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From Robert Samuel Newson <rnew...@apache.org>
Subject Re: Bigcouch vs couchbase
Date Thu, 27 Mar 2014 07:34:43 GMT
Hi,

A read or write to a BigCouch cluster (this will be true for CouchDB soon too) happens to
all 3 replicas of a shard in parallel. By default, BigCouch waits for the first 2 results,
so long as they match, before returning the result to the user (two 201’s is a 201, obviously).
In the even that they don’t match, BigCouch waits for the third response.

With N=3,R=W=2, BigCouch does not, by default, return "stale data" if you mean "I won’t
see my write if I’m too quick". But do note that BigCouch is explicitly AP not CP, so there
are conditions where this will happen. If the network is split (strictly, if the nodes think
the network is split) all nodes will still honor read and write requests. When the network
heals, CouchDB replication runs between all the pairs of replicas (aka Eventual Consistency).

BigCouch does use consistent hashing, so we can always calculate which nodes should hold a
copy of a particular document if it exists. Concurrent updates to the same document have similar
semantics to CouchDB but it is possible for both writes to succeed (at different replicas)
without succeeding at all replicas. The status codes of the requests will reflect that. For
a write, a 409 is returned if all replicas returned 409, a 201 if all replicas returned 201,
and a 202 if at least one replica returned 201 but the other replicas returned 409’s. The
replicas themselves run CouchDB replication (though an optimized, non-http version) whenever
they are updated, and this ensures all branches of all documents reach every replica.

Did I miss a question?

Some useful links;

https://cloudant.com/blog/dynamo-and-couchdb-clusters/

https://cloudant.com/blog/bigcouch-0-3/

https://cloudant.com/blog/bigcouch-zero-point-four/


On 27 Mar 2014, at 06:00, Stanley Iriele <siriele2x3@gmail.com> wrote:

> This is again extremely helpful...thanks... MVCC says... Hey...while you're
> writing... "Every one is still pointing to the old one for reads"... But
> the moment its done... Look at this new one" if it doesn't do that it has
> to do something about simultaneous read write right?
> 
> Also.. Does bigcouch return stale data... Ever?... Like the example I
> described above?... This mail thread really needs to be in a doc somewhere
> BTW.. Again many thanks
> On Mar 26, 2014 10:45 PM, "Benoit Chesneau" <bchesneau@gmail.com> wrote:
> 
>> On Thursday, March 27, 2014, Stanley Iriele <siriele2x3@gmail.com> wrote:
>> 
>>> Also... Jens... You said couchbase doesn't have MVCC ? All docs say That
>> it
>>> uses couchDB MVCC append only under the good on a single node... Could
>> you
>>> elaborate a tad on what you mean by doesn't have MVCC?... Also for the
>>> couchDB advocates...I think discussions about this are incredibly helpful
>>> for all parties involved..
>> 
>> 
>> 
>> couchabase has no revision tree / docs.  the mvcc has been removed  to not
>> impact the performances.
>> 
>> - benoit
>> 
>>> On Mar 26, 2014 2:58 PM, "Stanley Iriele" <siriele2x3@gmail.com
>> <javascript:;>>
>>> wrote:
>>> 
>>>> Thanks again Jens for the reply... Couchbase has documentation on
>>>> this...and gobs of marketing... But bigcouch does not...
>>>> 
>>>> In bigcouch...all nodes can handle every request... But let's say a
>> node
>>>> cannot reach the nodes that have a key..like node A knows it needs to
>>> read
>>>> from nodes B and C but cannot reach them.... But has a copy of the data
>>>> locally but was given r=2 for consent read... What does bigcouch do?
>>>> 
>>>> Potentially return stale data?.. Throw an error?....
>>>> 
>>>> I know that bigcouch is based on the dynamo white paper... But so is
>>>> Cassandra...and riak and you cab at least find out the answers to these
>>>> questions with a little googling. Again I'm eternally grateful for the
>>>> responses I'm getting I just wish there more docs....like the couchDB
>>> docs
>>>> page that even has a CAP theorem chart..(which is freaking awesome)..
>>>> On Mar 26, 2014 12:24 PM, "Jens Alfke" <jens@couchbase.com
>> <javascript:;>>
>>> wrote:
>>>> 
>>>>> 
>>>>> On Mar 26, 2014, at 9:31 AM, Stanley Iriele <siriele2x3@gmail.com
>> <javascript:;>>
>>> wrote:
>>>>> 
>>>>>> Why would you say that couchbase scales better?...
>>>>> 
>>>>> That's getting way off-topic for this list, but
>> http://couchbase.comhas
>>>>> a bunch of marketing materials and white papers and such, and we have
>>> sales
>>>>> engineers you can talk to if you really want to dive into it. But as
I
>>>>> said, Couchbase Server and BigCouch are very different.
>>>>> 
>>>>>> And does bigcouch ever return conflicts to the user? How is the
>> "which
>>>>> write won" problem solved?
>>>>> 
>>>>> AFAIK BigCouch's document/revision/conflict model is identical to
>>>>> CouchDBs. MVCC, revision IDs, revision trees, conflicts represented as
>>>>> branched trees, conflict resolution by deleting unwanted branches,
>> etc.
>>>>> 
>>>>> --Jens
>>>> 
>>>> 
>>> 
>> 


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