Handling multiple mapping type index templates

[dreamer-89]

Index templates are preserved over rolling upgrades, so it is possible to multiple type templates in the cluster state. Post removal of mapping types from 2.0; nodes running OpenSearch 2.0 or higher should refuse to join a cluster containing multiple type templates.

[dreamer-89]

As per discussion with @nknize, we should have tests in place to capture upgrade behavior with multiple type index templates.

The current bwc testing framework limits the previous ES/OS versions against which bwc tests can be run (earliest allowed is 7.10.0 which doesn't support multiple type mappings). So, probably need to start with 5.x version and perform series of upgrades to 2.0.0.

[dreamer-89]

Discussed it offline with @nknize , who advised to perform manual upgrades and verify if multi-type index templates survive rolling upgrades to 7.x / OS1.x cluster.

Verified that mapping types is removed silently during 6.x -> 7.x upgrade. The template remain same other than type removal under mappings and only one (user in example below) is kept in the template. When an index (test-index) matching template regex is created; it has settings correctly applied from the template. This behavior also applies when cluster upgraded to OS 1.3.0

5.0

curl -X PUT "localhost:9200/_template/template_multi_type?pretty" -H 'Content-Type: application/json' -d'
{
  "template": "te*",
  "settings": {
    "number_of_shards": 1
  },
  "mappings": {
    "tweet": {
      "properties": {
        "message": {
          "type": "keyword"
        }
       }
     },
     "user": {
      "properties": {
        "name": {
          "type": "keyword"
        }
       }
     }
    }
  }
}
'

➜  elasticsearch-5.0.2 curl -X GET "localhost:9200/_template/template_multi_type?pretty"
{
  "template_multi_type" : {
    "order" : 0,
    "template" : "te*",
    "settings" : {
      "index" : {
        "number_of_shards" : "1"
      }
    },
    "mappings" : {
      "user" : {
        "properties" : {
          "name" : {
            "type" : "keyword"
          }
        }
      },
      "tweet" : {
        "properties" : {
          "message" : {
            "type" : "keyword"
          }
        }
      }
    },
    "aliases" : { }
  }
}

7.10.2/7.17

# 7.10/7.17 removes any mapping type info from the index template
➜  ~ curl -X GET "localhost:9200/_template/template_multi_type?pretty"
{
  "template_multi_type" : {
    "order" : 0,
    "index_patterns" : [
      "te*"
    ],
    "settings" : {
      "index" : {
        "number_of_shards" : "1"
      }
    },
    "mappings" : {
      "properties" : {
        "name" : {
          "type" : "keyword"
        }
      }
    },
    "aliases" : { }
  }
}

[dreamer-89]

From manual steps above, it is verified that multi-type templates doesn't persist beyond 6.x. So, it is not possible to have multi-type templates in 7.x+ clusters. Hence, there is no need of handling multi-type templates in OS 2.0