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From "Adam Kocoloski (JIRA)" <j...@apache.org>
Subject [jira] Commented: (COUCHDB-968) Duplicated IDs in _all_docs
Date Tue, 14 Dec 2010 15:55:02 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-968?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12971289#action_12971289
] 

Adam Kocoloski commented on COUCHDB-968:
----------------------------------------

I can definitely come up with a case where the path-based merging will fail to fully collapse
the tree, but it depends on the ordering of the paths.  For example, consider a tree with
_revs_limit = 2 that has 3 branches which share a common trunk that has been stemmed away,
like so (o for an available revision, x for one that has been stemmed):

    o
    o
    x
    x o o
o o x
    x

Now I'll up the _revs_limit to 10 and replicate in path that fills in all the Xs.  If I skip
the non-recursive merge I'll be left with 3 completely separate branches, when in reality
they all share the first two revisions.  When I go to do the stemming the results will depend
on the order in which the paths are merged.  If I sort the paths by how much they're cut off
and add the longest one first, I'll be in good shape.  Eash subsequent path will attach the
original one correctly.  If on the other hand I do the longest one last I'll be left with
two distinct branches after the merge.

If sorting the paths before merging them in the stemmer is all that's required this is a simple
fix.  In fact, it looks like it already does this.  It doesn't go a global sort of all the
paths, but it does appear to sort the paths that have nothing in common in order of how much
they are cut off.  I think that may actually be sufficient.

> Duplicated IDs in _all_docs
> ---------------------------
>
>                 Key: COUCHDB-968
>                 URL: https://issues.apache.org/jira/browse/COUCHDB-968
>             Project: CouchDB
>          Issue Type: Bug
>          Components: Database Core
>    Affects Versions: 0.10.1, 0.10.2, 0.11.1, 0.11.2, 1.0, 1.0.1, 1.0.2
>         Environment: any
>            Reporter: Sebastian Cohnen
>            Assignee: Adam Kocoloski
>            Priority: Blocker
>             Fix For: 0.11.3, 1.0.2, 1.1
>
>
> We have a database, which is causing serious trouble with compaction and replication
(huge memory and cpu usage, often causing couchdb to crash b/c all system memory is exhausted).
Yesterday we discovered that db/_all_docs is reporting duplicated IDs (see [1]). Until a few
minutes ago we thought that there are only few duplicates but today I took a closer look and
I found 10 IDs which sum up to a total of 922 duplicates. Some of them have only 1 duplicate,
others have hundreds.
> Some facts about the database in question:
> * ~13k documents, with 3-5k revs each
> * all duplicated documents are in conflict (with 1 up to 14 conflicts)
> * compaction is run on a daily bases
> * several thousands updates per hour
> * multi-master setup with pull replication from each other
> * delayed_commits=false on all nodes
> * used couchdb versions 1.0.0 and 1.0.x (*)
> Unfortunately the database's contents are confidential and I'm not allowed to publish
it.
> [1]: Part of http://localhost:5984/DBNAME/_all_docs
> ...
> {"id":"9997","key":"9997","value":{"rev":"6096-603c68c1fa90ac3f56cf53771337ac9f"}},
> {"id":"9999","key":"9999","value":{"rev":"6097-3c873ccf6875ff3c4e2c6fa264c6a180"}},
> {"id":"9999","key":"9999","value":{"rev":"6097-3c873ccf6875ff3c4e2c6fa264c6a180"}},
> ...
> [*]
> There were two (old) servers (1.0.0) in production (already having the replication and
compaction issues). Then two servers (1.0.x) were added and replication was set up to bring
them in sync with the old production servers since the two new servers were meant to replace
the old ones (to update node.js application code among other things).

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