[elephant-devel] Learning about Elephant

Tue Jan 27 20:22:53 UTC 2009

Slot associations appear to work reflexively, by the way.

(defpclass self-assoc ()
	    ((friends-of :accessor friends-of
		     	 :associate (self-assoc friended-me)
		         :many-to-many t)
	     (friended-me :accessor friended-me
		       :associate (self-assoc friends-of)
		       :many-to-many t)))

This simply says I have an association among self-assoc instances.   
Any (setf (friends-of person1) person2) says that person2 is a friend- 
of person1.  Automatically, you get the inverse mapping which is that  
person1 friended person2 (assuming these associations are directional).

If they are not directional, then you can do this:
(defpclass person ()
   ((friends :accessor friends-of
	:associate (person friends) :many-to-many t)))

Now if you (setf (friends-of person1) person2), you will get:

(friends-of person1) => person2
(friends-of person2) => person1

This is a non-directional relationship between two entities so there's  
no need for two slots.

This all seems to work well in the current Alpha 2.

Ian

On Jan 27, 2009, at 1:32 PM, Ian Eslick wrote:

>
> On Jan 27, 2009, at 11:00 AM, John wrote:
>>
>> 1) Every read/write of pclass slots is done directly from/to the
>> database, so no in-memory "copy" exists (unless some sort of
>> transient cache slot model is used). This is good. However, is
>> Elephant "intelligent" enough so that if you attempt to setf a slot
>> value with the same value stored in the slot, Elephant will "avoid"
>> the writing to the database, since the value hasn't really changed?
>> If that's not the out-of-the-box behavior, I pressume that it would
>> be relatively easy to add this functionality using MOP. I also
>> assume that doing this will require another read before the write in
>> order to compare the value to be written, but then again, Elephant
>> claims that reads are much cheaper than writes.
>>
>
> Such a thing would be possible, but you would need to have a local
> copy of the value in memory and this could create more problems than
> it solves.  It's also unclear to me how common this case.  I don't
> write the same value to the same slot very often!
>
>> 2) According to Oracle: "Berkeley DB includes support for building
>> highly available applications based on replication..." (http://www.oracle.com/technology/documentation/berkeley-db/db/ref/rep/intro.html
>> ) So, I assume that in the event that a single machine becomes
>> unable to handle the load of a busy application, BDB supports
>> replication to multiple machines with near-instant replication
>> effects (performance is inversly proportional to replication speed).
>> So, the question is, can Elephant accommodate to this model? If I
>> read correctly, the manual states that elephant maintains a weak
>> hash of persistent objects. So, if BDB is deployed in a distributed
>> model and Elephant is running in each separate machine, we could in
>> theory "trust" that the data read/written from/to disk will be all
>> in sync across all machines (as long as database IO on same object/
>> slot occurs at a frequency greater than the replication rate). The
>> question I have is with the weak hash. If a write is made in one
>> machine, the data on disk is updated across all machines. However,
>> the weak hash remains stale in the machines were the data was only
>> replicated to. Is this an actual problem or does Elephant use the
>> weak hash "intelligently" to recover in the event that the weak hash
>> becomes "unexpectedly" stale?
>
> The weak hash is simply used to speed up the 'recreation' of a
> persistent reference when it is deserialized from the underlying
> storage medium (BDB in this case) so fits into this model.  I've
> looked several times at building-in the support to enable a full
> replication model for BDB, but it's a fair bit of work to deal with
> the single-master requirement for replication and distributed
> transactions for global coherence. The write performance and
> contention performance may decline noticeably, but conflict-free read
> performance should be the same as in the non-replicated case.  I don't
> fully understand all these issues yet and am prioritizing a lisp-only
> Prevalence style backend in my development roadmap.
>
>> 3) I come from a RDBMS world, so I'm still learning the modalities
>> of connected objects vs just related rows. So, reading the tutorials
>> you describe a friends model using PSets. So, imagine a concept
>> similar to Facebook in terms of a friends database. I have millions
>> of people created in the system and they all create their list of
>> friends. Some people may have "few" or no friends while others could
>> have hundreds of thousands of friends (e.g. Pres. Obama). Are PSets
>> the correct way to model this for larger number of objects or is
>> there a more appropriate methodology recommended in Elephant?
>> Obviously the idea behind this is so that you could perform
>> manipulations on these "friends" relatively easy, such as add/remove
>> friends or perform global queries as to list all friends of people
>> who are friends of Pres. Obama. There are references on the list
>> about a query system being worked on and some vanilla version being
>> available, but independently of that, I think my question is more
>> related to the object model implementation. Maybe I'm wrong.
>
> Psets today are a convenience API around the BTree that can be
> overridden by data stores later for more efficient implementation
> (e.g. hash vs. tree) as appropriate for that store.  BTrees are cheap
> for BDB to create and use so use of them at any size is fine.  You
> might want to wrap an abstraction around the BTree directly since the
> pset API is an un-ordered set.  If you want to do range extraction or
> set intersection you'll want the btree's support for ordered objects.
>
> The query system currently doesn't support psets, although it should
> eventually.
>
> Slot-associations currently don't deal with reflexive references
> (associations among instances of a single class), but that would also
> address the 'friends-of' problem.
>
> I hope you enjoy elephant!  If you want to add/implement or spec out
> some of these changes you are thinking about, I'd be happy to support
> you.
>
> Cheers,
> Ian
>
>> Anyway, thank you for your help in advanced. Look forward to hearing
>> back from anyone soon and keep learning more about Elephant.
>>
>> Thanks,
>> JD
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