Wednesday, October 21, 2009

Armstrong Ch2

Joe Armstrong's definition of what descriptions a software architecture should be composed of are quite good a characterizing a system.  One thing that I think should be moved out of the problem domain section into its own is that of performance constraints and requirements.  In this case, the problem domain clearly states that a telecom system should exhibit certain behavior.  But in other cases, the problem domain might not be so explicit--a system designed to revolutionize an industry by bringing priorly unheard of speed to solving problems might not characterize the performance requirements in the problem domain--this would be more an attribute of the system that we are trying to build to suit the problem domain, rather than an intrinsic property of it.   Required performance guidelines are certainly a central tenet of how a system should be architected.

Messaging for parallelism makes a lot of sense.  It helps to reduce the affect of unnecessary assumptions that can be easily imposed by shared memory type systems.  I have worked with a number of web services oriented applications which essentially use messaging, and it certainly does enforce strong separation.  However, in most of the systems I have worked on with web services, the calls have been blocking in nature, hence no parallelism gains were realized.

"Fail fast" kind of scares me.  The idea that an entire process should terminate its execution upon reaching an error case leads me to believe that performance could be detrimentally affected.  For example, in the case that a process uses a great deal of cached data, assumedly this cache would be flushed when the process "fast fails", and therefore there could be quite a bit of a performance hit by reloading this cache each time the process restarts.

I think that concurrency oriented programming makes sense in certain problem domains.  In others, I could see this style as an impediment from getting the job done.  I suppose that as the supporting tools for this type of programming evolve, the barrier to entry of this programming style will be reduced, but it seems to me that unless concurrency is priority one in the system, adopting this model would potentially make more work for you.  Having said this, more and more systems are setting concurrency as priority one (especially in the server software world), so I am by no means discrediting this style.  Rather, I am proposing that it should be adopted judiciously--only when concurrency is of high enough priority to warrant the overhead.

An unreliable messaging system makes a ton of sense to me.  Just look at how the internet has evolved, and worked well (arguably, I guess) for a wide variety of applications.  To impose a reliable messaging system would be to burden certain types of communication with overhead that they don't require.  Furthermore, as reliable messaging can be built on top of unreliable messaging with proven techniques, I believe that this is the best choice for a messaging system.

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