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
