Add minSize to pool to maintain a minimum # of connections

[kdubb]

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Motivation:

Adds a minSize parameter to pool options to ensure a minimum # of connection is always connected and available.

Conformance:

You should have signed the Eclipse Contributor Agreement as explained in https://github.com/eclipse/vert.x/blob/master/CONTRIBUTING.md Please also make sure you adhere to the code style guidelines: https://github.com/vert-x3/wiki/wiki/Vert.x-code-style-guidelines

[chandramouli-r]

Hi @tsegismont - this change looks good to me. Do you think someone more familiar with this code can approve it? Thank you.

[kdubb]

@tsegismont is out until May 2nd

@vietj you have any thoughts on this one?

[chandramouli-r]

Hi, @vietj - any thoughts on this PR? I think it looks ready to merge. Thanks!

[chandramouli-r]

Hi @tsegismont - any further thoughts on this PR?

[vietj]

I am wondering if that should not be a feature that is directly in the pool instead of being specific to this implementation

[tmonney]

Is this feature still considered for inclusion ? We would also need it in our applications, since we often experiment steep peaks where the concurrent establishment of many new DB connections can become a bottleneck. So far we have to resort to a scheduled task that periodically checks for the pool size and opens connections so that it reaches the min size. This is of course best-effort only and by no means perfect, but it still helps keeping some connections open.

[vietj]

@tmonney yes it is considered, I haven't looked back at this PR but according to my comments, I had doubts about the actual PR