The draft for the first chapter of my SOA Patterns book is available on-line from Manning Publications Co.

The first chapter talks about  software architecture and the inputs the architect can/should use to design one (emphasizing Quality Attributes); Explains the challenges of distributed systems and takes a look at the SOA from an architectural perspective.

You can download the chapter from here

Any comments are welcome (you can also leave your comments at soa@rgoarchitects.com)

Roy Osherov recommended this site today - but he also urged me to write more frequently.

This is probably a good opportunity to explain how posts are divided between my 3 blogs

• First theres the blog on Dr. Dobb's Journal. This blog is published on the "Architecture & Design" section of DDJ portal. I blog there about 3 times a week. Jon (my Editor @ DDJ) prefers a steady stream of blogs over longer posts which means that I break down large subjects (like OO principles, fallacies of distributed computing and the currently running series on the Architect's soft skills) into many parts.

• The second blog is a new one on Microsoft's Israel blogs site. The aim of this site is to bring Architecture content in Hebrew to the Israeli architects (As can be imagined, most of the technical content available is in English, I thought it was important to generate some content in Hebrew as well)

• The last blog is this one. My current plan for this blog is as follows

• • Cross posting selected posts from the DDJ site
  • I am posting here complete articles made by editing and aggregating multi-part blogs posts (again such as the fallacies etc.)
  • Pointers to presentations and articles I publish
  • In the near future, I'll start posting bits of my upcoming SOA patterns book (I am currently writing chapter 3). I've already documented 8 patterns (of more than 50 patterns and about 30 anti-patters). I plan to publish here at least some of the patterns here for review (I am still crossing the t's and dotting the i's with my publisher but I expect this to be finalized soon)

so Roy, does seven (7) posts in seven days (including 3 on Ms Israel site, 3 on DDJ and this one) qualify as posting often enough? :)

[Will also be cross-posted on my DDJ blog]

Working on my SOA patterns book, I thought of this rule for contract versioning which my shameless ego wanted to dub Arnon's Contract Versioning Principle. I was happy playing with this thought, until I realized that there isn't some profound new understanding here, this is just an application of LSP for service contracts.

Liskov Substitution Principle (LSP) which I recently blogged about here as part of a series of blogs on Object Oriented Principles, basically states that a subclass should be usable instead of it parent class. To put this in other words you could say that a subclass should meet the expectations that users of the parent class have come to expect from the parent class's observable behavior.

So LSP applied to SOA would state that:

When changing the internal behavior of a service, you don't need to create a new version of the contract if for each operation defined in the contract the preconditions are the same or weaker and the postconditions (i.e. the outcome of the request) are the same or stronger or in other words the to retain the same contract version, the new version of the service should meet the expectations that consumers of the service have come to expect from the old version's observable behavior

For example, let's say you have a customer service and the contract lets you get a customer's VIP status. If you changed the way the VIP status is calculated (e.g. in the old version the customer had to have 1 million dollars in her account, but now she must have 10 million dollars) there's no need to create a new contract version. However if you introduced a new level of VIP status (e.g. 1 Million = Gold, 10 Million = Platinum) you do need a new version for the contract

I've added a new section on the site www.rgoarchitects.com/Papers to allow easy access to all the papers, presentations and articles I published (and will be publishing e.g. I'll add a paper on architect soft skills in a month or so etc.)