When you need to expire a cached query, you can either:
1. Delete the cache entry
2. Regenerate the cache entry with updated data
3. Use a key scheme that allows you to lazy expire that model. Ex: Setting
your key to pix:id:version. The challenge of this key is knowing what the
newest version is.
On Mon, Nov 23, 2009 at 7:42 PM, dreadjr <drea...@gmail.com> wrote:
> so what is the best practice if something is added to the datamodel,
> and you have already cached it, just to remove the memcache key and
> reload?
>
> On Nov 21, 6:41 am, dburns <drrnb...@gmail.com> wrote:
> > I thought I'd share this, since I'm sure there are others that have
> > fallen into the same trap using this very common pattern (in this
> > sample, Pix derives from db.Model; get_pics is called on every page
> > load):
> >
> > def get_pics(self):
> > pics = memcache.get("pics")
> > if pics is None:
> > pics = Pix.gql("LIMIT 100")
> > memcache.add("pics", pics, 300) #
> Good for 5 minutes
> > return pics
> >
> > See the bug? Here, memcache is actually HURTINGperformancesince the
> > overhead of memcache is there but it saves nothing at all. The query
> > is still executed on every page load when the calling code iterates
> > through the result.
> >
> > http://code.google.com/appengine/docs/python/datastore/queryclass.htm...
> > mentions this by saying "creating a new iterator from the Query object
> > will re-execute the query", but it doesn't highlight this pitfall.
> > The issue here is that entities are not fetched on the Pix.gql line.
> > Instead, that simply returns a Query object. The results are actually
> > fetched when the calling code begins to iterate (in Python-speak, the
> > __iter__() method on the Query is what actually fetches entities).
> >
> > To fix this, you'd change the gql line to :
> > pics = list(Pix.gql("LIMIT 100"))
> > Putting a list() around the Pix.gql forces the query to happen at that
> > moment. Then the list of entities is stored in memcache, not the
> > Query object itself.
> >
> > I'm not sure if this applies to the Java API too, but it's worth a
> > heads-up.
> >
> > Comments welcome...
>
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--
Ikai Lan
Developer Programs Engineer, Google App Engine
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