Thanks again - I'm busy with a few things at the moment, but will have
a play with some code over the next week or two and report back here.
On Jul 22, 2:41 am, Mark Wyszomierski <mar...@gmail.com> wrote:
> Cool, if you structure your entity class like this though:
>
> class Entity {
> List<String> tags;
> String foo;
> }
>
> how are you going to query which Entity instances match a given tag? I think
> that all the tags need to be indexed so your query statement can find them -
> I'm not 100% sure on this, if you can somehow match on that List<String>,
> that would be new to me! I was thinking you might have to decide how many
> tags an Entity can have, and explicitly declare each like:
>
> class Entity {
> String tag1, tag2, tag3;
> String foo;
> }
>
> only the first tag is required, the other two can be null. Then your query
> statement would have to be something like:
>
> select from Entity where tag1 == 'cow' || tag2 == 'cow' || tag3 ==
> 'cow';
>
> not sure if there's a better way to do it, let us know what you try because
> this is probably a pattern that can be reused for others wanting to do the
> same thing,
>
> Thanks,
> Mark
>
>
>
> On Wed, Jul 21, 2010 at 3:15 PM, planetjones <jon.r.jo...@gmail.com> wrote:
> > Mark,
>
> > Thanks very much for this - it's useful. I'm slowly getting used to
> > this non-relational way of thinking.
>
> > The count of the tags doesn't need to be 100% accurate, as it will
> > only be used to approximate how many entries exist for a given tag -
> > the tag cloud aim is to show tags associated with many entities in a
> > larger font and tags associated with fewer entities in a smaller
> > font. If the insert, update or delete of Entity succeeds and gets
> > committed, but the persistence action against the TagCounter fails in
> > a few instances that would not be a big deal.
>
> > I'll probably allow three tags per Entity, so was planning on:
>
> > class Entity {
> > List<String> tags;
> > String foo;
> > }
>
> > for flexibility in of the number of tags.
>
> > So when inserting a new entity I would need to:
>
> > Create Entity
> > For each tag in Entity select TagCounter
> > If tag counter exists for tag increment count by 1
> > If tag counter does not exist for tag create TagCounter with count of
> > 1
>
> > When deleting an entity I would need to:
>
> > For each tag in Entity select TagCounter
> > Decrease count by 1 of TagCounter
> > Delete Entity
>
> > This sounds ok to me. When a user clicks a tag the query is simple
> > too.
>
> > I'll give this a try and see how it looks.
>
> > Thanks again.
>
> > On Jul 21, 12:46 pm, Mark <mar...@gmail.com> wrote:
> > > I probably have a naive understanding of what you're doing, maybe
> > > explain a little more, but to me it looks like this:
>
> > > class TagCounter {
> > > int count;
> > > String label; // primary key, unique tag name.
> > > }
>
> > > class Entity {
> > > String tag;
> > > String foo;
> > > }
>
> > > whenever the user creates a new entity, record its tag(s), then
> > > increment the appropriate TagCounter instance. Keeping counters like
> > > this introduces a bottleneck and goes against what you want to be
> > > doing for a highly scalable web service, so you may want to shard them
> > > (imagine you have thousands of users all trying to increment a single
> > > tag counter at once). Since the above two classes won't be in the
> > > same entity group, you wouldn't be able to perform both operations in
> > > a single transaction. You can either accept the fact that your tag
> > > count may be out of sync with the actual number of entities with that
> > > tag, or you can have a multistep process and clean up dangling tag
> > > counts with a task (eventual consistency might be the term here). I'm
> > > referring to Nick Johnson's article on distributed transactions which
> > > I think you can modify to do what you're looking for:
>
> > > http://blog.notdot.net/2009/9/Distributed-Transactions-on-App-Engine
>
> > > I'm still learning app engine so take the above with a big grain of
> > > salt. To me, it seems that if you want to write a highly scalable web
> > > service, you need to accept that you may lose some data precision, or
> > > you have to put in additional work to get what you want (compared to
> > > assuming a single mysql instance etc).
>
> > > Anyway, after you implement something like the above, your original
> > > two queries should be easy to do. You probably want to define how many
> > > tags an Entity can have. If an Entity can have more than one tag, I'd
> > > explicitly state what that max is. Write out everything you need when
> > > the user submits an entity, so that your reads will be fast as other
> > > users browse the site.
>
> > > Mark
>
> > > On Jul 19, 9:16 am, planetjones <jon.r.jo...@gmail.com> wrote:
>
> > > > Hi,
> > > > I'm looking to store entities using GAE Java which have 1-many tags.
> > > > I
> > > > would like to display a tag cloud, so will need to know how many
> > > > times
> > > > each tag occurs (can't use aggregate functions and group by on GAE to
> > > > do this like I would in SQL). And when I click a tag I would like to
> > > > retrieve all entities with the selected tag.
> > > > Does anyone have any examples of what my Classes should look like to
> > > > do this optimally and efficiently? Sample JPA code would be great
> > > > too.
> > > > Cheers - Jonathan
>
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