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From Jan Lehnardt <...@apache.org>
Subject Re: [DISCUSS] couchdb 4.0 transactional semantics
Date Wed, 15 Jul 2020 14:14:54 GMT


> On 15. Jul 2020, at 16:12, Robert Newson <rnewson@apache.org> wrote:
> 
> 
> Thanks Jan
> 
> I would prefer not to have the configuration switch, instead remove what we don’t want.
As you said there’ll be a 3 / 4 split for a while (and not just for this reason). 

I’d support an effort for folks to ease into 4.x, as long as it is not the default behaviour.
I haven’t thought about this enough to have a definite opinion about what that looks like.

Best
Jan
—
> -- 
>  Robert Samuel Newson
>  rnewson@apache.org
> 
> On Wed, 15 Jul 2020, at 14:46, Jan Lehnardt wrote:
>> 
>>> On 14. Jul 2020, at 18:00, Adam Kocoloski <kocolosk@apache.org> wrote:
>>> 
>>> I think there’s tremendous value in being able to tell our users that each
response served by CouchDB is constructed from a single isolated snapshot of the underlying
database. I’d advocate for this being the default behavior of 4.0.
>> 
>> I too am in favour of this. I apologise for not speaking up in the 
>> earlier thread, which I followed closely, but never found the time to 
>> respond to.
>> 
>> From rnewson’s options, I’d suggest 3. the mandatory limit parameter. 
>> While this does indeed mean a BC break, it teaches the right semantics 
>> for folks on 4.0 and onwards. For client libraries like our own nano, 
>> we can easily wrap this behaviour, so the resulting API is mostly 
>> compatible still, at least when used in streaming mode, less so when 
>> buffering a big _all_docs response).
>> 
>>> If folks wanted to add an opt-in compatibility mode to support longer responses,
I suppose that could be OK. I think we should discourage that access pattern in general, though,
as it’s somewhat less friendly to various other parts of the stack than a pattern of shorter
responses and a smart pagination API like the one we’re introducing. To wit, I don’t think
we’d want to support that compatibility mode in IBM Cloud.
>> 
>> Like Adam, I do not mind a compat mode, either through a different API 
>> endpoint, or even a config option. I think we will be fine in getting 
>> people on this path when we document this in our update guide for the 
>> 4.0 release. I don’t think this will lead to a Python 2/3 situation 
>> overall, because the 4.0+ features are compelling enough for relatively 
>> small changes required, and CouchDB 3.x in its then latest form will 
>> continue to be a fine database for years to come, for folks who can’t 
>> upgrade as easily. So yes, I anticipate we’ll live in a two-versions 
>> world a little longer than we did during 1.x to 2.x, but the reasons to 
>> leave 1.x behind were a little more severe than the improvements of 4.x 
>> over 3.x (while still significant, of course).
>> 
>> Best
>> Jan
>> —
>> 
>>> 
>>> Adam
>>> 
>>>> On Jul 14, 2020, at 10:18 AM, Robert Samuel Newson <rnewson@apache.org>
wrote:
>>>> 
>>>> Thanks Nick, very helpful, and it vindicates me opening this thread.
>>>> 
>>>> I don't accept Mike Rhodes argument at all but I should explain why I don't;
>>>> 
>>>> In CouchDB 1.x, a response was generated from a single .couch file. There
was always a window between the start of the request as the client sees it and CouchDB acquiring
a snapshot of the relevant database. I don't think that gap is meaningful and does not refute
our statements of the time that CouchDB responses are from a snapshot (specifically, that
no change to the database made _during_ the response will be visible in _this_ response).
In CouchDB 2.x (and continuing in 3.x), a CouchDB database typically consists of multiple
shards, each of which, once opened, remain snapshotted for the duration of that response.
The difference between 1.x and 2.x/3.x is that the window is potentially larger (though the
requests are issued in parallel). The response, however much it returned, was impervious to
changes in other requests once it has begun.
>>>> 
>>>> I don't think _all_docs, _view or a non-continuous _changes response should
allow changes made in other requests to appear midway through them and I want to hear the
opinions of folks that have watched over CouchDB from its earliest days on this specific point
(If I must name names, at least Adam K, Paul D, Jan L, Joan T). If there's a majority for
deviating from this semantic, I will go with the majority.
>>>> 
>>>> If we were to agree to preserve the 'single snapshot' behaviour, what would
the behaviour be if we can't honour it because of the FoundationDB transaction limits?
>>>> 
>>>> I see a few options.
>>>> 
>>>> 1) We could end the response uncleanly, mid-response. CouchDB does this when
it has no alternative, and it is ugly, but it is usually handled well by clients. They are
at least not usually convinced they got a complete response if they are using a competent
HTTP client.
>>>> 
>>>> 2) We could disavow the streaming API, as you've suggested, attempt to gather
the full response. If we do this within the FDB bounds, return a 200 code and the response
body. A 400 and an error body if we don't.
>>>> 
>>>> 3) We could make the "limit" parameter mandatory and with an upper bound,
in combination with 1 or 2, such that a valid request is very likely to be served within the
limits.
>>>> 
>>>> I'd like to hear more voices on which way we want to break the unachievable
semantic of old where you could read _all_docs on a billion document database over, uptime
gods willing, a snapshot of the database.
>>>> 
>>>> B.
>>>> 
>>>>> On 13 Jul 2020, at 21:15, Nick Vatamaniuc <vatamane@gmail.com>
wrote:
>>>>> 
>>>>> Thanks for bringing the topic up for the discussion!
>>>>> 
>>>>> For background, this topic was discussed on the mailing list starting
>>>>> in February, 2019
>>>>> https://lists.apache.org/thread.html/r02cee7045cac4722e1682bb69ba0ec791f5cce025597d0099fb34033%40%3Cdev.couchdb.apache.org%3E
>>>>> 
>>>>> The primary reason for restart_tx option is to provide compatibility
>>>>> for _changes feeds to allow older replicators to handle 4.0 sources.
>>>>> It starts a new transaction after 5 seconds or so (a current FDB
>>>>> limitation, might go up in the future) and transparently continues to
>>>>> stream data where it left off. Ex, streaming [a,b,c,d], times out
>>>>> after b, then it will continue with c, d etc. Currently this is also
>>>>> used for other streaming APIs as an alternative to returning mangled
>>>>> JSON after emitting a 200 response and streaming some of the rows.
>>>>> However it is not used for paginated responses, the new APIs developed
>>>>> by Ilya. So users have an option to get the guaranteed snapshot
>>>>> behavior option as well.
>>>>> 
>>>>> And for completeness, if we decide to remove the option, we should
>>>>> specify what happens if we remove it and get a transaction_too_old
>>>>> exception. Currently the behavior would be to restart the transaction,
>>>>> resend all the headers and all the rows again down the socket, which
I
>>>>> don't think anyone wants, but is what we'd get if we just make
>>>>> {restart_tx, false}
>>>>> 
>>>>>> I understand that automatically resetting the FDB txn during a response
is an attempt to work around that and maintain "compatibility" with CouchDB < 4 semantics.
I think it fails to do so and is very misleading.
>>>>> 
>>>>> It is a trade-off in order to keep the same API shape as before. Sure,
>>>>> streaming all the docs with _all_docs or _changes feeds is not a great
>>>>> pattern but many applications are implemented that way already.
>>>>> Letting them migrate to 4.0 without having to rewrite the application
>>>>> with the caveat that they might see a document updated in the
>>>>> _all_docs stream after the request has already started, is a nicer
>>>>> choice, I think, than forcing them to rewrite their application, which
>>>>> could lead to a python 2/3 scenario.
>>>>> 
>>>>> Due to having multiple shards (Q>1), as discussed in the original
>>>>> mailing thread by Mike
>>>>> (https://lists.apache.org/thread.html/r8345f534a6fa88c107c1085fba13e660e0e2aedfd206c2748e002664%40%3Cdev.couchdb.apache.org%3E),
>>>>> we don't provide a strict read-only snapshot guarantee in 2.x and 3.x
>>>>> anyway, so users would have to handle scenarios where a document might
>>>>> appear in the stream that wasn't there at the start of the request
>>>>> already. Though, granted, a much smaller corner case but I wonder how
>>>>> many users care to handle that...
>>>>> 
>>>>> Currently users do have an option of using the new paginated API which
>>>>> disables restart_tx behavior
>>>>> https://github.com/apache/couchdb/blob/prototype/fdb-layer/src/chttpd/src/chttpd_db.erl#L947,
>>>>> though I am not sure what happens when transaction_too_old exception
>>>>> is thrown then (emit a bookmark?)
>>>>> 
>>>>> So based on the compatibility consideration, I'd vote to keep the
>>>>> restart_tx option (configurable perhaps if we figure out what to do
>>>>> when it is disabled) in order to allow users to migrate their
>>>>> application to 4.0. At least informally we promised users to keep a
>>>>> strong API compatibility when we released 3.0 with an eye towards 4.0
>>>>> (https://blog.couchdb.org/2020/02/26/the-road-to-couchdb-3-0/). I'd
>>>>> think not emitting all the data in a _changes or _all_docs response
>>>>> would break that compatibility more than using multiple transactions.
>>>>> 
>>>>> As for what happens when a transaction_too_old is thrown, I could see
>>>>> an option passed in, something like, single_snapshot=true, and then
>>>>> use Adam's suggestion to accumulate all the rows in memory and if we
>>>>> hit the end of the transaction return a 400 error. We won't emit
>>>>> anything out while rows are accumulated, so users don't get partial
>>>>> data, it will be every row requested or a 400 error (so no chance of
>>>>> perceived data loss). Users may retry if they think it was a temporary
>>>>> hiccup or may use a small limit number.
>>>>> 
>>>>> Cheers,
>>>>> -Nick
>>>>> 
>>>>> On Mon, Jul 13, 2020 at 2:05 PM Robert Samuel Newson <rnewson@apache.org>
wrote:
>>>>>> 
>>>>>> Hi All,
>>>>>> 
>>>>>> I'm concerned to see the restart_fold function in fabric2_fdb (https://github.com/apache/couchdb/blob/prototype/fdb-layer/src/fabric/src/fabric2_fdb.erl#L1828)
in the 4.0 development branch.
>>>>>> 
>>>>>> The upshot of doing this is that a CouchDB response could be taken
across multiple snapshots of the database, which is not the behaviour of CouchDB 1 through
3.
>>>>>> 
>>>>>> I don't think this is ok (with the obvious and established exception
of a continuous changes feed, where new snapshots are continuously visible at the end of the
response).
>>>>>> 
>>>>>> FoundationDB imposes certain limits on transactions, the most notable
being the 5 second maximum duration. I understand that automatically resetting the FDB txn
during a response is an attempt to work around that and maintain "compatibility" with CouchDB
< 4 semantics. I think it fails to do so and is very misleading.
>>>>>> 
>>>>>> Discuss.
>>>>>> 
>>>>>> B.
>>>>>> 
>>>> 
>>> 
>> 
>> 


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