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 <iil...@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 <gar...@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 <iil...@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 <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
