- 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 <rnew...@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 <iil...@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 <vatam...@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
> >>
>
>
