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From Joan Touzet <woh...@apache.org>
Subject Re: [DISCUSS] Streaming API in CouchDB 4.0
Date Thu, 23 Apr 2020 18:02:34 GMT
I realise this is bikeshedding, but I guess that's kind of the point... 
Everything below is my opinion, not "fact."

It's unfortunate we need a new endpoint for all of this. In a vacuum I 
might have just suggested we use the semantics we already have, perhaps 
with ?from= instead of ?since= .

"page" only works if the size of a page is well known, either by server 
preference or directly in the URL. If I ask for:

   GET /{db}/_all_docs?limit=20&page=3

I know that I'm always going to get document 41 through 60 in the 
default collation order.

There's a *fantastic* summary of examples from popular REST APIs here:

 
https://medium.com/@ignaciochiazzo/paginating-requests-in-apis-d4883d4c1c4c

We are *pretty close* to what a cursor means in those other examples, 
except for the fact that our cursor can go stale/invalid after a short time.

Bob, could you be a bit more detailed in your explanation how our 
definition isn't close to these? Or did you mean SQL CURSOR (which is 
something entirely different?) If so, I'm fine with this being a REST 
API cursor - something clearly distinct.

I come back to wanting to preserve the existing endpoint syntax and 
naming, without new endpoints, but specifying this new FDB token via 
?cursor= and this being the trigger for the new behaviour. At some 
point, we simply stop accepting ?since= tokens. This seems inline with 
other popular REST APIs.

-Joan "still sick and not sleeping right" Touzet


On 2020-04-23 12:30, Robert Newson wrote:
> cursor has established meaning in other databases and ours would not be very close to
them. I don’t think it’s a good idea.
> 
> B.
> 
>> On 23 Apr 2020, at 11:50, Ilya Khlopotov <iilyak@apache.org> wrote:
>>
>> 
>>>
>>> The best I could come up with is replacing page with
>>> cursor - {db}/_all_docs/cursor or possibly {db}/_cursor/_all_docs
>> Good idea, I like {db}/_all_docs/cursor (or {db}/_all_docs/_cursor).
>>
>>> On 2020/04/23 08:54:36, Garren Smith <garren@apache.org> wrote:
>>> I agree with Bob that page doesn't make sense as an endpoint. I'm also
>>> rubbish with naming. The best I could come up with is replacing page with
>>> cursor - {db}/_all_docs/cursor or possibly {db}/_cursor/_all_docs
>>> All the fields in the bookmark make sense except timestamp. Why would it
>>> matter if the timestamp is old? What happens if a node's time is an hour
>>> behind another node?
>>>
>>>
>>>> On Thu, Apr 23, 2020 at 4:55 AM Ilya Khlopotov <iilyak@apache.org>
wrote:
>>>>
>>>> - page is to provide some notion of progress for user
>>>> - timestamp - I was thinking that we should drop requests if user would
>>>> try to pass bookmark created an hour ago.
>>>>
>>>> On 2020/04/22 21:58:40, Robert Samuel Newson <rnewson@apache.org> wrote:
>>>>> "page" and "page number" are odd to me as these don't exist as concepts,
>>>> I'd rather not invent them. I note there's no mention of page size, which
>>>> makes "page number" very vague.
>>>>>
>>>>> What is "timestamp" in the bookmark and what effect does it have when
>>>> the bookmark is passed back in?
>>>>>
>>>>> I guess, why does the bookmark include so much extraneous data? Items
>>>> that are not needed to find the fdb key to begin the next response from.
>>>>>
>>>>>
>>>>>> On 22 Apr 2020, at 21:18, Ilya Khlopotov <iilyak@apache.org>
wrote:
>>>>>>
>>>>>> Hello everyone,
>>>>>>
>>>>>> Based on the discussions on the thread I would like to propose a
>>>> number of first steps:
>>>>>> 1) introduce new endpoints
>>>>>> - {db}/_all_docs/page
>>>>>> - {db}/_all_docs/queries/page
>>>>>> - _all_dbs/page
>>>>>> - _dbs_info/page
>>>>>> - {db}/_design/{ddoc}/_view/{view}/page
>>>>>> - {db}/_design/{ddoc}/_view/{view}/queries/page
>>>>>> - {db}/_find/page
>>>>>>
>>>>>> These new endpoints would act as follows:
>>>>>> - don't use delayed responses
>>>>>> - return object with following structure
>>>>>> ```
>>>>>> {
>>>>>>     "total": Total,
>>>>>>     "bookmark": base64 encoded opaque value,
>>>>>>     "completed": true | false,
>>>>>>     "update_seq": when available,
>>>>>>     "page": current page number,
>>>>>>     "items": [
>>>>>>     ]
>>>>>> }
>>>>>> ```
>>>>>> - the bookmark would include following data (base64 or protobuff???):
>>>>>> - direction
>>>>>> - page
>>>>>> - descending
>>>>>> - endkey
>>>>>> - endkey_docid
>>>>>> - inclusive_end
>>>>>> - startkey
>>>>>> - startkey_docid
>>>>>> - last_key
>>>>>> - update_seq
>>>>>> - timestamp
>>>>>> ```
>>>>>>
>>>>>> 2) Implement per-endpoint configurable max limits
>>>>>> ```
>>>>>> _all_docs = 5000
>>>>>> _all_docs/queries = 5000
>>>>>> _all_dbs = 5000
>>>>>> _dbs_info = 5000
>>>>>> _view = 2500
>>>>>> _view/queries = 2500
>>>>>> _find = 2500
>>>>>> ```
>>>>>>
>>>>>> Latter (after few years) CouchDB would deprecate and remove old
>>>> endpoints.
>>>>>>
>>>>>> Best regards,
>>>>>> iilyak
>>>>>>
>>>>>> On 2020/02/19 22:39:45, Nick Vatamaniuc <vatamane@apache.org>
wrote:
>>>>>>> Hello everyone,
>>>>>>>
>>>>>>> I'd like to discuss the shape and behavior of streaming APIs
for
>>>> CouchDB 4.x
>>>>>>>
>>>>>>> By "streaming APIs" I mean APIs which stream data in row as it
gets
>>>>>>> read from the database. These are the endpoints I was thinking
of:
>>>>>>>
>>>>>>> _all_docs, _all_dbs, _dbs_info  and query results
>>>>>>>
>>>>>>> I want to focus on what happens when FoundationDB transactions
>>>>>>> time-out after 5 seconds. Currently, all those APIs except _changes[1]
>>>>>>> feeds, will crash or freeze. The reason is because the
>>>>>>> transaction_too_old error at the end of 5 seconds is retry-able
by
>>>>>>> default, so the request handlers run again and end up shoving
the
>>>>>>> whole request down the socket again, headers and all, which is
>>>>>>> obviously broken and not what we want.
>>>>>>>
>>>>>>> There are few alternatives discussed in couchdb-dev channel.
I'll
>>>>>>> present some behaviors but feel free to add more. Some ideas
might
>>>>>>> have been discounted on the IRC discussion already but I'll present
>>>>>>> them anyway in case is sparks further conversation:
>>>>>>>
>>>>>>> A) Do what _changes[1] feeds do. Start a new transaction and
continue
>>>>>>> streaming the data from the next key after last emitted in the
>>>>>>> previous transaction. Document the API behavior change that it
may
>>>>>>> present a view of the data is never a point-in-time[4] snapshot
of the
>>>>>>> DB.
>>>>>>>
>>>>>>> - Keeps the API shape the same as CouchDB <4.0. Client libraries
>>>>>>> don't have to change to continue using these CouchDB 4.0 endpoints
>>>>>>> - This is the easiest to implement since it would re-use the
>>>>>>> implementation for _changes feed (an extra option passed to the
fold
>>>>>>> function).
>>>>>>> - Breaks API behavior if users relied on having a point-in-time[4]
>>>>>>> snapshot view of the data.
>>>>>>>
>>>>>>> B) Simply end the stream. Let the users pass a `?transaction=true`
>>>>>>> param which indicates they are aware the stream may end early
and so
>>>>>>> would have to paginate from the last emitted key with a skip=1.
This
>>>>>>> will keep the request bodies the same as current CouchDB. However,
if
>>>>>>> the users got all the data one request, they will end up wasting
>>>>>>> another request to see if there is more data available. If they
didn't
>>>>>>> get any data they might have a too large of a skip value (see
[2]) so
>>>>>>> would have to guess different values for start/end keys. Or impose
max
>>>>>>> limit for the `skip` parameter.
>>>>>>>
>>>>>>> C) End the stream and add a final metadata row like a "transaction":
>>>>>>> "timeout" at the end. That will let the user know to keep paginating
>>>>>>> from the last key onward. This won't work for `_all_dbs` and
>>>>>>> `_dbs_info`[3] Maybe let those two endpoints behave like _changes
>>>>>>> feeds and only use this for views and and _all_docs? If we like
this
>>>>>>> choice, let's think what happens for those as I couldn't come
up with
>>>>>>> anything decent there.
>>>>>>>
>>>>>>> D) Same as C but to solve the issue with skips[2], emit a bookmark
>>>>>>> "key" of where the iteration stopped and the current "skip" and
>>>>>>> "limit" params, which would keep decreasing. Then user would
pass
>>>>>>> those in "start_key=..." in the next request along with the limit
and
>>>>>>> skip params. So something like "continuation":{"skip":599, "limit":5,
>>>>>>> "key":"..."}. This has the same issue with array results for
>>>>>>> `_all_dbs` and `_dbs_info`[3].
>>>>>>>
>>>>>>> E) Enforce low `limit` and `skip` parameters. Enforce maximum
values
>>>>>>> there such that response time is likely to fit in one transaction.
>>>>>>> This could be tricky as different runtime environments will have
>>>>>>> different characteristics. Also, if the timeout happens there
isn't a
>>>>>>> a nice way to send an HTTP error since we already sent the 200
>>>>>>> response. The downside is that this might break how some users
use the
>>>>>>> API, if say the are using large skips and limits already. Perhaps
here
>>>>>>> we do both B and D, such that if users want transactional behavior,
>>>>>>> they specify that `transaction=true` param and only then we enforce
>>>>>>> low limit and skip maximums.
>>>>>>>
>>>>>>> F) At least for `_all_docs` it seems providing a point-in-time
>>>>>>> snapshot view doesn't necessarily need to be tied to transaction
>>>>>>> boundaries. We could check the update sequence of the database
at the
>>>>>>> start of the next transaction and if it hasn't changed we can
continue
>>>>>>> emitting a consistent view. This can apply to C and D and would
just
>>>>>>> determine when the stream ends. If there are no writes happening
to
>>>>>>> the db, this could potential streams all the data just like option
A
>>>>>>> would do. Not entirely sure if this would work for views.
>>>>>>>
>>>>>>> So what do we think? I can see different combinations of options
here,
>>>>>>> maybe even different for each API point. For example `_all_dbs`,
>>>>>>> `_dbs_info` are always A, and `_all_docs` and views default to
A but
>>>>>>> have parameters to do F, etc.
>>>>>>>
>>>>>>> Cheers,
>>>>>>> -Nick
>>>>>>>
>>>>>>> Some footnotes:
>>>>>>>
>>>>>>> [1] _changes feeds is the only one that works currently. It behaves
as
>>>>>>> per RFC
>>>> https://github.com/apache/couchdb-documentation/blob/master/rfcs/003-fdb-seq-index.md#access-patterns
>>>> .
>>>>>>> That is, we continue streaming the data by resetting the transaction
>>>>>>> object and restarting from the last emitted key (db sequence
in this
>>>>>>> case). However, because the transaction restarts if a document
is
>>>>>>> updated while the streaming take place, it may appear in the
_changes
>>>>>>> feed twice. That's a behavior difference from CouchDB < 4.0
and we'd
>>>>>>> have to document it, since previously we presented this point-in-time
>>>>>>> snapshot of the database from when we started streaming.
>>>>>>>
>>>>>>> [2] Our streaming APIs have both skips and limits. Since FDB
doesn't
>>>>>>> currently support efficient offsets for key selectors
>>>>>>> (
>>>> https://apple.github.io/foundationdb/known-limitations.html#dont-use-key-selectors-for-paging
>>>> )
>>>>>>> we implemented skip by iterating over the data. This means that
a skip
>>>>>>> of say 100000 could keep timing out the transaction without yielding
>>>>>>> any data.
>>>>>>>
>>>>>>> [3] _all_dbs and _dbs_info return a JSON array so they don't
have an
>>>>>>> obvious place to insert a last metadata row.
>>>>>>>
>>>>>>> [4] For example they have a constraint that documents "a" and
"z"
>>>>>>> cannot both be in the database at the same time. But when iterating
>>>>>>> it's possible that "a" was there at the start. Then by the end,
"a"
>>>>>>> was removed and "z" added, so both "a" and "z" would appear in
the
>>>>>>> emitted stream. Note that FoundationDB has APIs which exhibit
the same
>>>>>>> "relaxed" constrains:
>>>>>>>
>>>> https://apple.github.io/foundationdb/api-python.html#module-fdb.locality
>>>>>>>
>>>>>
>>>>>
>>>>
>>>
> 

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