
> >
> > ResourceManager concept:
> >
> >ml/boost/persistent/ResourceManager.html
> I'm not sure if the interface of resource_manager is enough to implement a
> two-phase commit protocol, but we will see this once we talk about the
> protocol that ensure that the transaction is ACID for all the resources.

right, the interface for distributed transactions is missing.
there has to be at least a prepare_transaction(), a recover() and a
transaction_id with a way to obtain it from a transaction object, so the
transaction manager can log the IDs of the resource transactions in its
distributed transaction log.

I also need a finish_transaction() but this is a minor issue. I'm quite sure
this can be an empty function for your libraries.

> I see that there are TransactionManager::transaction and a
> ResourceManager::transaction types. Could you point on the documentation
> (if already written) how these types are correlated, and when these
> function are called?

I don't think it's documented except for the Reference.
TransactionManager::transaction is sometimes called the "global transaction",
and ResourceManager::transaction the "local transaction" in other systems.

the details are as follows:

the user creating a transaction scope:

transaction tx;

calls TransactionManager::begin_transaction and stores a
TransactionManager::transaction. note that no resource manager is called at
this point.

accessing a resource, e.g. by calling Locator::operator*, obtains
ResourceManager::transaction by calling
TransactionManager::resource_transaction(), and uses it to access the object.

TransactionManager::resource_transaction() returns the
ResourceManager::transaction that is associated with the
TransactionManager::transaction. if there is none associated yet,
ResourceManager::begin_transaction is called to create one.

the point of all this is that if there are multiple resources used by a
transaction manager, a local transactions in a resource manager is only
started if the resource is actually used in this global transaction.
for example most of my transactions may access Boost.STM transactional memory
only, and only from time to time I access Boost.STM and Boost.Persistent in
the same transaction.

if more than one ResourceManager::transaction's were created in the course of
a global transaction, a two-phase-commit has to be performed.

> > 3.
> > transactions are bound to the current thread when they are created,
> > but transaction managers are bound to the whole application.
> >
> > my_transaction_manager t1;
> > my_transaction_manager t2;
> >
> > //t2 is the current transaction manager, even when another thread is
> > started. bind() can be used to bind another transaction manager for
> > all running threads.
> >
> > creating and binding of transaction managers is not thread-safe.
> Sorry. I'm a little bit lost. Why do you need to change the instance of the
> transaction manager. I thought that there was only a transaction manager.
> When an application will need to change it?

yes, one at a time. this might be rarely used, but it is possible to have 2
transaction managers of the same type if the right one is active at the time
you access e.g. objects that use it.
my (minor) point of this was that transactions are bound to threads, and
transaction managers are bound to applications.

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