为什么Elasticsearch允许我在无法满足仲裁要求的单节点集群中索引文档?

从…起 https://www.elastic.co/guide/en/elasticsearch/guide/2.x/distrib-write.html :

请注意,number\u of\u replicas是在索引设置中指定的副本数,而不是当前活动的副本数。如果指定索引应包含三个副本,则仲裁如下:

int((主副本+3个副本)/2)+1=3

但如果只启动两个节点,则活动碎片副本将不足,无法满足仲裁要求,并且无法索引或删除任何文档。

我在单节点集群上运行了以下命令,并且我能够成功地为文档编制索引,尽管上面的数学表示我不应该能够为文档编制索引。

curl -X DELETE http://localhost:9200/a/?pretty
curl -X PUT -siH 'Content-Type: application/json' \
     http://localhost:9200/a?pretty -d '{

    "settings": {
        "number_of_replicas": 3
    }
}'
curl -sH 'Content-Type: application/json' -X PUT http://localhost:9200/a/a/1?pretty -d '{"a": "a"}'
curl -si http://localhost:9200/_cluster/health?pretty
curl -si http://localhost:9200/a/a/1?pretty

以下是输出:

$ curl -X PUT -siH 'Content-Type: application/json' \
     http://localhost:9200/a?pretty -d '{

    "settings": {
        "number_of_replicas": 3
    }
}'
HTTP/1.1 200 OK
content-type: application/json; charset=UTF-8
content-length: 77

{
  "acknowledged" : true,
  "shards_acknowledged" : true,
  "index" : "a"
}

$ curl -sH 'Content-Type: application/json' -X PUT http://localhost:9200/a/a/1?pretty -d '{"a": "a"}'
{
  "_index" : "a",
  "_type" : "a",
  "_id" : "1",
  "_version" : 1,
  "result" : "created",
  "_shards" : {
    "total" : 4,
    "successful" : 1,
    "failed" : 0
  },
  "_seq_no" : 0,
  "_primary_term" : 1
}

$ curl -si http://localhost:9200/_cluster/health?pretty
HTTP/1.1 200 OK
content-type: application/json; charset=UTF-8
content-length: 468

{
  "cluster_name" : "docker-cluster",
  "status" : "yellow",
  "timed_out" : false,
  "number_of_nodes" : 1,
  "number_of_data_nodes" : 1,
  "active_primary_shards" : 5,
  "active_shards" : 5,
  "relocating_shards" : 0,
  "initializing_shards" : 0,
  "unassigned_shards" : 15,
  "delayed_unassigned_shards" : 0,
  "number_of_pending_tasks" : 0,
  "number_of_in_flight_fetch" : 0,
  "task_max_waiting_in_queue_millis" : 0,
  "active_shards_percent_as_number" : 25.0
}

$ curl -si http://localhost:9200/a/a/1?pretty
HTTP/1.1 200 OK
content-type: application/json; charset=UTF-8
content-length: 124

{
  "_index" : "a",
  "_type" : "a",
  "_id" : "1",
  "_version" : 1,
  "found" : true,
  "_source" : {
    "a" : "a"
  }
}

即使我已经将索引配置为具有3个副本碎片,并且数学上说我必须至少具有3个节点,但我如何能够仅使用1个节点为文档编制索引?