Code Abstractions and Babushka Dolls

There are various ways of encapsulating program code. We encapsulate code so that it can be invoked from some other program code. The encapsulated code can be used in its abbreviated form. I will also refer to it as an abstraction in this posting.

There are some ways of encapsulating code that encapsulate a small amount of code and are fine grained. There are some ways of encapsulating a large amount of code or other abstractions of code and are coarse grained. I would like to be able to list all the forms of encapsulation from smallest to largest and arrange them like Russian Babushka dolls with a clear scheme for which goes inside which.

At the finest level of code abstraction I will start with functions, procedures and methods. They are pretty simple. The name 'function' has a mathematical significance the pre-dates computers. Procedures, subroutines and methods are all variations on the theme of a group of instructions.

The next level of abstraction is the 'class' or the instance of that class the 'object'. Object is an unimaginative name. We may as well have used the synonymous term 'thing' but that would have sounded like we just did not know what to call this abstraction of code. "Let's build a thing to access our database" sounds a lot less specific than "Let's build an object to access our database".

Then we have services. These should be reasonably coarse grained. They may contain classes which may contain methods. I skip java beans in this abstraction discussion that would seem to inhabit the same level in the hierarchy as services.

My metaphor of the Babushka dolls is already looking a bit shaky. There are methods that can contain methods, classes that contain classes. Recursion does not work very well with Babushka dolls. Also there are abstractions like code libraries and include files that cut-across the above abstractions, but I will press on anyway.

The level of abstraction I would like to look at is the component as described by the Service Component Architecture (SCA). Component is another synonym for 'thing'. Instead of 'object' and 'component' we may as well have used 'thing 1' and 'thing 2' and apologized to Dr Seuss for using his naming convention. According to the SCA we should then consider encapsulating our components in 'composites' and our 'composites' in 'domains'.

'composites' and 'domains' at least are getting way from being synonyms of 'things'. A composite is just a group of things and a domain is somewhere you find things in, a bit like a container. Of course a 'container' is typically seen as more a runtime environment than a way of collecting together code or abstractions.

'Domain' is a rather overused term in IT at present. Do not try to draw any parallel between an SCA domain, a Domain Name Service (DNS) or Domain Driven Design. There is very little connection.

I would like to bring the concept of Roger Sessions' 'Software Fortress' in at this point. At least Roger has come up with an imaginative name for his abstraction. His explanation of the various roles that software plays in relation to the Fortress is also imaginative as these are illustrated by various goblins, fairies and ogres in his documentation.

Definition: Software Fortress
A software fortress is a conglomerate of software systems consisting of potentially many processes running on potentially many machines spread out over a potentially wide geographic area that:
I. Serve a common purpose
II. Are owned by a cohesive group of individuals
III. Work together in a trust relationship.
IV. Are built on a common technology base.

Roger could improve his definition by indicating whether "or" or "and" separated the different lines of his definition. Definition line IV suggests the Fortress is quite similar to the Domain concept from SCA. If we include the other three definition lines we may find that a Fortress has finer granularity than the domain and may be closer to a composite or component.

The recommendation provided in a couple of presentations I have seen from Glenn Smyth is that web services should be built if and only if they are required on the border of a software fortress. The essence for the SCA is that a service appears on the border of all components. Therefore the finer grained interpretation of 'Software Fortress' that incorporates an 'and' between the last lines of the definition would seem more consistent with this view.

One difference between ways of encapsulating program logic is the stage in the software development life cycle. At runtime then objects, services and containers encapsulate running logic. At development time classes, include files and functions are relevant. At requirements definition time the domains in domain driven design have relevance and the "common purpose" aspect of a fortress helps define the encapsulation. We therefore would more correctly have three sets of Babushka dolls.

SCA is all about runtime and development time. There is nothing that suggests what part of the design a component represents. Domain driven design is all about business requirements. There is nothing in this process that advises how you are to encapsulate your code to represent the domain. SOA attempts to span all three stages of the development life cycle both by suggesting business services and then keeping to this granularity in the developed code and the runtime.

The encapsulation and abstraction of logic is a complicated area. I probably have not made it any simpler with this posting. It is increasingly vital in software. There is not much excitement as there used to be around programming languages but how we bundle together program instructions is still an area of active interest. The point to take away from this is that we have to define our terms clearly and use them in the context of the stage of the software development lifecycle. A consistent approach from design through to runtime would be nice, but at least your software development methodology should identify the mapping between different ways of abstracting business logic thus linking your three or more sets of Babushka dolls.

References

David Chappell, Introducing SCA,

http://www.davidchappell.com/articles/Introducing_SCA.pdf

Eric Evans, Domain Driven Design,(WebCast)
http://www.infoq.com/presentations/model-to-work-evans

Eric Evans, Domain Drive/Model Driven
http://domaindrivendesign.org/discussion/blog/evans_eric_ddd_and_mdd.html

Roger Sessions has book on Software Fortresses which I confess I have not read yet but the following article incorporates and overview of this concept and the newsletter below explains some of its origins.
Roger Sessions, Modeling Software Architectures
http://www.objectwatch.com/PaperAndSpreadsheetVersion1-0.zip

Roger Sessions, The Software Fortress Model; A Next Generation Model for Describing Enterprise Software Architectures, ObjectWatch Newsletter # 36, November, 2001

http://www.objectwatch.com/newsletters/issue_36.htm

Glen Smyth's presentation to the ACS is now available at
http://sa.acs.org.au/it_arch/index.php/Presentations_Past%2C_Present_and_Future