reroute命令报错

我执行下面的命令报错：
POST /_cluster/reroute
{
  "commands" : [ {
    "allocate" : {
    "index" : "kid_book_v3",
    "shard" : 0,
    "node" : "mDHDv4jaRpmD515eIqnJ8g",
    "allow_primary" : true
    }
  }]
}
 
报错内容：
{
  "error": {
    "root_cause": [
      {
        "type": "unknown_named_object_exception",
        "reason": "Unknown AllocationCommand [allocate]",
        "line": 3,
        "col": 18
      }
    ],
    "type": "parsing_exception",
    "reason": "[cluster_reroute] failed to parse field [commands]",
    "line": 3,
    "col": 18,
    "caused_by": {
      "type": "unknown_named_object_exception",
      "reason": "Unknown AllocationCommand [allocate]",
      "line": 3,
      "col": 18
    }
  },
  "status": 400
}