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From Ilya Khlopotov <iil...@apache.org>
Subject Re: [DISCUSS] Streaming API in CouchDB 4.0
Date Wed, 22 Apr 2020 20:18:57 GMT
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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