Elasticsearch is built to be always available, and to scale with your needs. Scale can come from buying bigger servers (vertical scale, or scaling up ) or from buing more servers (horizontal scale, or scaling out).
    While Elasticsearch can benefit from more powerful hardware, vertical scale has its limits. Real scalability comes from horizontal scale — the ability to add more nodes to the cluster and to spread load and reliability between them.
    With most databases, scaling horizontally usually requires a major overhaul of your application to take advantage of these extra nodes. In contrast, elasticsearch is distributed by nature: it knows how to manage mutiple nodes to provide scal and high availability. This alse means your application doesn’t need to care about it.
    In this chapter, we show how to set up your cluster , nodes and shards to scale with you needs and to ensure that your data is safe from hardware failure.

    扩容方式 => [
    垂直扩容 => [ 购买更强性能的服务器， 局限] ，
    水平扩容 => [ 购买更多的服务器，组成更多节点的服务器集群，可执行性高]
    ];

    数据库 => [
    大多数传统数据库 => 实现集群和分布式需要对应用程序进行大幅度更改，
    elasticsearch => 天生就是分布式的
    ];