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From "Mclean, Adam" <adam.mcl...@rbc.com>
Subject RE: Views with include_docs=true including old or incomplete results.
Date Thu, 31 Oct 2013 13:59:57 GMT
Thanks for the pointer.  This seems a little bit different to me, as the fetched document is
an older version that wouldn't match the view in the first place.  There is no change to the
document after it matches the view.

If I have a document:
{
  "_id": "1234"
  "_rev": 1-
  "match": false
}

And a view defined as:
if(doc.match) {
  emit();
}

Revision 1 of the document does not match the view.

If I write:
{
  "_id": "1234"
  "_rev": 2-
  "match": true
}

At about the same time (within a few ms) as I query the view, I get a return showing the document
now matches, however the included "doc": {} is "_rev": 1-

There are no additional changes made to the document, and if I add a few more ms between the
write, and the query the included document is correctly "_rev": 2-


Either way it sounds like calling the view without include_docs, then retriving using the
_all_docs with a keys = [] is a better way to go?

-----Original Message-----
From: Robert Newson [mailto:rnewson@apache.org] 
Sent: 2013, October, 31 9:39 AM
To: user@couchdb.apache.org
Subject: Re: Views with include_docs=true including old or incomplete results.

https://wiki.apache.org/couchdb/HTTP_view_API

The include_docs option will include the associated document. However, the user should keep
in mind that there is a race condition when using this option. It is possible that between
reading the view data and fetching the corresponding document that the document has changed.
If you want to alleviate such concerns you should emit an object with a _rev attribute as
in emit(key, {"_rev": doc._rev}). This alleviates the race condition but leaves the possibility
that the returned document has been deleted (in which case, it includes the "_deleted":
true attribute). Note: include_docs will cause a single document lookup per returned view
result row. This adds significant strain on the storage system if you are under high load
or return a lot of rows per request. If you are concerned about this, you can emit the full
doc in each row; this will increase view index time and space requirements, but will make
view reads optimally fast.

B.


On 31 October 2013 13:09, Mclean, Adam <adam.mclean@rbc.com> wrote:
> I have a busy database that I'm querying with a view like "_design/[doc]/_view/[view]?include_docs=true"
>
> I'm having trouble with old or incomplete documents being included in the result.
>
> I see that either:
> a) include_docs is including the previous version of the document - a version of the
document that wouldn't actually match the view.
> b) include_docs is not including the document.  It includes the 'key', and 'value', but
the 'doc' is empty in output.
>
> This only occurs if I'm querying the view at roughly the same time (within a few ms)
of the incorrectly included document being written.  The document being written/updated DOES
match the view criteria, however the include_docs seems to have a timing issue where it retrieves
an old copy of the document, or doesn't include anything in the "doc": {} portion of the result.
>
> So in summary I think the view logic is fine, but include_docs seems to retrieve incorrect
documents when the timing lines up.
>
> I'm using CouchDB 1.3.1 with ERL R15B02 and Spidermonkey 1.8.5 on Linux.
>
> Is this a caveat of using include_docs?  Would I be smarter to get the keys, then make
a second query to couch to get the documents?  Was this something 'known' in 1.3.1 and fixed
in 1.4?
>
> Thanks!
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