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From "Mutton, James" <jmut...@akamai.com>
Subject Re: Could CouchDB 2.0 fix actual read quorum?
Date Wed, 01 Apr 2015 05:55:07 GMT
For at least my part of it, I agree with Adam. Bigcouch has made an effort to inform in the
case of a failure to apply W. I've seen it lead to confusion when the same logic was not applied
on R.

I also agree that W and R are not binding contracts. There's no agreement protocol to assure
that W is met before being committed to disk. But they are exposed as a blocking parameter
of the request, so notification being consistent appeared to me to be the best compromise
(vs straight up removal).

</JamesM>


> On Mar 31, 2015, at 13:15, Robert Newson <rnewson@apache.org> wrote:
> 
> 
> If a way can be found that doesn't break things that can be sent in all or most cases,
sure. It's what a user can really infer from that which I focused on. Not as much, I think,
as users that want that info really want. 
> 
> 
>> On 31 Mar 2015, at 21:08, Adam Kocoloski <kocolosk@apache.org> wrote:
>> 
>> I hope we can all agree that CouchDB should inform the user when it is unable to
satisfy the requested read "quorum".
>> 
>> Adam
>> 
>>> On Mar 31, 2015, at 3:20 PM, Paul Davis <paul.joseph.davis@gmail.com> wrote:
>>> 
>>> Sounds like there's a bit of confusion here.
>>> 
>>> What Nathan is asking for is the ability to have Couch respond with some
>>> information on the actual number of replicas that responded to a read
>>> request. That way a user could tell that they issued an r=2 request when
>>> only r=1 was actually performed. Depending on your point of view in an MVCC
>>> world this is either a bug or a feature. :)
>>> 
>>> It was generally agreed upon that if we could return this information it
>>> would be beneficial. Although what happened when I started implementing
>>> this patch was that we are either only able to return it in a subset of
>>> cases where it happens, return it inconsistently between various responses,
>>> or break replication.
>>> 
>>> The three general methods for this would be to either include a new
>>> "_r_met" key in the doc body that would be a boolean indicating if the
>>> requested read quorum was actually met for the document. The second was to
>>> return a custom X-R-Met type header, and lastly was the status code as
>>> described.
>>> 
>>> The _r_met member was thought to be the best, but unfortunately that breaks
>>> replication with older clients because we throw an error rather than ignore
>>> any unknown underscore prefixed field name. Thus having something that was
>>> just dynamically injected into the document body was a non-starter.
>>> Unfortunately, if we don't inject into the document body then we limit
>>> ourselves to only the set of APIs where a single document is returned. This
>>> is due to both streaming semantics (we can't buffer an entire response in
>>> memory for large requests to _all_docs) as well as multi-doc responses (a
>>> single boolean doesn't say which document may have not had a properly met
>>> R).
>>> 
>>> On top of that, the other confusing part of meeting the read quorum is that
>>> given MVCC semantics it becomes a bit confusing on how you respond to
>>> documents with different revision histories. For instance, if we read two
>>> docs, we have technically made the r=2 requirement, but what should our
>>> response be if those two revisions are different (technically, in this case
>>> we wait for the third response, but the decision on what to return for the
>>> "r met" value is still unclear).
>>> 
>>> While I think everyone is in agreement that it'd be nice to return some of
>>> the information about the copies read, I think its much less clear what and
>>> how it should be returned in the multitude of cases that we can specify an
>>> value for R.
>>> 
>>> While that doesn't offer a concrete path forward, hopefully it clarifies
>>> some of the issues at hand.
>>> 
>>> On Tue, Mar 31, 2015 at 1:47 PM, Robert Samuel Newson <rnewson@apache.org>
>>> wrote:
>>> 
>>>> 
>>>> It’s testament to my friendship with Mike that we can disagree on such
>>>> things and remain friends. I am sorry he misled you, though.
>>>> 
>>>> CouchDB 2.0 (like Cloudant) does not have read or write quorums at all, at
>>>> least in the formal sense, the only one that matters, this is unfortunately
>>>> sloppy language in too many places to correct.
>>>> 
>>>> The r= and w= parameters control only how many of the n possible responses
>>>> are collected before returning an http response.
>>>> 
>>>> It’s not true that returning 202 in the situation where one write is made
>>>> but fewer than 'r' writes are made means we’ve chosen availability over
>>>> consistency since even if we returned a 500 or closed the connection
>>>> without responding, a subsequent GET could return the document (a
>>>> probability that increases over time as anti-entropy makes the missing
>>>> copies). A write attempt that returned a 409 could, likewise, introduce a
>>>> new edit branch into the document, which might then 'win', altering the
>>>> results of a subsequent GET.
>>>> 
>>>> The essential thing to remember is this: the ’n’ copies of your data
are
>>>> completely independent when written/read by the clustered layer (fabric).
>>>> It is internal replication (anti-entropy) that converges those copies,
>>>> pair-wise, to the same eventual state. Fabric is converting the 3
>>>> independent results into a single result as best it can. Older versions did
>>>> not expose the 201 vs 202 distinction, calling both of them 201. I do agree
>>>> with you that there’s little value in the 202 distinction. About the only
>>>> thing you could do is investigate your cluster for connectivity issues or
>>>> overloading if you get a sustained period of 202’s, as it would be an
>>>> indicator that the system is partitioned.
>>>> 
>>>> In order to achieve your goals, CouchDB 2.0 would have to ensure that the
>>>> result of a write did not change after the fact. That is, anti-entropy
>>>> would need to be disabled, or somehow agree to roll forward or backward
>>>> based on the initial circumstances. In short, we’d have to introduce strong
>>>> consistency (paxos or raft or zab, say). While this would be a great
>>>> feature to add, it’s not currently present, and no amount of twiddling
the
>>>> status codes will achieve it. We’d rather be honest about our position
on
>>>> the CAP triangle.
>>>> 
>>>> B.
>>>> 
>>>> 
>>>>>> On 30 Mar 2015, at 22:37, Nathan Vander Wilt <nate-lists@calftrail.com>
>>>>> wrote:
>>>>> 
>>>>> A technical co-founder of Cloudant agreed that this was a bug when I
>>>> first hit it a few years ago. I found back the original thread here — this
>>>> is the discussion I was trying to recall in my OP:
>>>>> It sounds like perhaps there is a related issue tracked internally at
>>>> Cloudant as a result of that conversation.
>>>>> 
>>>>> JamesM, thanks for your support here and tracking this down. 203 seemed
>>>> like the best status code to "steal" for this to me too. Best wishes in
>>>> getting this fixed!
>>>>> 
>>>>> regards,
>>>>> -natevw
>>>>> 
>>>>> 
>>>>>> On Mar 25, 2015, at 4:49 AM, Robert Newson <rnewson@apache.org>
wrote:
>>>>>> 
>>>>>> 2.0 is explicitly an AP system, the behaviour you describe is not
>>>> classified as a bug.
>>>>>> 
>>>>>> Anti-entropy is the main reason that you cannot get strong consistency
>>>> from the system, it will transform "failed" writes (those that succeeded
on
>>>> one node but fewer than R nodes) into success (N copies) as long as the
>>>> nodes have enough healthy uptime.
>>>>>> 
>>>>>> True of cloudant and 2.0.
>>>>>> 
>>>>>> Sent from my iPhone
>>>>>> 
>>>>>>> On 24 Mar 2015, at 15:14, Mutton, James <jmutton@akamai.com>
wrote:
>>>>>>> 
>>>>>>> Funny you should mention it.  I drafted an email in early February
to
>>>> queue up the same discussion whenever I could get involved again (which I
>>>> promptly forgot about).  What happens currently in 2.0 appears unchanged
>>>> from earlier versions.  When R is not satisfied in fabric,
>>>> fabric_doc_open:handle_message eventually responds with a {stop, …}  but
>>>> leaves the acc-state as the original r_not_met which triggers a read_repair
>>>> from the response handler.  read_repair results in an {ok, …} with the
only
>>>> doc available, because no other docs are in the list.  The final doc
>>>> returned to chttpd_db:couch_doc_open and thusly to chttpd_db:db_doc_req is
>>>> simply {ok, Doc}, which has now lost the fact that the answer was not
>>>> complete.
>>>>>>> 
>>>>>>> This seems straightforward to fix by a change in
>>>> fabric_open_doc:handle_response and read_repair.  handle_response knows
>>>> whether it has R met and could pass that forward, or allow read-repair to
>>>> pass it forward if read_repair is able to satisfy acc.r.  I can’t speak
for
>>>> community interest in the behavior of sending a 202, but it’s something
I’d
>>>> definitely like for the same reasons you cite.  Plus it just seems
>>>> disconnected to do it on writes but not reads.
>>>>>>> 
>>>>>>> Cheers,
>>>>>>> </JamesM>
>>>>>>> 
>>>>>>>> On Mar 24, 2015, at 14:06, Nathan Vander Wilt <
>>>> nate-lists@calftrail.com> wrote:
>>>>>>>> 
>>>>>>>> Sorry, I have not been following CouchDB 2.0 roadmap but
I was
>>>> extending my fermata-couchdb plugin today and realized that perhaps the
>>>> Apache release of BigCouch as CouchDB 2.0 might provide an opportunity to
>>>> fix a serious issue I had using Cloudant's implementation.
>>>>>>>> 
>>>>>>>> See
>>>> https://github.com/cloudant/bigcouch/issues/55#issuecomment-30186518 for
>>>> some additional background/explanation, but my understanding is that
>>>> Cloudant for all practical purposes ignores the read durability parameter.
>>>> So you can write with ?w=N to attempt some level of quorum, and get a 202
>>>> back if that quorum is unment. _However_ when you ?r=N it really doesn't
>>>> matter if only <N nodes are available…if even just a single available
node
>>>> has some version of the requested document you will get a successful
>>>> response (!).
>>>>>>>> 
>>>>>>>> So in practice, there's no way to actually use the quasi-Dynamo
>>>> features to dynamically _choose_ between consistency or availability —
when
>>>> it comes time to read back a consistent result, BigCouch instead just
>>>> always gives you availability* regardless of what a given request actually
>>>> needs. (In my usage I ended up treating a 202 write as a 500, rather than
>>>> proceeding with no way of ever knowing whether a write did NOT ACTUALLY
>>>> conflict or just hadn't YET because $who_knows_how_many nodes were still
>>>> down…)
>>>>>>>> 
>>>>>>>> IIRC, this was both confirmed and acknowledged as a serious
bug by a
>>>> Cloudant engineer (or support personnel at least) but could not be quickly
>>>> fixed as it could introduce backwards-compatibility concerns. So…
>>>>>>>> 
>>>>>>>> Is CouchDB 2.0 already breaking backwards compatibility with
>>>> BigCouch? If true, could this read durability issue now be fixed during the
>>>> merge?
>>>>>>>> 
>>>>>>>> thanks,
>>>>>>>> -natevw
>>>>>>>> 
>>>>>>>> 
>>>>>>>> 
>>>>>>>> 
>>>>>>>> 
>>>>>>>> * DISCLAIMER: this statement has not been endorsed by actual
uptime
>>>> of *any* Couch fork…
>> 
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