Shard awareness #2
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This required supplying std::optional from a 3rd party implementation (by Andrzej Krzemienski). The selection of `optional` is done on the basis of C++ standard version, e.g.: cmake -DCASS_CPP_STANDARD=17 .. will use std::optional, while `-DCASS_CPP_STANDARD=11` will fall back to the 3rd party impl.
At this point metrics indicate that cross-shard ops are reduced, but the implementation is still raw. The reasons: When connection-to-the-right-shard is being searched among per-host connections, it is done by linear search. Connecting logic does not attempt to reconnect until all shards are hit. Topology change events are not accounted for.
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The reconnection is being scheduled until we reach the desired number of connections per shard. The other change is that instead of picking the 'least busy' connection from the host's pool, we pick the least busy one from the "shards pool".
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Design is based on shard-awareness in java driver.