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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> >>>> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> >>>>> 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 < >>>>> [email protected]> >>>>>>>>> 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 < >>>>> [email protected]> >>>>>>>>>>> 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 <[email protected]> >>>>> 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 <[email protected]> >>>>> 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 < >>>>>>>>>> [email protected]> 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… >>>>>>>> >>>>> >>>>> >>> >> >
