Thursday, March 22, 2007

SD West Classes and Keynotes: Day 2

Agile Architectures (Granville Miller)

8.30 and the room is packed! Architecture sure is a subject that attracts people and make them react.

After reminding us that canonical agile does not mandate any architect role, Granville explained how the community has evolved towards the recognition of the need for architecture. This said, the agile myth that "refactoring is enough to make architecture useless" is still alive. Though experienced developers will have good design reflexes, there are at least two compelling reasons for them to err on the side of architecture:
  • when they will need to step back in order to keep on progressing (ditto Ward Cunningham),

  • to ensure smooth integration of the different parts of a large project.

Hence when a developer will wear the architect's hat, he will typically:
  1. partition the system on a whiteboard,

  2. discuss this partition to reach an agreement among the team,

  3. write unit tests, fa├žades, scaffold to lay out the baseline of the architecture,

  4. then take his hat off and code.
How long will the architecture defined at step 1 be valid? Well, about an iteration, which is by the way the timeline of these 4 steps. The main lesson of this is that agile architecture is not carved on stone but evolves as the rest of the system in order to satisfy the most important of the customer rights: to change his mind without paying an exorbitant cost.

To visualize this reality, Granville presented the notion of shadow architecture on a chart that shows the transfer between leading shadows and trailing shadows. At the start of the iteration, leading shadows represent the majority of the architecture shadow: they are the products of white board design sessions. At the end of the iteration, trailing shadows totally replace the leading shadows: they are the actual reification of the architecture, possibly visualized in code analysis tools. If some leading shadows remain, it means more work to do on the next iteration. This is a great tribute to Punxsutawney Phil and the doom a visible shadow represents for this innocent animal and the rest of us, Spring addicts (!).

Creating a Domain-Specific Language (Juha-Pekka Tolvanen)

During this class, Juha-Pekka explored the creation of a domain-specific language (DSL) in the particular concept of interactive TV. The main goal to achieve in this process was to provide content producers with a limited language (15 concepts maximum, compared to UML hundreds of concepts) that they could use to actually build interactive TV applications.

How to proceed? Here are four main steps:
  1. Identify abstractions (ask the producer to "mind map" the different use cases in order to build a metamodel),

  2. Define modeling concepts and rules (notation),

  3. Specify notation,

  4. Implement generators & test with reference cases.
The overall presentation was then focused on creating the DSL using the speaker's company tool, MetaEdit+, which unfortunately do not currently run on Intel Macs so I could not do the exercise on my machine.

To my sense, like all vendor sponsored talks, it was too much focused on a particular tool and not enough on the available options out there. I was, for example, expecting to see lighter-less-box-and-arrows-and-XML-outcome things similar to what Martin Fowler is currently exploring in his bliki or to Drools DSL.

Agile Principles of Object-Oriented Class Design (Uncle Bob)

Uncle Bob right after lunch is probably the best cure for food drowsiness. His capacity to captivate the audience is amazing: I suspect he irradiates some mystical fluid that make our body redirect energy from food processing to brain power. Or he is simply a plainly bright orator.

After a quick and obviously unconventional introduction to OO, Uncle Bob immediately exposed the root cause why our application code starts to rot after time until it becomes an unmaintainable and tangled Gordian knot. And it is... drum roll... poor dependency management! Of course not (only) in the Maven sense of it but in the broader sense of dependency between objects, packages and components.

Poor dependency management leads to rigid, fragile and not re-usable code. If any change implies going through the whole structure, if touching one place breaks the system in many places, if no bit of code is re-purposable easily, then you surely have tight coupling which is a manifestation of a poor management of dependencies. How to write solid code? Tattoo "solid" on your forehead? Nope. But use it as a mnemonic for the five principles of software development craftsmanship wisdom:

  • Single responsibility principle: a class should have one and only one reason to change (it must not support features that have completely different evolution life-cycles).

  • Open closed principle: adding a new feature must be done by adding new code, not by modifying existing one.

  • Liskov substitution principle: derived classes must be valid substitutes for their base classes.

  • Interface segregation principle: avoid fat interfaces that offer methods relevant to many use cases but mainly irrelevant to its users.

  • Dependency inversion principle: details should depend on abstractions, never the opposite.

In this quest for good dependency management, abstraction is the most important keyword. Everything should depend on abstractions and not on implementations in order to reach the ultimate goal of independently deployable units. Because abstractions are critical to protect our application from the changes that will inevitably occur, how is it possible to figure them out in order to be safe? Should it be an upfront task? Is a crystal ball needed?

This is where the agile word come into play, not as hollow buzz but as a reality: build the abstractions as you need them, which can be as often as you get feedback from your users. Hence expose your system early and frequently to them...

What's New in XML in Java 5 and Java 6 (Elliotte Rusty Harold)

XML is not only for girls and can actually be proven useful, mind you! But parsing or processing XML never came for free in Java and have always required careful development practices to limit its impact on CPU or RAM. In this class, Elliotte presented the latest addition of Java 5 and 6 and how they can make XML related development easier and more efficient.

Here is what Elliotte had in his grab bag for us today:

Java 5

JAXP: The API is at version 1.3 in Java 5 with Crimson finally kicked out and replaced by Xerces and at version 1.4 in Java 6.

DOM3: In the many sweet additions of DOM level 3, Elliotte insisted on the addition of get/setTextContent on the Node interface. I can not agree more: time and again I have seen developers using an extra XML APIs just because writing or reading text was too painful with plain old DOM Level 2. Now the JDK offers everything you need for regular XML twiddling: that is one dependency to remove, which is always good.

I also find Tree Position interesting as you sometimes want to refer to a node without having to enter into the realm of XPath. Other additions of note contain Node Similarity and Equality methods, better namespace support and the possibility to bind any custom user data to any node.

DOM has also evolved as a framework with the capacity to control how documents are represented in memory (DOMConfiguration), new ways of creating documents (DOMImplementation) that avantageously replace the factory based JAXP builders and load and save features (LSParser and LSSerializer).

In the matter of parsing files, the newly added LSParserFilter can be leveraged to accept only certain nodes hence reduce the memory footprint of a document, a good strategy to consider for large XML instances you do not fully need in memory. LSParserFilter can also be used to alter a document at load time (for example to change a namespace into an other).

XPath: is now available in v1.0 with namespace support (NamespaceContext) and custom XPathFunction support.

Validation: has been added in a very versatile way as you can select the schema language you intend to use (Schema, Relax NG, DTD...). A validator has also the interesting ability of augmenting an document by adding optional elements discovered in its associated schema. It is now also possible to discover the type of a node based on a the validator of the document.

Java 6

XML Digital Signature: which is apparently a pretty involved API that is not limited to XML signing...

StAX: which is a promising pull parsing technology that is fast, memory efficient, streamable, read only. Sounds like SAX? Yes but think of StAX as an inversed SAX where the application actually calls for event via method invocations instead of being called back when they occur. This allows to control the flow of events hence to orient parsing for a greater efficiency.

And what about the future? Elliotte mentioned that XQuery API will probably make it to Java 7 while XML Encryption will probably wait a little longer before showing up in the JDK.

OK, I managed to conclude my blogging frenzy earlier today. Note that my main issue in this field trip blogging is the short live of my MacBook Pro battery...


zepag said...

3 small comments:

- First thanks for your post about Usurper ;) (this should be a comment of this precise post, but anyway...)
- Then if something like an architect hat exists, I want it ;);)
- Finally Stax rules, we do use it and it's SAX for speed + easy to use + write abilities. Woodstox is a good OSS implementation BTW.


David Dossot said...

Thanks for the feedback on StAX: I am looking forward to be able to use it!

Here is the hat you are looking for: at least it fits me perfectly.


jf said...

I remember having used StAX a ong time ago to split huge xml files into smaller ones.