Describe how load balancing differs from the legacy CPP Driver's

[wprzytula]

Load balancing in CPP-RS Driver is implemented on top of Rust Driver's DefaultPolicy. This results in a few differences in behaviour that are worth mentions in cassandra.h and in the documentation.

Differences

• There's no round-robin in DefaultPolicy - instead, replicas are randomly chosen.

  This is deliberate, as it prevents imbalance inherent to round-robin. If some node were down, then round-robin would make the next node in the ring/tablet list receive both its own load and all load of the down node. Random selection of replica from the set of alive replicas solves the imbalance issue.

  Symptoms:

  • cass_cluster_set_load_balance_round_robin does not set round-robin load balancing.

• more difference TODO