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From: Wout Mertens <wmertens@cisco.com>
To: user@couchdb.apache.org
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Subject: reserving resources in Couch
Date: Thu, 12 Feb 2009 18:11:34 +0100

Ok,

(no actual code yet, I don't have time to code right now :( )

I have a project currently using an RDBMS and I'd like to port it to  
CouchDB. One of the things I do is lock a table, choose a free  
resource from a query on a static table and the session list, assign  
the resource to a new session and unlock the table.

How would I be able to do the same thing with CouchDB given that 2  
sessions could start at the same time? I do have the advantage that  
simultaneous starters would contact the same CouchDB instance.

I was thinking of using sums: make a view that calculates the sum of  
resources. A resource record would count as +1 and an in-use record  
would be -1.

Then when you reserve a resource, you save the in-use record. After  
saving, look up the sum for the resource you reserved. If it's not  
equal to 0, then use a stable algorithm to determine who has to  
release the resource again.

Would this close the race condition? Note that no documents are  
overwritten at reservation time, each reservation doubles as the event  
log. When the session clears up, the document that represents it is  
updated to release the resource.

Does this work? Is there a better way to do it?

Thanks,

Wout.