[ZODB-Dev] ZEO and relstporage performance

[Benji York]

On Wed, Oct 14, 2009 at 3:38 PM, Ross J. Reedstrom <reedstrm at rice.edu> wrote:
> On Wed, Oct 14, 2009 at 02:20:50PM -0400, Benji York wrote:
> We're actually set up w/ squid in front of the zope FEs, using IPC to
> talk to them all. The default behavior is just to respond w/ a "CACHE
> MISS" and use network access timings to select. This is non-optimal,
> since it does little true load-balancing, until the FE is completely
> hammered (very non-linear response-time curve). I'd love to see an
> example where someone replaced the default response w/ something more
> meaningful.

You'd probably be interested in the last four paragraphs of
http://benjiyork.com/blog/2008/03/icp-for-faster-web-apps.html.

> The shoal that replying "HIT' for ZEO cached data breaks on
> is that the IPC request contains a URL, not ZODB object refs. And
> converting one to the other is what the whole dang machine _does_.

Right, but in many situations you can guess effectively.  See below.

> And you have very little time to answer that IPC query, lest you destroy
> the gains you hope to get from having a hot-cache. So some ad-hoc
> approximation, like keeping the last couple hundred URLs served, and
> responding 'HIT' for those, might get some part of the benefit.

I'm thinking about something more specific.

For example, lets say that your data is a dictionary application and
URLs are of the structure: my-site.com/WORD, where word is the word you
want to show the definition of.

If you generate an IPC HIT if the process has served a word beginning
with the same letter recently, then particular letters will begin to be
associated with particular servers, increasing the ZEO (and ZODB) cache
hit rates.

If you take this a step further and design your app so that you load all
the words that start with a letter as one giant object, you'll get an
even greater effect.  (Note that in real life that object would be too
giant, but you get the idea.)

> This is probably the wrong list for it, but does anyone know of a
> published example of replacing Zope's default icp-server response? Last
> time I looked, I couldn't find one.

Other than my blog post and zc.icp the only other ICP and Zope info I
know of is http://www.zope.org/Members/htrd/icp/intro (very old).

Note that I haven't used zc.icp in a long time because we decided to
move away from ICP and (eventually) move to a load balancer that
implements our desired policy instead.
-- 
Benji York
Senior Software Engineer
Zope Corporation