query from Mongo with FS2

The mongo driver exposes query cursors as reactive publisher.

When using FS2 interoperability with reactive streams, I could observe that we query elements one by one, leading to a lot of DB queries, leading to bad performances.

with FS2:

Mongo log:

command":{"getMore":3736877399621840227,"collection":"test","batchSize":2

with akka-streams:

Mongo log:

"command":{"getMore":5259084578442247860,"collection":"test","batchSize":8

Try it yourself

start mongo

docker-compose up -d

Inject some data

in sbt:

runMain mongo_fs2.Populate

Read data

in sbt:

run

and select the main class to run

Fix

After several discussions, we can up with a way to demand several elements at the same time: typelevel/fs2#2666