[Zope-dev] ZCatalog caching with memcached

[Tres Seaver]

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Roché Compaan wrote:
> On Sat, 2008-10-25 at 09:20 +0200, Hedley Roos wrote:
>>> Have you measures the time needs for some "standard" ZCatalog queries
>>> used with a Plone site with the communication overhead with memcached?
>>> Generally spoken: I think the ZCatalog is in general fast. Queries using a
>>> fulltext index are known to be more expensive or if you have to deal with
>>> large resultsets or complex queries.
>>>
>> No I haven't. Roche Compaan has done extensive benchmarking using
>> funkload testing plain catalog vs module level cache vs memcached, but
>> the tests are more about page serving than catalog query time. I'll
>> ask him to comment more on that.
> 
> I actually did some profiling as well and catalog searches were just too
> damn slow. The average execution time for searchResults was 100
> milliseconds and this is why I told Hedley we should do some caching at
> query level in the first place. I experimented with this idea a couple
> of years back but wasn't successful due to inexperience. I was trying to
> cache brains which obviously leads to persistency bugs. This time around
> it was obvious to me that we should cache the IISet result sets.
> 
> I suspect specific indexes are just performing suboptimally and needs to
> be improved. ExtendPathIndex in Plone seems to be one of them.
> 
> The effect on performance is really awesome, now we just need to fine
> tune the implementation.

Before (or while) we work on caching, can we try to improve the
underlying indexes, and the way that applications use them?  I'm pretty
sure that there is a lot of room for improvement:

 - Plone uses too many indexes, and in particular, uses multiple text
   indexes.  Having extra indexes around "just in case" is a sure lose
   a write time, and may even be expensive at query time (depending on
   the query).

 - Particular indexes have performance characteristics based on their
   designed purpose:  for instance, the stock FieldIndex implementation
   assumes that the number of documents indexed will be >> the number of
   discrete indexable values.  Using such an index in an application
   domain with a very large set of indexable values probably loses, and
   in ways which don't show up in early / small-scale testing.

 - I'm pretty sure that we haven't yet found the best data structure for
   "hierarchy indexes" (e.g., the Plone EPI index, or the stock Zope2
   PathIndex, etc.).  Something like a 'trie' might be optimal for
   pure prefix searching of hierarchies.

 - I am confident that the TopicIndex is underutiliized:  it does *all*
   the work for a given query at write time, and can thus be blindingly
   fast at query time.

 - Other special-purpose indexes (e.g., a "recent items" index) would
   be worth a look, especially for applications with large volumes of
   content.


Tres.
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Tres Seaver          +1 540-429-0999          tseaver at palladion.com
Palladion Software   "Excellence by Design"    http://palladion.com
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