Hi Nick, Thank you for extensive answer.
> API versioning idea in principle sounds good, but can't think of a > clean way to do it. /_v2/_all_dbs pattern might work, See separate discussion here https://lists.apache.org/thread.html/rcc742c0fdca0363bb338b54526045720868597ea35ee6842aef174e0%40%3Cdev.couchdb.apache.org%3E Best regards, iilyak On 2020/04/27 15:54:11, Nick Vatamaniuc <vatam...@gmail.com> wrote: > It's good to see more activity in the thread. I thought everyone had > lost interest :-) Nice work, Ilya, on the prototype. I think you > picked what I had initially called option D and E. With the exception > that we don't force clients to specify a limit when a max limit is > configured in the settings. > > API versioning idea in principle sounds good, but can't think of a > clean way to do it. /_v2/_all_dbs pattern might work, but what would > it look like for views, perhaps /{db}/_design/{ddoc}/_v2/_view/{view} > is not that bad? The idea, I guess, would be to allow users the option > to use the old APIs to migrate their application to 4.x without having > to worry about rewriting everything in addition to having to configure > and maintain new FDB backend. > > Also if we go the API versioning route, I like the uniformity of using > {"items":[...]} in the response instead of {"rows": []}, or > {"docs":[]}. Also +1 on adding _changes in there as well. > > On the other hand, if we wanted to keep things as compatible as > possible, we could have a new parameter like `&use_cursor=true` for > view-like endpoints (excluding _all_dbs, etc), and, if that is > specified the user should expect the response to not return all the > rows sometimes and then look for a "cursor"/"bookmark" field in the > response. We could have an option to enforce `&use_cursor=true` usage > on some API points, kind of in the same vein as using a max limit > (option E). The point would be that user would explicitly acknowledge > that they are expecting this behavior. Otherwise I don't think it is a > good idea to keep the shape of the response the same, and only emit an > extra bookmark/cursor and then skip some of the rows from the results. > > Regarding not using delayed response and having configurable limits. I > think we'd have to see how those interact with each other. Can users > still set set _all_docs limit = infinity and get the current behavior? > They can currently (even on the prototype/fdb-layer branch). Sometimes > it is useful to stream all the data at once, say when doing a backup, > or other maintenance. Using a proper client with a cursor helper, > which hides the iteration loop, or a custom shell script would work, > but it might be nice to retain the option to be able to do it straight > from the API. > > There was some discussion whether we should include more or less > fields in the bookmark, timestamp might have issues with time skew, > but we could include the instance id (uuid) of the database. We do > that with the shard uuids currently in the update sequence. We don't > have to use the whole uuid but maybe just the 5-6 bytes to ensure that > if database is recreated and users are "streaming" data from it they > don't all of the sudden end up from a completely different db > instance. Including the update_seq is also interesting, but I think > the discussion so points to not do it in the first version, though it > would allow a user to know if, say, they backed up consistent point in > time snapshot of the database using a sequence of cursor requests. > > I am also +0 on returning "completed: true", I kind of like the idea. > Maybe we could just return a null cursor then without using yet > another field. "If you have a cursor you can iterate until the end, if > you don't you're done" kind of idea? > > Cheers, > -Nick > > > On Wed, Apr 22, 2020 at 4:19 PM 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 > > > >
