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From: Troy Kruthoff <tkruthoff@blit.com>
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Subject: Re: reserving resources in Couch
Date: Thu, 12 Feb 2009 11:56:30 -0800
References: <7528BFCF-2533-4A6F-AE60-C6968E219F88@cisco.com>

If I understand you correctly, what you need is already baked in with  
revision #'s.

1) Get a doc that is not assigned a resource
2) Flag the doc as being in-use and then save it.
2a)  If the save fails because of conflict, you can then verify the  
new rev is in use and forget about it
2b)  If save is success, you know that process has secured the "in- 
use" lock

-- troy


On Feb 12, 2009, at 9:11 AM, Wout Mertens wrote:

> 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.