Add maxOpenRequests param in DynamoConfig

[williamho]

Pull Request Checklist

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Fixes

Fixes #1542

Purpose

Allows the maxOpenRequests of the underlying host connection pool to
be configured separately from the maxConnections value (as specified
by the parallelism value in the config).

Background Context

To preserve old behavior, I made it an optional field, where if it's
absent, then it will use the parallelism value as before. However the
one thing I'm not sure of is whether I should have the Option exposed
in the public interface (since this is meant to be used in Java as
well). Glad to accept any suggestions on how to handle optional values
like this.

References

#1542 - Can't specify maxOpenRequests in DynamoClient

[2m]

Thanks for the PR. It is looking very good. Regarding exposing Option in the public API - it is fine. It serves as a getter for Scala users. However you need to add a getter for Java users called getGaxOpenRequests which will return an Optional.

[williamho]

Thanks, that makes sense.

Now that I'm thinking about it though, while maxOpenRequests works for me, I can imagine that in the future, someone else will want to be able to set other fields on the underlying ConnectionPoolSettings.

Rather than adding just one configurable setting, would it be better to just accept the whole ConnectionPoolSettings as an optional param? This way it has maximum flexibility. Not sure if that would leak too much of the implementation details to the public API though.

edit: disregard that, I forgot those other params can still be set with the usual config

[williamho]

Actually, disregard my last comment. I forgot that the other connection pool settings can still be set with the usual akka.http.host-connection-pool config.

I've updated the PR to allow getting/setting with java Optionals, as well as using hyphens in the config for consistency with other config fields

[2m]

LGTM

[2m]

Awesome stuff. Thanks a lot!