Preference

preference 参数用于控制搜索操作应该在哪些分片副本上执行。默认情况下，Scope 会根据一系列规则（如分配感知和自适应副本选择配置）来选择分片副本，但有时候可能需要对搜索进行更精细的控制，这时就可以使用 preference 参数。

例如，使用 _shards:2,3 可以限制搜索操作只在指定的分片 2 和 3 上执行。

GET <host>:<port>/<index_name>*/_search?pretty&preference=_shards:2,3

curl -X GET "localhost:9200/preference_index/_search?pretty&preference=_shards:2,3" -H 'Content-Type: application/json' -d'
{
    "query": {
        "term": {
            "hobby": { "value": "cloud" }
        }
    }
}'

{
  "took" : 239,
  "timed_out" : false,
  "terminated_early" : false,
  "_shards" : {
    "total" : 2,
    "successful" : 2,
    "skipped" : 0,
    "failed" : 0
  },
  "_clusters" : {
    "total" : 2,
    "successful" : 2,
    "skipped" : 0
  },
  "hits" : {
    "total" : 2,
    "max_score" : 0.48436219,
    "hits" : [
      {
        "_index" : "preference_index",
        "_type" : "default_type_",
        "_id" : "1",
        "_score" : 0.48436219,
        "_source" : {
          "name" : "Emma Edgar",
          "age" : 32,
          "hobby" : "Look through the moon to see cloud and snow",
          "job" : {
            "name" : "Product Manager",
            "salary" : 35000,
            "work desc" : "designing product about apples and pinapples"
          }
        }
      },
      {
        "_index" : "preference_index",
        "_type" : "default_type_",
        "_id" : "4",
        "_score" : 0.4132582,
        "_source" : {
          "name" : "Rivera Interpreter",
          "age" : 26,
          "hobby" : "The cloak of golden armour and dark silk has broken tens of thousands of clouds",
          "job" : {
            "name" : "JavaScript Programmer",
            "salary" : 22000
          }
        }
      }
    ]
  }
}