elasticsearch版本升级方案
常用的滚动升级过程（Rolling Upgrade）如下：
$ curl -XPUT '$es_server:9200/_cluster/settings?pretty' -H 'Content-Type: application/json' -d '{"transient":{"cluster.routing.allocation.enable":"none"}}'
$ curl -XPOST '$es_server/_flush/synced?pretty'
$ systemctl stop elasticsearch.service# upgrade elasticsearch
$ systemctl start elasticsearch.service
$ curl -XGET '$es_server:9200/_cat/nodes?pretty'
$ curl -XPUT '$es_server:9200/_cluster/settings?pretty' -H 'Content-Type: application/json' -d '{"transient":{"cluster.routing.allocation.enable":"all"}}'
$ curl -XGET '$es_server:9200/_cat/health?pretty'
以上为单个节点升级过程，逐个节点操作即可
参考：
https://www.elastic.co/guide/en/elasticsearch/reference/current/setup-upgrade.html
https://www.elastic.co/guide/en/elasticsearch/reference/current/rolling-upgrades.html