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From Robert Newson <rnew...@apache.org>
Subject Re: replicating docs with tons of conflicts
Date Thu, 14 Mar 2013 18:44:17 GMT
Conflicts are *not* removed during compaction, CouchDB has no way of
knowing which ones it would be ok to delete.

CouchDB does struggle to process documents with lots of conflicts,
we've encountered this at Cloudant a fair bunch. We resolve the
conflicts via http if possible or, if that consistently fails, with a
direct erlang manipulation. It's certainly something we need to
improve.

B.

On 14 March 2013 13:09, Riyad Kalla <rkalla@gmail.com> wrote:
> Stephen,
> I am probably wrong here (someone hop in and correct me), but I thought
> Compaction would remove the old revisions (and conflicts) of docs.
>
> Alternatively a question for the Couch devs, if Stephen set _revs_limit to
> something artifically low, say 1, and restarted couch and did a compaction,
> would that force the DB to smash down the datastore to 1 rev per doc and
> remove the long-tail off these docs?
>
> REF: http://wiki.apache.org/couchdb/Compaction
>
> On Thu, Mar 14, 2013 at 2:02 AM, Stephen Bartell <snbartell@gmail.com>wrote:
>
>> Hi all,
>>
>> tldr; I've got a database with just a couple docs.  Conflict management
>> went unchecked and these docs have thousands of conflicts each.
>>  Replication fails.  Couch consumes all the server's cpu.
>>
>> First the story, then the questions.  Please bear with me!
>>
>> I wanted to replicate this database to another, new database.  So I
>> started the replication.  beam.smp took 100% of my cpu and the replicator
>> status held steady at a constant percent for quite a while.  It eventually
>> finished.
>>
>> I thought maybe I should handle the conflicts and then replicate.
>>  Hopefuly it'll go faster next time.  So I cleared all the conflicts.  I
>> replicated again but this time I could not get anything to replicate.
>>  Again, cpu held steady, topped out. I eventually restarted couch.
>>
>> I dug throughout the logs and saw that the POSTS were failing.  I figure
>> that the replicator was timing out when trying to post to couch.
>>
>> I have a replicator that I've been working on thats written in node.js.
>>  So I started that one up to do the same thing.  I drew inspiration from
>> Pouchdb's replicator and from Jens Alkes amazing replication algorithm
>> documentation, so my replicator follows more or less the same story.  1)
>> consume _changes with style=all_docs.  2) revs_diff on the target database.
>>  3) get each revision from source with revs=true.  4) bulk post with
>> new_edits=false.
>>
>> Same thing.  Except now I can kind of make sense of whats going on.
>>  Sucking the data out of the source is no problem.  Diffing the revs
>> against the target is no problem.  Posting the docs is THE problem.  Since
>> the database is clean, thousands of docs are being thrown at couch at once
>> to build up the revision trees.  Couch is just taking forever in finishing
>> the job.  It doesn't matter if I bulk post the docs or post them
>> individually, couch sucks 100% of my cpu every time and takes forever to
>> finish. (I actually never let it finish).
>>
>> So that is is the story. Here are my questions.
>>
>> 1) Has anyone else stepped on this mine?  If so, could I get pointed
>> towards some workarounds?  I don't think it is right to make the assumption
>> that users of couchdb will never have databases with huge conflict sausages
>> like this. So simply saying manage your conflicts won't help.
>>
>> 2) Lets say I did manage my conflicts.  I still have the
>> _deleted_conflicts sausage.  I know that _deleted and _deleted_docs must be
>> replicated to maintain consistency across the cluster.  If the replicator
>> throws up when these huge sausages come through, how is the data ever going
>> to replicate?  Is there a trade secret I don't know about?
>>
>> 3) Is there any limit on the resources that CouchDB is allowed to consume?
>>  I can get that we run into these cases where theres tons of data to move
>> and its just going to take a hell of a long time.  But I don't get why its
>> permissible for CouchDB to eat all my cpu.  The whole server should never
>> grind to a halt because its moving lots of data.  I feel like it should be
>> like the little train who could.  Just chug along slow and steady until it
>> crests the hill.
>>
>> I would really like to reply on the erlang replicator, but I can't.  At
>> least with the replicator I wrote I have a chance with throttling the posts
>> so CouchDB doesn't render my server useless.
>>
>> Sorry for wrapping more questions into those questions.  I'm pretty tired,
>> stumped, and have machines in production crumbling.
>>
>> Best,
>> Stephen

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