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From Robert Samuel Newson <>
Subject Re: Could CouchDB 2.0 fix actual read quorum?
Date Sat, 04 Apr 2015 10:12:44 GMT
I’ve made branch 2655-r-met2 in fabric which will indicate the consistency of the response.
I’ve kept the is_r_met and r_met names for now, but if this is the right direction we will
want to change that.

When fabric returns r_met:"consistent" it means complete agreement among all R responses
When fabric returns r_met:"divergent" it means we saw more than one distinct revision from
the R responses but all divergent copies are ancestors (i.e, they’re missing an update rather
being an alternate branch)
When fabric returns r_met:"disagreement" we saw truly divergent responses. Fabric blocks for
the repair, so the response is "healed", but nevertheless it indicates an issue like a recent
partition not yet fully healed by anti-entropy.

Obviously these names are terrible and we’ll need to brainstorm on those, but let’s first
establish if this is the right kind of metadata.


> On 4 Apr 2015, at 10:41, Robert Samuel Newson <> wrote:
> Ok, most of those make sense to me (I think the last two, and particularly the last one,
are confounded by the fact couch will initiate read repair if it sees a lack of convergence,
i.e, R to N* different revisions, and will perform the usual arbitrary-but-consistent winner
algorithm right there).
> So, what we want is not really r_met in the sense that fabric means it; which is the
minimum number of responses to wait for before returning, regardless of whether they are the
same revision or not.
> It’s as you said, did we see at least R responses with the same revision? Would we
want additional nuance like whether the responses were so inconsistent that we ran read repair?
This would distinguish the case where there are simply fewer than R responses (for nodes down
/ slow / partitioned) that are returning the same revision versus the case where all R to
N* responses return different revisions.
> I’ll see how easy it is to return the first value while we ponder the other question.
> * I say "R to N" to mean fabric will wait for at least R responses (or timeout) but up
to N responses (or timeout) if the responses vary.
> B.
>> On 4 Apr 2015, at 02:08, Mutton, James <> wrote:
>> * Report the number of r_met failed conditions to a statistical aggregator for alerting
or trending on client-visible behavior.
>> * Pause some operation for a time if possible, retry later.
>> * Possibly re-resolve and use another cluster that is more healthy or less loaded
>> * Indicate some hidden failure or bug in how shards got moved around/restored from
down nodes
>> </JamesM>
>> On Apr 3, 2015, at 17:27, Robert Samuel Newson <> wrote:
>>> I’ve pushed an update to the fabric branch which accounts for when the r= value
is higher than the number of replicas (so that it returns r_met:false)
>>> Changing this so that r_met is true only if R matching revisions are seen doesn’t
sound too difficult.
>>> Where I struggle is seeing what a client can usefully do with this information.
When you receive the r_met:false indication, however we end up conveying it, what will you
do? Retry until r_met:true?
>>> B.
>>>> On 4 Apr 2015, at 00:55, Mutton, James <> wrote:
>>>> Based on Paul’s description it sounds like we may need to decide 3 things
to close this out:
>>>> * What does satisfying R mean?
>>>> * What is the appropriate scope of when R is applied?
>>>> * How do we most appropriately convey the lack of R?
>>>> I’m basing my opinions of R on W.  W is satisfied when a write succeeds
to W nodes.  For behavior to be consistent between R and W, R should be considered to be met
when R “matching” results have been found, if we treat “matching” == “successful”.
 I believe this to be a more-correct interpretation of R-W consistency then treating R-satisfied
as “found-but-not-matching” since it matches the complete positive of W's “successfully-written”.
 For scope, W acts for both current versions and historical revision updates (e.g. resolving
conflicts).  W also functions in bulk operations so R should function in multi-key requests
as well if it’s to be consistent.
>>>> The last question is how to appropriately convey lack of R.  I tested these
branches to see that the _r_met was present, that worked.  I also made some quick modifications
to return a 203 to see how some clients behaved.  Here are my test results:
>>>> I tested a few clients including an old version of couchdbkit and all worked
while the server was returning a 203 and/or the meta-field.  A quick test-with replication
was mixed.  I did a replicate into a couchdb 1.6 machine and although I did see some errors,
replication succeeded (the errors were related to checkpointing the target and my 1.6 could
have been messed up).  All that to say that where I tested it, returning a 203 on R was accepted
behavior by clients, just as returning a 202 on W.  By no means is that extensive but at least
indicative.  So, I think both approaches, field and status-code, are possible for single key
requests (more on that in a second) and whether it’s status or field, I favor at least having
consistency with W.  We could also have consistency by converting W’s 202 to a to be an
in-document meta field like _w_met and only present when ?is_w_met=true is present on the
query string.  That feels more drastic.
>>>> So the last issue is for the bulk/multi-doc responses.  Here the entire approach
of reads and writes diverges.  Writes are still individual doc-updates, whereas reads of multi-docs
are basically a “view” even if it’s all_docs.  IMHO, views could be called  out of scope
for when R is Applied.  It doesn’t even descend into doc_open to apply R unless “keys”
are specified and normal views without include_docs would do the same IIRC.  This approach
of calling all views out of scope because they could only even be in scope under certain circumstances,
leaves the door open still for either a status-code or field (and again, if using a field
it would be more consistent API behavior to switch W to behave the same)
>>>> Cheers,
>>>> </JamesM>
>>>> On Apr 2, 2015, at 3:51, Robert Samuel Newson <>
>>>>> To move this along I have COUCHDB-2655 and three branches with a working
>>>>> All three branches are called 2655-r-met if you want to try this locally
(and please do!)
>>>>> Sample output;
>>>>> curl -v 'foo:bar@localhost:15984/db1/doc1?is_r_met=true'
>>>>> {"_id":"doc1","_rev":"1-967a00dff5e02add41819138abb3284d","_r_met":true}
>>>>> By making it opt-in, I think we avoid all the collateral damage that
Paul was concerned about.
>>>>> B.
>>>>>> On 2 Apr 2015, at 10:36, Robert Samuel Newson <>
>>>>>> Yeah, not a bad idea. An extra query arg (akin to open_revs=all,
conflicts=true, etc) would avoid compatibility breaks and would clearly put the onus on those
supplying it to tolerate the presence of the extra reserved field.
>>>>>> +1
>>>>>>> On 2 Apr 2015, at 10:32, Benjamin Bastian <>
>>>>>>> What about adding an optional query parameter to indicate whether
or not
>>>>>>> Couch should include the _r_met flag in the document body/bodies
>>>>>>> (defaulting to false)? That wouldn't break older clients and
it'd work for
>>>>>>> the bulk API as well. As far as the case where there are conflicts,
>>>>>>> seems like the most intuitive thing would be for the "r" in "_r_met"
>>>>>>> have the same semantic meaning as the "r" in "?r=" (i.e. "?r="
means "wait
>>>>>>> for r copies of the same doc rev until a timeout" and "_r_met"
would mean
>>>>>>> "we got/didn't get r copies of the same doc rev within the timeout").
>>>>>>> Just my two cents.
>>>>>>> On Thu, Apr 2, 2015 at 1:22 AM, Robert Samuel Newson <>
>>>>>>> wrote:
>>>>>>>> Paul outlined his previous efforts to introduce this indication,
and the
>>>>>>>> problems he faced doing so. Can we come up with an acceptable
>>>>>>>> A different status code will break a lot of users. While
the http spec
>>>>>>>> says you can treat any 2xx code as success, plenty of libraries,
etc, only
>>>>>>>> recognise 201 / 202 as successful write and 200 (and maybe
204, 206) for
>>>>>>>> reads.
>>>>>>>> My preference is for a change that "can’t" break anyone,
which I think
>>>>>>>> only leaves an "X-CouchDB-R-Met: 2" response header, which
isn’t the most
>>>>>>>> pleasant thing.
>>>>>>>> Suggestions?
>>>>>>>> B.
>>>>>>>>> On 1 Apr 2015, at 06:55, Mutton, James <>
>>>>>>>>> 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,
>>>>>>>> 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 <>
>>>>>>>>>> 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 <>
>>>>>>>>>>> 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 <>
>>>>>>>> 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 <
>>>>>>>>>>>> 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
>>>>>>>>>>>>> The r= and w= parameters control only
how many of the n possible
>>>>>>>> responses
>>>>>>>>>>>>> are collected before returning an http
>>>>>>>>>>>>> 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 <
>>>>>>>>>>>>>> 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 <>
>>>>>>>> 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 <>
>>>>>>>> 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.
>>>>>>>> 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
>>>>>>>>>>>>>>>> Cheers,
>>>>>>>>>>>>>>>> </JamesM>
>>>>>>>>>>>>>>>>> On Mar 24, 2015, at 14:06,
Nathan Vander Wilt <
>>>>>>>>>>>>>> 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
>>>>>>>>>>>>>>>>> See
>>>>>>>> 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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