DynamoDB: Fix parallelism #876

[gregbeech]

This PR fixes #876 by changing the parallelism in DynamoDB so that it is applied to the HTTP connection pool.

I upped the default parallelism to 32 as this is the nearest power of 2 below 50, which is the default that AWS used in their own SDK before deprecating the methods. I've run up to 128 connections in production (by setting the akka-http config properties) without any problems so 32 seems perfectly safe.

[ennru]

Makes sense to me.
Where does the need for "power of two" come from?

[gregbeech]

The power of two is from the akka-http requirement for max open requests:

    # Must be a power of 2 and > 0!
    max-open-requests = 32

It's an intriguing requirement but I haven't dug into the code enough to see why it's implemented that way.

[gregbeech]

One thing that occurs to me is that with this change parallelism is still used for mapAsync and it's likely way higher than needed.

For example, we use 128 connections from our production servers but don't need anything like that parallelism for mapAsync as it's so much faster (we still use the default of 10 for that). Having another setting for that value seems messy though, as that's really an implementation detail that the user of the library shouldn't have to know/care about.

Any ideas? Perhaps leaving them the same is fine as it'll never get that high, or we could compute a value for it that seems more reasonable such as parallelism/8.

[ennru]

I think its OK to use the same high value for the mapAsync instead of complicating the settings or the explanations thereof.

[ennru]

LGTM

[ennru]

Thank you for this improvement. Good to see you funnelling quite some data through it!