Friday, December 01, 2006

Service Architecture: JBOWS, SOA or WOA

A few days ago, I got into a discussion of whether the Amazon Web Services are true SOA or just a programming model exposed using web-services. The discussion started when I said that CRUD style operations are not according to SOA best practices; and that operating on parts of a domain object of type aggregate root should be avoided, favoring actions on the complete entity to be exposed as service operations.

Before making my SOA-vs-JBOWS case, a short introduction to WOA: as adhering to all the SOA service design principles and the different WS-* technologies can be quite daunting and complex, a lot of service-oriented solutions have emerged that take a simpler approach (REST, SOAP, POX, etc). Gartner has coined the acronym WOA (Web-Oriented Architecture) for these kinds of solutions that implement and expose services using more pragmatic techniques following the WOA tenets.

First the "what's in an operation name" issue:

It is not best practice to use CRUDy style operation names that conveys only that the domain object state is going to change in the repository (e.g. UpdateCustomerAddress). You should rather use operation names that reveals the event in the business process that caused the action (e.g. CustomerHasMoved). The point is that your service will most likely perform some business logic on the domain object in conjunction with storing it in the database. The operation name should convey the fact that the state of the real-world entity represented by the domain object has changed.

Note that you need not change the operation names very much, sometimes renaming UpdateXxx to ChangeXxx can be sufficient to convey the correct semantics. Focus on creating business process driven services rather than data-driven services.

The discussion I had was about if names such as AddXxx, ModifyXxx and DeleteXxx are CRUDy style names or not. I think they are to closely named after what the code is going to do with the domain object repository rather than reflecting changes to the real-life entity, i.e. it is more a programming model (WOA) than a SOA operation.

Note that CRUDy operations and names can be quite ok, especially for information services, which will still be needed to manage data in your repositories. There will still be a need for creating new customers, even in SOA.

Then on to the operations on different types of domain objects issue:

A common metaphor for designing SOA operation and data contracts is to think of paper forms being passed around to clerks to fulfil a business process. Another metaphor is mail orders, the point is that the 'document' contains all data needed to perform the business process.

Documents can be simple domain objects or aggregate root objects. An order is an example of an aggregate root object - it contains general order data and a set of order items. The order items belong to the order, i.e. it is an identifying relationship and not just a relation. As a general rule, an operation should process a whole document and never mess directly with 'identified items' within the document. Such operations would be very CRUDy style, and breaks the "boundaries are explicit" tenet.

Note that it is perfectly legal to operate on normal relations, such as adding orders to a customer. It is the nature of the domain and its business processes that decide if a relation is identifying or not, thus there is no hard rule to help you decide whether incremental operations are ok or not.

Using the 'document' metaphor, you would implement a 'RegisterOrder' operation to add new orders. You should, however, not implement operations for changing a subset of the order items; you must rather implement a 'ChangeOrder' or a 'CancelOrder' operation.

In my opinion, the Amazon shopping cart is analogous to the above order domain object. Thus, according to SOA best practices, they should not expose operations such as ChartModify that allows you to do incremental changes to the items in the chart. I understand that they do this for simplicity and performance reasons, and that the shopping cart is really not registered until it is submitted; but some developers use the AWS as general best practices for SOA and apply the same programming model blindly.

I don't propose that Amazon Web Services are not properly designed; they are just WOA rather than SOA.
A closing note: there is no "one or the other" between SOA and WOA, in fact they are both central in web 2.0. Read more about it in this report from the Web 2.0 Summit.

1 comment:

Kjell-Sverre Jerijærvi said...

Read about the core categories of data contracts in service-oriented solutions: Five core data contract categories