Tuesday, January 31, 2012

##what are(not) part of Architecture (too late to change

Dogfight - When you develop a dogfight jet over 30 years, you stick to your initial design. You try to build on the strengths of your design and protect the weaknesses. You don't suddenly abandon your design and adopt your rival's design because you would be years behind on that path. Software architecture is similar.

Q1: what Features define a given architecture? I'm a fellow architect. If you must describe in 10 phrases your system architecture and provide maximum Insight, which 10 aspects would you focus on?

Q2: (another side of the same coin) What technical Decisions must be made early when creating a system from scratch? 
Q2b: in other words, what design decisions will be expensive to change later? Why expensive?

Disclaimer -- In this context, "later" means somewhere during those pre-UAT phases, not LATE into the release process. Obviously after release any config/DB/code change requires regression test and impact analysis, but that's not our topic here.

A colleague of mine liked to discuss in terms of layering. In OO architecture, A layer consists of a bunch of similar classes (or interfaces) dedicated to a logically coherent high-level task. 2 interacting layers can exist in the same OS process or (less common but very critical) across processes. Intra-process layering basically means the 2 interacting classes (belonging to 2 layers) have well-defined responsibilities and a well-defined contract between, where one is always a Service-provider ("dependency") to the service-consumer. Though popular among architects, I'm not sure these are the most important architectural features.

Here are my answers to the question "Is the decision costly to change later in SDLC?"

? anything in the wsdl/schema of a web service? yes if i'm not the only developer of this web service's clients. A change would need these fellow programmers to change their code and test, test, test
? universal and fundamental data constraints like "if this is positive then that must be non-null"? Yes because other developers would assume that and design code flows accordingly.
? database schema, the ER between a bunch of tables? Yes since it usually requires code change "upstairs". DB schema is usually a Foundation layer.
? choose a DB vendor? yes if you happen to have truly skillful database developers in the team. They will use proprietary vendor extensions behind the manager's back when the temptation is too strong. Such non-portable business logic will be expensive to re-implement in another database.
? use MOM or RPC-style communication? yes
? Serialization format? yes. "Later" means implementation effort wasted
? XML or binary or free text format? yes probably
? make a task re-runnable? yes. Adding this feature later on can be hard
? make a bunch of objects immutable/read-only/const? yes. Adding this feature can be hard. Look into C++ literature.
? shared vs unshared cache? yes. effort is non-trivial
? What logic to put into SQL vs java/C#? sometimes No. You can change things later on, but usually wasting sizable implementation effort
? MVC (the highest-level design pattern there is)? yes, though you can adjust this design later on
? Choose programming language? Yes. Implementation effort wasted if switching late in the game.
--

? where to implement authentication, entitlement? Many say yes, but I feel no. Make the damn system work first, then worry about securing it. On fast-paced trading floor, Business usually can tolerate this feature being not so perfect. Business and upper management worry more about ..... You got it -- Functionality.

? Which commercial(or free) product to choose? sometimes NO. You can use mysql or activemq and then change to commercial products. In fast projects, We can often delay this decision. Start coding without this particular commercial product.

? make a module thread safe? no, even though adding this feature can be hard
? how many staging tables? no
? choose a decorator/factor/singleton (or any of the classic) design pattern? no. They are usually too  low-level to be among the "10 architectural features". I don't know why a lot of architect interview questions hit this area.

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