An Elixir struct to specify when to split/merge shards in a keyspace.

:raft_kv uses the following 3 stats to judge whether splitting/merging should be done:

1. number of keys in a shard
2. aggregated data size (in an arbitrary unit) in a shard
3. current load (computing resource required, in an arbitrary unit) of a shard

Fields:

• :max_shards Maximum number of shards. If number of shards is the same as this value, no further split occurs.
• :min_shards Minimum number of shards. If number of shards is the same as this value, no further merge occurs.
• :max_keys_per_shard Threshold number of keys for shard split. Shards that contains more keys than this value become candidates for split. If nil, shards are not split due to number of keys. Defaults to nil.
• :max_size_per_shard Threshold size for shard split. Shards whose aggregated size exceeds this value become candidates for split. If nil, shards are not split due to size. Defaults to nil.
• :max_load_per_shard Threshold load for shard split. Shards that have been experiencing load above this value become candidates for split. If nil, shards are not split due to load. Defaults to nil.
• :load_per_query_to_missing_key RaftKV.query/4 returns {:error, :key_not_found} if target key does not exist. This value is used as the "load" for such queries. Defaults to 0.
• :merge_threshold_ratio: For each of the 3 types of split thresholds above (:max_keys_per_shard, :max_size_per_shard and :max_load_per_shard), merge thresholds are calculated by multiplying this ratio. Consecutive 2 shards become candidates for merge if they together (1) contain less keys than the threshold, (2) contain smaller size than the threshold, and (3) experience less load than the threshold. Defaults to 0.5.