The approach I take to performance issues while coding is that
performance issues should be in the back of your mind at all times. Not
to ignore them, but also to resist the temptation to focus on
performance too much during the design and initial implementation of a
feature, and planning to revisit the issue if performance problems
become apparent later.
This philosophy has both strengths and weaknesses and recent events have showcased both of these.
cmScribe uses a complex and flexible fine-grained permissioning
mechanism where permissions can be granted to all kinds of actions on
all kinds of objects. Having certain permissions can cause others to be
granted implicitly, and the rules for this kind of implication can be
any arbitrary C# code. Since the permissions are so fine-grained, any
given page hit can require a large number of permissions to be
evaluated. Furthermore, the implication rules mean that evaluating one
permission may require a number of others to be evaluated as well.
The system is so complex, in fact, that I struggled quite a lot during
the initial design process to come up with a way of meeting all the
requirements at all. (Is it over-engineered? I don't know. I do know
that after using it for a year there's only one feature I'd have cut,
and that's never been used and doesn't add any complexity) The first
and biggest advantage of keeping performance issues on the backburner
is that if I'd had to juggle performance along with all the other
constraints I was trying to meet, I don't know whether I'd have been
able to produce a working system in the first place. In this case,
deferring performance for later may have made the difference between
impossible and possible.
Since then I've had to revisit this code for performance reasons on two
or three separate occasions. You could look at this as a disadvantage
of the approach I took: surely if performance had been designed in from
the very beginning then I wouldn't have had to repeatedly fix
performance problems later. But you can also look at it as a strength:
the code worked adequately to start with without spending the time on
performance. Later, as more demanding scenarios came up, it was
possible to fix it without too much trouble to again perform
adequately, by a combination of caching frequently-used information in
memory, tweaking the order of operations to make the common cases use
less steps, and micro-optimizing the individual steps to eliminate
avoidable database hits and other expensive operations. I'm in the
middle of an iteration of that process right now, and I'm entirely
confident that I can have it performing adequately again shortly.
The weakness of the approach, however, is that an architecture designed
without considering performance (since I was struggling so much with
all the other issues, performance was probably further back in my mind
even than usual) has turned out to have some performance bottlenecks
that simply can't be removed without changing the architecture itself.
There are situations where it's possible to know based on fixed
information that there's no way a user could possibly have a particular
permission, but that fixed information isn't available within the
architecture, so the code will still chase down a number of dead ends
before it arrives at the answer. And there's no way to make that
information available with little caching tweaks and
micro-optimizations. It needs a whole new structure.
For now, I can continue to tweak the heck out of the existing
architecture and I'm confident it will perform adequately for quite
some time. Which leads to the final advantage - even when you do reach
the point where there's nothing to be done but throw out the whole
thing and start over with performance at the very front of your mind,
the experience gained from the first attempt will be invaluable in
designing the system the right way. Every tweak I make to the existing
code will be designed into the next version from day one.
Sounds a lot better than being stuck a year ago unable to write the
thing at all because I couldn't get my head around how to make it fast, doesn't it? :)
