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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 13:41:03 GMT

    [ https://issues.apache.org/jira/browse/COUCHDB-968?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12971259#action_12971259
] 

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

After mulling it over a bit more this morning I think I might have hit upon a simpler solution.
 The problems I discovered last night arose from the fact that we take the result of a successful
merge and try to merge that branch into the remaining branches of the tree.  I think we might
able to skip that step because

a) a "double-merge" must be really rare.  The only way I could come up with a successful double-merge
would be to lower the _revs_limit and introduce two distinct edit branches that used to share
a common trunk, then raise the _revs_limit and replicate in a version of the document which
has the common trunk and shares at least one revision with each branch.  But more importantly,

b) revision stemming should do the second merge for us.  Stemming involves exploding the tree
into a list of paths, taking the Limit sublist of each one, and then merging them all back
together one by one.  I'm pretty sure the "double-merge" gets covered here, i.e. the final
result of the stem will be fully merged.  We always stem before completing a write, so the
revision tree on disk would never be in a not-completely-merged state.

I'll think it over some more and see if I can come up with a diabolical case in which the
recursive merge would work but the path-based merging in the stemmer would not.  I'd prefer
not to have to deal with sibling insert branches in the merge code; the code which merges
just a single path into the list of paths is both cleaner and faster.

> 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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