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From Adam Kocoloski <kocol...@apache.org>
Subject Re: Questions on durability
Date Tue, 23 Apr 2013 20:01:23 GMT
Hi Mark, answers inline … 

On Apr 23, 2013, at 3:37 PM, Mark van Cuijk <mark@van-cuijk.nl> wrote:

> Hi,
> 
> For a new project, we're considering CouchDB as an interesting candidate for storage,
mainly because of the good durability properties, support for replication and the continuous
changes feed.
> 
> Regarding durability I have three questions. I found the durability matrix on the wiki,
but the page isn't finished yet. Our setup will be a single client writing to a single CouchDB
server, which is replicated to several other CouchDB servers.
> 
> 1. Can I access the continuous changes feed on each of the servers and will those feeds
serve the same events in the same order?

No, the replication may re-order them.

> 2. Will each server only send events to the changes feed after the data is durably persisted
to hardware storage?

Yes.  If you want to ensure that the data is not only written but also fsynced before its
available to readers you need to configure your server with

[couchdb]
delayed_commits = false

Otherwise the fsync may occur a second or two after the documents are visible to readers.

> 3. The batch=ok parameter influences the response to the writing client, but does it
have any influence regarding the answer to the previous question?

No.  Your _changes reader will not see the documents until they've been durably stored, which
may be well after the batch=ok write has been acknowledged to the client.

> 
> Thanks,
> Mark


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