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From Adam Kocoloski <kocol...@apache.org>
Subject Re: [DISCUSS] Streaming API in CouchDB 4.0
Date Tue, 28 Apr 2020 17:22:41 GMT
Hi Ilya, 

Initial reaction — there’s a lot to like here. It seems like a pragmatic step forward
for the current API that handles the corner case of large responses while maintaining compatibility
for the large majority of API requests that don’t exceed this limit.

I think the `limit` parameter might be overloaded, though. If we add this pagination capability
then `limit` could be used to refer to the size of a “page” or to the size of the overall
result set. In the current proposal there’s no easy way to say “give me 5,000 results
in blocks of 100”. Maybe it makes sense to add `page_size` as a new parameter and keep `limit`
as the total number of results returned (i.e., once the `limit` is reached the final response
will not have a “next” bookmark).

If we do introduce a `page_size` I would not impose a maximum on `limit`. It should be possible
to use bookmarks to page through an entire database.

To your other questions:

> - `bookmark` vs `token`?

Prefer `bookmark` since that’s already used for _find and _search

> - should we prohibit setting other fields when bookmark is set?

As a general rule, yes. I can see an exception around allowing users to say whether they want
to retrieve the next page of results using the same FDB read version as the previous request,
and how the server should proceed if that read version is now too old. But that’s for later.

> - `previous`/`next`/`first` as href vs token value itself (i.e. `{"previous": "983uiwfjkdsdf",
"next": "12343tyekf3", "first": "iekjhfwo034"}`)

I think I would avoid including the scheme and server. I could see including the full path
instead of just the bookmark, particularly as you’re including the “first” bit (which
is a nice touch).

A couple of other comments:

> Latter on we would introduce API versioning and deal with `{db}/_changes` and `_all_docs`
endpoints. 

I think you mean `_all_dbs` instead of `_all_docs` here, and I agree that’s fine to defer.

> - don't use delayed responses when `bookmark` field is provided
> - don't use delayed responses when `limit` query key is specified and when it is below
the max limit
> - return 400 when limit query key is specified and it is greater than the max limit
> - return 400 when we stream rows (in case when `limit` query key wasn't specified) and
reach max limit

If we add the `page_size` parameter does this become

- don't use delayed responses when `bookmark` field is provided
- don't use delayed responses when `page_size` query key is specified and when it is below
the max
- return 400 when `page_size` query key is specified and it is greater than the max

I feel like the discussion of using delayed response or not is just a performance optimization,
assuming we’re able to identify the right status code before streaming. So it could be omitted.

Choosing a default `page_size` requires some thought. I think we want to have a default, even
though it is a behavior change. At the same time maybe we can minimize the number of times
where pagination needs to show up with something like the following:

- user supplies `limit` less than max page size -> set `page_size` = `limit`
- user omits `limit`or sets one that exceeds max page size -> set `page_size`= ~100

Regards, Adam

> On Apr 28, 2020, at 7:56 AM, Ilya Khlopotov <iilyak@apache.org> wrote:
> 
> Hello, 
> 
> I would like to introduce second proposal.
> 
> 1) Add new optional query field called `bookmark` (or `token`) to following endpoints
>  - {db}/_all_docs
>  - {db}/_all_docs/queries
>  - _dbs_info
>  - {db}/_design/{ddoc}/_view/{view}
>  - {db}/_design/{ddoc}/_view/{view}/queries
> 2) Add following additional fields into response:
>   ```
>    "first": {
>        "href": "https://myserver.com/myddb/_all_docs?limit=50&descending=true"
>    },
>    "previous": {
>         "href": "https://myserver.com/myddb/_all_docs?bookmark=983uiwfjkdsdf"
>    },
>    "next": {
>        "href": "https://myserver.com/myddb/_all_docs?bookmark=12343tyekf3"
>     },
>     ```
> 3) Implement per-endpoint configurable max limits 
>   ```
>   [request_limits]
>  _all_docs = 5000
>  _all_docs/queries = 5000
>  _all_dbs = 5000
>  _dbs_info = 5000
>  _view = 2500
>  _view/queries = 2500
>  _find = 2500
>  ```
> 4) Implement following semantics:
>   - The bookmark would be opaque token and would include information needed to ensure
proper pagination without the need to repeat initial parameters of the request. In fact we
might prohibit setting additional parameters when bookmark query field is specified.
>   - don't use delayed responses when `bookmark` field is provided
>   - don't use delayed responses when `limit` query key is specified and when it is below
the max limit
>   - return 400 when limit query key is specified and it is greater than the max limit
>   - return 400 when we stream rows (in case when `limit` query key wasn't specified)
and reach max limit
>   - the `previous`/`next`/`first` keys are optional and we omit them for the cases they
don't make sense
> 
> Latter on we would introduce API versioning and deal with `{db}/_changes` and `_all_docs`
endpoints. 
> 
> Questions:
> - `bookmark` vs `token`?
> - should we prohibit setting other fields when bookmark is set?
> - `previous`/`next`/`first` as href vs token value itself (i.e. `{"previous": "983uiwfjkdsdf",
"next": "12343tyekf3", "first": "iekjhfwo034"}`)
> 
> Best regards,
> iilyak
> 
> On 2020/04/22 20:18:57, 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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