Sunday, May 23, 2010

Nested transactions

Here's another periodic entry on our road to discussing the STM work we're doing around JBossTS. We're going to build on the Transactional Objects for Java toolkit that I described before. But before doing so it's worth stressing that everything I'm talking about here is based on the ArjunaCore module, so it doesn't need an ORB or SOAP stack. In fact it doesn't actually need anything from an application server, whether or not that is J(2)EE based. So yes, you could take that module and embed it within your application, framework or whatever and gain full ACID transaction capabilities.

Hopefully if you're reading these entries then you already understand the concept of traditional transactions, which we'll refer to as top-level transactions here. If not then there are enough articles, presentations and books to get you there, and the JBossTS documentation will help too. But suffice it to say that with a top-level transaction, you get the all-or-nothing (Atomic) affect: no matter how many resources are updated within the scope of that transaction, if one of them commits the work then all of them will and likewise, if one of them rolls back the work then all of the others will also roll back their work.

Now this is a great capability to have, especially if you have to do very little as a user to get there. However, for some applications and objects it can be restrictive. For instance, if your transactions last for a long period of time (say minutes), updating different resources throughout, and just at the very end there's a failure, then everything has to be done again. This and other limitations with top-level transactions lead us and others to look at various extended transaction models, and one of the earliest approaches was nested transactions from 1981, also known as subtransactions (not to be confused with subordinate coordinators).

Transactions which are contained within the other transaction are said to be nested (or subtransactions), and the resulting transaction is referred to as the enclosing transaction. The enclosing transaction is sometimes referred to as the parent of a nested (or child) transaction. A hierarchical transaction structure can thus result, with the root of the hierarchy being referred to as the top-level transaction. An important difference exists between nested and top-level transactions: the effect of a nested transaction is provisional upon the commit/roll back of its enclosing transaction(s), i.e., the effects will be recovered if the enclosing transaction aborts, even if the nested transaction has committed.

Subtransactions are a useful mechanism for two reasons:

  • fault-isolation: if subtransaction rolls back (e.g., because an object it was using fails) then this does not require the enclosing transaction to rollback, thus undoing all of the work performed so far.

  • modularity: if there is already a transaction associated with a call when a new transaction is begun, then the transaction will be nested within it. Therefore, a programmer who knows that an object require transactions can use them within the object: if the object’s methods are invoked without a client transaction, then the object’s transactions will simply be top-level; otherwise, they will be nested within the scope of the client’s transactions. Likewise, a client need not know that the object is transactional, and can begin its own transaction.

Imagine what you could do with nested transactions, particularly if you get to control them programmatically. You can start transactions within your application or objects without having to be concerned about whether or not there are transactions already in scope. If you do know and care, then you can utilize them for application needs. For instance, nested transactions give you the ability to try alternate paths in a decision tree without having to worry about the impact on transactional resources; you try each option in a separate nested transaction and roll back the paths you don't chose, relying on the transaction system to undo that work automatically, and then you simply commit the selected path (nested transaction).

So how do you use them? Well in JBossTS there are no special constructs for nesting of transactions: if a transaction is begun while another is running on the thread then it is automatically nested. This allows for a modular structure to applications, whereby objects can be implemented using transactions within their operations without the application programmer having to worry about the applications which use them, i.e., whether or not the applications will use transactions as well. Thus, in some applications those transactions may be top-level, whereas in others they may be nested. Objects written in this way can then be shared between applications and JBossTS will guarantee their consistency. If a nested transaction is rolled back then all of its work will be undone, although strict two-phase locking means that any locks it may have obtained will be retained until the top-level transaction commits or rolls back. As mentioned before, if a nested transaction commits then the work it has performed will only be committed by the system if the top-level transaction commits; if the top-level transaction aborts then all of the work will be undone.

What all of the above means is that if you want to use nested transactions in your applications then all you need to is start them programmatically as we saw earlier. So for example, if we wanted to use the AtomicObject we defined earlier within a top-level transaction, the code could look like:

AtomicAction A = new AtomicAction();
AtomicObject obj = new AtomicObject();




The fact that the AtomicObject incr method uses a transaction internally is hidden from the invoker. In this case it'll be nested. If the code looked like ...

AtomicObject obj = new AtomicObject();


... then the transaction would be top-level. Hopefully the differences in behaviour are pretty obvious by now, but just in case: in the former case we get a chance to undo the increment if we were to roll back the enclosing transaction, whereas in the latter, assuming the incr method didn't fail, then there is no opportunity to roll back; we would have to do an application driven undo which isn't guaranteed to be equivalent from an ACID perspective (e.g., another user could have seen the state between the increment and the decrement). And of course that leads us towards compensating transactions, but they are definitely out of scope for this article.

So I hope I've given you a flavour to nested transactions and some of the power they could bring to applications and objects. Why not give them a go?
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