1,Document 被存到哪个shard上？
shard = hash(routing) % number_of_primary_shards
2 Routing a Document to a Shard
3 法定人数 quorum
int( (primary + number_of_replicas) / 2 ) + 1
如果你设置了3副本，而只启动了2个Node， 默认情况下，你将无法 增减document。
4，
A new index has 1 replica by default, which means that two active shard copies should be required in order to satisfy the need for a quorum. However, these default settings would prevent us from doing anything useful with a single-node cluster. To avoid this problem, the requirement for a quorum is enforced only when number_of_replicas is greater than 1.
只有number_of_replicas 大于1 ，才启用投票机制。
5，
