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From Adam Kocoloski <kocol...@apache.org>
Subject Re: _changes resource
Date Tue, 07 Jul 2009 16:58:52 GMT
Hi Eric,

On Jul 7, 2009, at 3:43 AM, eric casteleijn wrote:

> Adam Kocoloski wrote:
>> On Jul 6, 2009, at 3:58 PM, Chris Anderson wrote:
>>>> == Deleted and Conflicts==
>>>>
>>>> _all_docs_by_seq includes a 'deleted' flag and a list of  
>>>> 'conflicts'.
>>>> Should the _changes API to do the same?
>>>
>>> The plan is to drive replication from changes, so anything needed by
>>> replication is on the roadmap. I don't think it'd hurt to have any  
>>> of
>>> those but Damien would be better to answer this one.
>> The deleted=true flag probably won't be needed by the replicator,  
>> because the _changes feed includes the deletion revid.  I expect  
>> that the replicator will just download this revision like any  
>> other, find the _deleted:true bit set in the document, and delete  
>> the document on the target.
>
> Note that replication is an important user of _changes, but by no  
> means the only one, if update notifiers go away. (Which I think  
> everyone agrees would be a good idea.) I would like to have the  
> option to not only see when a document was deleted, but in addition  
> when one was first created on the node in question, which in my  
> application would require special action, over and above what needs  
> to happen for an update to a document.

I can see the motivation behind adding created and deleted flags to  
the "changeset" included in each row.  I certainly didn't mean to  
imply that the replicator was the only consumer of the _changes feed.

>> _conflicts and _deleted_conflicts are more interesting.  When one  
>> of these occurs, the document shows up in the _changes feed, but  
>> the revision in that row is the latest revision of the document,  
>> not the conflict/deleted_conflict rev.  Unlike _all_docs_by_seq,  
>> it's not possible for the replicator to determine the list of  
>> revisions to replicate solely by analyzing the _changes feed.
>> I think the most efficient solution is to start including conflict  
>> and deleted_conflict revisions in the revlist in the _changes row.  
>> I don't know the revision tree well enough to know if it's possible  
>> to identify the set of all conflict revisions that were saved after  
>> update_sequence N, but if it is that would be a neat restriction.
>
> This sounds like a good idea.
>
>> Another option might be to configure a metadata-only request so  
>> that the replicator could check what revisions exist on the source  
>> for each updated document.  Could be a useful thing to have in  
>> general.
>
> And so does this.
>
> What I would like to add:
>
> Right now the _changes feeds are per db, and while that is great in  
> some use cases (like replication.) In others, where there are many  
> thousands of databases, one global _changes feed would be much more  
> practical. It is also how the current update notifiers work, so not  
> having this option would break existing applications, at least it  
> would mine. ;)

Adding a global _changes feed seems useful and a very easy thing to  
implement from my end.

> One last thing that would be great to have is a way to configure  
> what information goes into a particular _changes feed or an option  
> to write your own _changes-like feeds in javascript, like you could  
> a view, so that one could have a feed of changes to the values of a  
> particular field, for instance. It would make it so that processes  
> that act on updates from the db never have to query back into the db  
> for additional data, which could be a performance win. (That is if  
> the configurable _changes feeds aren't too much of a performance  
> loss.)

>
> I've also filed a feature request in JIRA with these suggestions:
>
> https://issues.apache.org/jira/browse/COUCHDB-390

Cool, thanks!  We can move the discussion there.  Best,

Adam

> but more discussion, here or on that issue, is most welcome.
>
> -- 
> - eric casteleijn
> http://www.canonical.com


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