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From Paul Davis <>
Subject Re: couch sequence numbers and _all_docs_by_seq
Date Wed, 29 Apr 2009 16:16:19 GMT
Adam Kocoloski wrote:
> On Apr 29, 2009, at 4:08 AM, eric casteleijn wrote:
>> Paul Davis wrote:
>>> Eric,
>>> Implementation isn't a problem, but I would fear that replication
>>> could screw views up. As in, your views would depend on the order of
>>> document modification which could be impacted quite heavily by
>>> replication etc.
>> Ah right, hadn't thought of that. So in a system using replication, 
>> views that would use the sequence number as a key would get a severe 
>> performance penalty? (Just checking to see if I understand the 
>> implications.) That might be a good enough reason not to expose this 
>> as a feature, lest people start depending on it and saying CouchDb is 
>> slow as a result.
>> If this is true, I suppose it also holds for the built in 
>> _all_docs_by_seq view?
> I'm not sure Paul's comment is performance-related, actually. I think 
> he was more concerned that a view which incorporates update_seq could 
> give different results on two servers that are using replication to 
> stay synchronized with one another. Paul, correct me if I'm 
> misinterpreting you.
Yep that's the issue. If you use the update sequence in your map view 
then it violates those math words about same input means same output. 
Because the update_sequence will change for every PUT etc, and 
replication could give it an entirely different update_seq.

A thought, you could look at _all_docs_by_seq and then use multi-get 
against your view.

Paul Davis
> At the moment the replicator doesn't try to update documents on the 
> target in the exact same order that they were updated on the source, 
> but I think it could be done without too much of a penalty if we 
> decided it was necessary. Regards,
> Adam

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