Sunday, March 13, 2011

Slightly alkaline transactions if you please ...

Given that the traditional ACID transaction model is not appropriate for long running/loosely coupled interactions, let’s pose the question, “what type of model or protocol is appropriate?” The answer to that question is that that no one specific protocol is likely to be sufficient, given the wide range of situations that transactions are likely to be deployed within. In fact trying to shoe-horn ACID transactions into a wide range of situations for which they were never designed is one of the reasons they've gotten such a bad reputation over the years.

There are a number of different extensions to the standard transaction model that have been proposed to address specific application needs, that may not be easily or efficiently addressed through the use of traditional transactions:

Nested transactions: permits a finer control over recovery and concurrency. The outermost transaction of such a hierarchy is referred to as the top-level transaction. The permanence of effect property is only possessed by the top-level transaction, whereas the commits of nested transactions (subtransactions) are provisional upon the commit/abort of an enclosing transaction. And yes, JBossTS supports nested transactions!

Type specific concurrency control: concurrent read/write or write/write operations are permitted on an object from different transactions provided these operations can be shown to be non-interfering. Oh and yes, we support this too!

Independent top-level transactions: with this model it is possible to invoke a top-level transaction from within another (possibly deeply nested) transaction. If the logically enclosing transaction rolls back, this does not lead to the rollback of the independent top-level transaction, which can commit or rollback independently. In the event that the enclosing transaction rolls back, compensation may be required, but this is typically left up to the application. Yes, we've got this one covered as well.

Structured top-level transactions: long-running top-level transactions can be structured as many independent, short-duration top-level transactions. This allows an activity to acquire and use resources for only the required duration. In the event of failures, to obtain transactional semantics for the entire duration may require compensations for forward or backward recovery. Now although we don't support this directly within JBossTS since it is really workflow, JBossTS has been used to implement workflow systems for many years.

What this range of extended transaction models illustrate is that a single model is not sufficient for all applications. Therefore, is it possible to develop a framework within which all of these models can be supported, and also facilitate the development of other models? This was the question asked by the Object Management Group when it began its work on attempting to standardise extended transaction models. In this paper we shall given an overview of the results of the work we performed with IBM, Iona and others in producing the final Activity Service OMG specification that attempts to answer that question. This also became a Jave specification with JSR 95.

Now I'm not going to go into this standard in any detail, since that could be the subject of several other blog posts, but I will summarise it as this: have a generic coordination infrastructure that allows the intelligence behind the protocol (e.g., two-phase or three-phase) as well as the trigger points for the protocol (e.g., at the end of the transaction or during the transaction) to be defined at runtime and in a pluggable manner. But why did I mention all of this? Because this standard formed the basis of the Web Services transactions standard (and the various competing specifications that came before it), with WS-Coordination as the pluggable infrastructure.

At this point in time there are only two types of transaction protocol supported by the Web Services standard and the REST-based approach that we're working on:

• ACID transactions: Web services are for interoperability in closely coupled environments such as corporate intranets as much as they are for the Web. Interoperability between heterogeneous transaction service implementations is a requirement and yet has been difficult to achieve in practice. This transaction model is designed to support interoperability of existing transaction processing systems via Web services, given such systems already form the backbone of enterprise class applications. Although ACID transactions may not be suitable for all Web services, they are most definitely suitable for some, and particularly high-value interactions such as those involved in the finance sector. For example, in a J2EE environment, JTA-to-JTA interoperability is supported through the JTS specification, but this is neither mandated nor universally implemented.

• Forward compensation based transactions: this model is designed for those business interactions that are long in duration, where traditional ACID transactions are inappropriate. With this model, all work performed within the scope of an application should be able to be compensated such that an application’s work is either performed successfully or undone. However, how individual Web services perform their work and ensure it can be undone if compensation is required, is an implementation choice. The model simply defines the triggers for compensation actions and the conditions under which those triggers are executed.

And the nice thing about our implementations of both of these is that they're based on the exact same transaction engine: ArjunaCore. Because it was designed with extensibility in mind and also with the requirement to be able to relax the various ACID properties, over the last decade we've been able to use it relatively untouched as the basis of pretty much every transaction protocol and implementation that's been out there. So if we do decide that we need to add another extended transaction protocol for other use cases, I'm pretty confident that it won't require us to start from scratch!
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