Amazon diagnosis

George Herbert george.herbert at gmail.com
Thu May 5 12:52:19 CDT 2011


On Thu, May 5, 2011 at 10:45 AM, Ryan Malayter <malayter at gmail.com> wrote:
>
>
> On May 1, 2:29 pm, Jeff Wheeler <j... at inconcepts.biz> wrote:
>
>> What it really boils down to is this: if application developers are
>> doing their jobs, a given service can be easy and inexpensive to
>> distribute to unrelated systems/networks without a huge infrastructure
>> expense.  If the developers are not, you end up spending a lot of
>> money on infrastructure to make up for code, databases, and APIs which
>> were not designed with this in mind.
>
> Umm... see the CAP theorem. There are certain things, such as ACID
> transactions, which are *impossible* to geographically distribute with
> redundancy in a performant and scalable way because of speed of light
> constraints.

That specific example depends on how order-dependent your consistency
constraint is; you can have time-asynchronous locally ACID changes
across databases which are widely separate.  If your consistency
requires order synchronicity across the geographic DB cluster then
this is a potential epic fail, of course.

The general point is valid.

Being able to tell if your application *really* does require strict
consistency or not, and if it requires strict ordering or not if it
requires strict consistency, is unfortunately beyond most line-level
system designers.  A lot of people guess wrong in both directions, and
either cripple the app's performance unnecessarily or end up with
dangerous failure modes inherent in the architecture.


-- 
-george william herbert
george.herbert at gmail.com




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