6 Search interface -- 12 Explain interface

Explain the score calculation process for the search. Such as:

GET /twitter/tweet/0/_explain
{
      "query" : {
        "match" : { "message" : "elasticsearch" }
      }
}

return

{
   "_index": "twitter",
   "_type": "tweet",
   "_id": "0",
   "matched": true,
   "explanation": {
      "value": 1.55077,
      "description": "sum of:",
      "details": [
         {
            "value": 1.55077,
            "description": "weight(message:elasticsearch in 0) [PerFieldSimilarity], result of:",
            "details": [
               {
                  "value": 1.55077,
                  "description": "score(doc=0,freq=1.0 = termFreq=1.0\n), product of:",
                  "details": [
                     {
                        "value": 1.3862944,
                        "description": "idf, computed as log(1 + (docCount - docFreq + 0.5) / (docFreq + 0.5)) from:",
                        "details": [
                           {
                              "value": 1.0,
                              "description": "docFreq",
                              "details": []
                           },
                           {
                              "value": 5.0,
                              "description": "docCount",
                              "details": []
                           }
                        ]
                     },
                     {
                        "value": 1.1186441,
                        "description": "tfNorm, computed as (freq * (k1 + 1)) / (freq + k1 * (1 - b + b * fieldLength / avgFieldLength)) from:",
                        "details": [
                           {
                              "value": 1.0,
                              "description": "termFreq=1.0",
                              "details": []
                           },
                           {
                              "value": 1.2,
                              "description": "parameter k1",
                              "details": []
                           },
                           {
                              "value": 0.75,
                              "description": "parameter b",
                              "details": []
                           },
                           {
                              "value": 5.4,
                              "description": "avgFieldLength",
                              "details": []
                           },
                           {
                              "value": 4.0,
                              "description": "fieldLength",
                              "details": []
                           }
                        ]
                     }
                  ]
               }
            ]
         },
         {
            "value": 0.0,
            "description": "match on required clause, product of:",
            "details": [
               {
                  "value": 0.0,
                  "description": "# clause",
                  "details": []
               },
               {
                  "value": 1.0,
                  "description": "*:*, product of:",
                  "details": [
                     {
                        "value": 1.0,
                        "description": "boost",
                        "details": []
                     },
                     {
                        "value": 1.0,
                        "description": "queryNorm",
                        "details": []
                     }
                  ]
               }
            ]
         }
      ]
   }
}