David Chappell


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The State of SCA: An Update  
# Friday, May 09, 2008
I moderated a panel on Service Component Architecture (SCA) at JavaOne last week. I was also the moderator for last year’s SCA panel, and several of the same people were on the panel with me this time. While the things we talked about were broadly similar, two things stand out about what's changed in a year.

The first is that SCA is real, or at least part of it is. One of the things the SCA specs define is an XML-based language called the Service Component Definition Language (SCDL). SCDL is meant to provide a vendor-neutral way to describe how components created in various technologies, such as Java, BPEL, and Spring, are configured and wired together to create applications. Vendors were showing SCDL in real products on the JavaOne floor—Oracle had an especially nice demo—and so it's clear that this part of SCA is seeing some success.

Whether SCDL will in fact provide much cross-vendor portability remains to be seen. As usual, this depends on how many proprietary extensions vendors add. Still, a standard language for describing the components and assembly of an application is a useful idea, and the signs so far are promising.

The second thing that stands out after a year is less promising: It’s the confusion around how to write SCA components. Along with SCDL, the SCA specs define how to create components using several different technologies. Yet the various SCA vendors and open source projects can’t agree on which of these to implement. SCA support for Spring components, for example, is hit or miss: some SCA offerings support it, some don’t. BPEL is much the same—Oracle is a big fan, while the open source Fabric3 currently has no BPEL support.

And just as it was a year ago, support for SCA’s new programming model for creating Java components is uneven. As I've written before, I believe that this aspect of the spec is really important--it unifies the diverse approaches of Java EE much as Microsoft's Windows Communication Foundation (WCF) unified the diverse programming models in the original .NET Framework. Yet this part of the SCA standard has always been contentious. At last year's SCA panel, for example, the SAP rep asked the audience who wants to see a new programming model for Java components and (unsurprisingly) got no hands raised. Accordingly, SAP has been a leader in defining an alternative way to create Java SCA components as EJB 3.0 session beans. This alternative is a superset of SCA's original component model, so it's not a wholly new thing. Still, the challenge for developers and decision makers is to choose among these various options, and so creating more of them is problematic.

Some existing SCA offerings, such as Fabric3, implement the original Java programming model for SCA components and apparently have no intention of supporting the EJB-based approach. SAP, by contrast, explicitly told me that they have no plans to support the original Java programming model; they're going with the EJB-based approach. IBM, ever the big tent, is supporting both.

The stated goal of SCA is to provide application portability. Widespread support for SCDL is an essential part of this, but so is agreeing on how to create SCA components. For SCA to really improve portability, the vendors and open source projects that support it need to agree on how their customers should create components. If they don’t, SCA risks becoming yet another standard that doesn’t provide much benefit to the people who use it.

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I went to the panel in question, and for the most part found it very educational. You asked some very hard questions which are great, but I don't quite follow your concern over the SCA programming model.

Java already has a multitude of programming models; example: Spring, jRuby, etc.

There is a strength in diversity found in the Java platform not found in some of the more proprietary competition. (ahem: Microsoft)

This allows Java to address a diverse set of concerns to a diverse set of users.

Secondly, the SCA programming model is a POJO programming model, it's based on annotations, XML configurations, and dependency injection. This means that service components developed using SCA can be easily plugged into different containers regardless of the model being supported, as long as it's a POJO one.

I posted on SCA and JavaOne, as well as the panel that you moderated in particular at http://agileconsulting.blogspot.com/2008/05/highlights-of-sca-at-javaworld-2008.html

Jeff Anderson

Hi, David. Very interesting article and as someone said before, very informative also. But there's something that I don't understand too much, and is the importante or relevance the assembly model has in all the SCA world. I ask this because I haven't seen too much about this model, or the viability to use it to model coarsed-grained components such as web services in a SOA architecture. What's your opinion about this?

Thank you very much!

There are two big reasons why the lack of vendor agreement on how to create SCA components matters so much. First, remember that the goal of SCA is to allow creating portable applications. If vendors don't support the same programming models for components, this goal can't be achieved. Although your comment refers to "the SCA programming model", there is actually no single programming model for SCA--there are several, and not all of them are based on POJOs (in fact, not all are based on Java).

The second reason why vendor support for a common programming model matters is that having too many choices will make SCA an unattractive choice for IT decision makers. While developers love having lots of choices, their managers usually hate this kind of diversity in their environment. It makes maintaining applications significantly more difficult and expensive, since they've got to keep people around who know all of these different technologies. Making these decision makers happy is a big part of why vendors bother to define programming model standards such as Java EE or SCA.

If SCA provides even a small set of standard programming models, it can make life better for IT decision makers who must choose an application platform. If each SCA vendor and open source project supports its own idiosyncratic group of programming models, however, the standard will have much less value. It's no better than letting each vendor define its own programming model.

Guzmán: The SCA assembly model certainly can be used to group coarse-grained components into a composite--this is one of its main goals. Remember, though, that this only works (today, at least) if all of the components are running on the same SCA infrastructure, i.e., one provided by a single vendor or open source project. Don't expect to be able to use SCA to assemble arbitrary Web services built on diverse vendor platforms into a single composite--it won't work.

For more on this, you might read the introduction to SCA that I wrote last year.

David ,
I am unsure if I entirely agree that the primary goal of SCA is to support the creation of portable applications. I would believe the primary goal of SCA is to provide a common assembly language that supports the wiring together of different technology assets into a cohesive component, using a consistent approach, regardless of technology platform.

Offering a consistent programming model to Java developers regardless of Java component "type" (EJB, JMS, etc.) seems like a secondary objective, but one that makes good sense because traditional Sun flavored approaches just don't seem to be offering this to Java developers.

I definitely agree that having Sun onboard is both useful and desirable, but the Java platform is increasingly supporting diverse models for diverse users.

IT managers definitely benefit from a consistent programming model, they still can, but they don't need to be restricted by the platform. Managers just need to choose the most suitable model for the project/program, and use good governance to evangelize or enforce.

But Jeff, you say that SCA's goal is to provide "a common assembly language". The reason a common assembly language is valuable is because it allows portability of applications and (potentially) developer skills. If portability isn't the goal, then each vendor should just define its own proprietary language for doing this.

Still, allowing portability of just the assembly part of an application makes no sense. The most valuable parts of a typical app--and thus the aspects that an organization would most want to move from one vendor platform to another--are its business logic. If the assembly language my app uses is standard, but the component technology I've chosen to implement this business logic on isn't widely supported, I'm no better off--the app is still locked into the platform it was built on.

Vendors love this kind of thing. They can claim--correctly--that they support standards while still locking customers into their platform. But users should always be wary of vendor attempts to do this. It goes against the whole purpose of creating standards for portability.

What is the state of SCA today? I notice that some of the newer ESBs like WS02 and Fuse ESB avoid it.

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