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From Adam Kocoloski <kocol...@apache.org>
Subject Re: Replication and new user questions
Date Tue, 25 Aug 2009 21:39:09 GMT
Hi Blair, all good questions, I'll try to answer inline:

On Aug 25, 2009, at 5:10 PM, Blair Zajac wrote:

> Hello,
>
> We're looking at using CouchDB's replication to allow us to easily  
> have multi-master replicating databases across multiple facilities,  
> (e.g. Los Angeles, Albuquerque, Bristol, England, etc).  It looks  
> like it'll be the perfect tool for the job.
>
> Some questions on the current implementation and the work that I've  
> read is going to be in forthcoming releases.
>
> 1) What's the most robust automatic replication mechanism?  While  
> continuous replication looks nice, I see there's some tickets open  
> with it and that it has issues with four nodes.  Is a more robust  
> solution, but a little slower and heavier, it to have an  
> update_notification that manually POSTs to _replicate?

We're committed to making continuous replication as robust and  
performant as possible.  The entire replication codebase went through  
a significant refactoring after 0.9, and what you're seeing is us  
ironing out a few of the kinks before 0.10 gets out the door.  I'd  
encourage you to give "continuous":true a shot, provided my answer to  
2) isn't a deal-breaker.

> 2) With the persistent continuous replication feature, is there a  
> way to stop continuous replication without restarting couchdb?

At the moment, no.  We just didn't have time to add that feature to  
0.10.  It's coming soon, though.

> Will there be a way to manage the list of replicant databases when  
> the persistent continuous replication feature is complete?

Absolutely yes.  It will probably be a special DB called _replication  
where you can PUT and DELETE documents that configure continuous  
replications.

> 3) How does continuous replication deal with network outages, say if  
> a link goes down between the Los Angeles and Bristol data centers?   
> Does CouchDB deal with a hanging TCP connection ok?

CouchDB retries requests using a timeout that doubles with every  
failure.  It does this for about 5 minutes, then gives up.

> 4) It would be nice for CouchDB to have in it a list of replicant  
> databases that it will automatically push changes to, so this list  
> could also be maintained in CouchDB, instead of with an external  
> script.  Is there any work on a feature like this?  This could be  
> easily done with an external update_notification script.

Yep, definitely planned for the near future (certainly by 0.11).

> 5) I wrote the following Bourne shell script and after running it  
> for an hour, it consumes 100% of a CPU.  This is even after stopping  
> the shell script and compacting both databases.  What would explain  
> this behavior?

I couldn't quite get that script to work ($HOST2 was undefined, and  
then something else failed), but can you try it again with a fresh  
checkout?  I fixed a bug last night that could very well have caused  
this.  Best,

Adam

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