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From Sebastian Cohnen <sebastiancoh...@googlemail.com>
Subject Re: Picking first free document from database
Date Fri, 04 Jun 2010 10:31:05 GMT
hmmm...

i would let the client fetch a available doc from couch and update it's status. in case another
process tries to update the document status in between couch will respond with 409 Conflict
(since it cannot provide the correct mvcc token). if you get a conflict take the next one.
the load for the server should be okay, since AFAIK detecting conflicts and rejecting updates
is relatively cheap.

another idea, that pops into my mind: if you really have a ton of load/requests, you could
use some kind of proxy, which ensures that every document id is only given out one time to
once client. this proxy would be your new bottleneck, but you also could use some queuing
technique here and fetch batches of available docs from couch...

On 04.06.2010, at 11:57, Attila Nagy wrote:

> Hello,
> 
> I would like to build an application on CouchDB, which allocates free documents to the
users. Each document would have a status, which indicates whether it's available for assigning,
or not.
> Because of the great number of new users wanting available documents, I think the default
model of writing a view, which lists available documents, and picking one from it (for efficiency
limiting the number of return values to one and picking that) won't work. The contention will
make applications do a lot of conflicts, which slows down (or even make it impossible) the
new assignments, and is a lot of unnecessary load for both the server and the client applications.
> 
> So I would need a method, which would pick the first document from the database which
has "available" in its status field, and immediately (atomically) set it to "unavailable",
and return the document ID to the caller.
> 
> I've found two posts, which do something similar (server side changes to documents):
> http://blog.couch.io/post/410290711/atomic-increments-in-couchdb
> http://wiki.apache.org/couchdb/Document_Update_Handlers
> 
> I'm very new to CouchDB, and I can't yet to see whether these can be adapted to the above
task (these two are tied to a given document).
> 
> So in short I would need: a server-side function, which traverses on all entries (uses
a view for efficiency?), picks the first one which has status: available, sets that to unavailable
and returns the docID to the client. And of course all of this atomically, meaning no two
calls to this method should give back the same docID.
> 
> Is this doable somehow with CouchDB?
> 
> Thanks,


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