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From Robert Samuel Newson <rnew...@apache.org>
Subject Re: Could CouchDB 2.0 fix actual read quorum?
Date Sat, 04 Apr 2015 00:27:33 GMT

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 <jmutton@akamai.com> 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: https://gist.github.com/jamutton/c823fdac328777e22646
> 
> 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 <rnewson@apache.org> wrote:
> 
>> To move this along I have COUCHDB-2655 and three branches with a working solution;
>> 
>> https://git-wip-us.apache.org/repos/asf?p=couchdb-chttpd.git;h=b408ce5
>> https://git-wip-us.apache.org/repos/asf?p=couchdb-couch.git;h=7d811d3
>> https://git-wip-us.apache.org/repos/asf?p=couchdb-fabric.git;h=90e9691
>> 
>> 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 <rnewson@apache.org> wrote:
>>> 
>>> 
>>> 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 <bbastian@apache.org> wrote:
>>>> 
>>>> 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, it
>>>> seems like the most intuitive thing would be for the "r" in "_r_met" to
>>>> 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 <rnewson@apache.org>
>>>> wrote:
>>>> 
>>>>> 
>>>>> Paul outlined his previous efforts to introduce this indication, and
the
>>>>> problems he faced doing so. Can we come up with an acceptable mechanism?
>>>>> 
>>>>> 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 <jmutton@akamai.com>
wrote:
>>>>>> 
>>>>>> 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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