GAX-JAVA: Allow users to customize ChannelPoolSettings RESIZE_INTERVAL and MAX_RESIZE_DELTA

[lkleeven]

We are creating our own in company Google Pub/Sub client. To improve performance we've recently switched to using the InstantiatingGrpcChannelProvider. This already gave a performance improvement, however to be able to deal with big spikes of load, we'd like to be able to also set the resize interval and max resize delta, currently present as package private static fields in ChannelPoolSettings. This would allow us to set clients up to respond faster to big spikes in load.

We've considered keeping a higher minimum channel count open, but that feels rather wasteful, especially if the spikes are few and far between.

We are willing to contribute if that helps to speed things up. And are also open to suggestions on how to otherwise deal with these load spikes from a client perspective.

[blakeli0]

Hi @lkleeven, thanks for reporting this! I think your request is reasonable, but we may not expose these two settings at this moment. Because

1. Channel resizing is expensive and could be error prone if misconfigured. The settings you mentioned were not exposed on purpose because we want the channel resizing process to have minimum impact on the normal operations, as vast of majority of clients are good with just 1 channel. If RESIZE_INTERVAL is exposed and it is set to something like 5 seconds, it may actually hurt the performance.
2. Channel pool was designed for moderate spikes(~2x), not big spikes(10x). If there is a big spike, I would first consider optimizing in other ways like adding more machines. If you are stuck with limited machines and the big spike is predictable, one possible way you can do in code is to create another client with a different number of channels.

That being said, if you still think this feature is of great value, please create an issue in the support console instead of filing on GitHub. This will ensure a timely response and proper prioritization.