search（3）- elastic4s-QueryDSL

  elastic4s是elasticsearch一个第三方开发的scala语言终端工具库（Elastic4s is a concise, idiomatic, reactive, type safe Scala client for Elasticsearch.）。scala用户可以用elastic4s提供的DSL用编程代码形式来构建ES服务请求。与字符型json文本直接编写请求不同的是：在编译DSL编写的ES服务请求时可以发现无论是语法上或者语意上的错误。一般来讲：elastic4s的程序流程相对直接、简单，如下：

  client.execute {
    indexInto("books" ).fields("title" -> "重庆火锅的十种吃法", "content" -> "在这部书里描述了火锅的各种烹饪方式")
  }.await

  val response = client.execute {
    search("books").matchQuery("title", "火锅")
  }.await

...

...

一项ES操作服务从构建请求到具体运行都是在execute(T)这个函数里进行的。值得注意的是这个T类型在上面的例子里可以是IndexRequest或者SearchRequest，如下：

   def indexInto(index: Index): IndexRequest
...
   def search(index: String): SearchRequest

实际上execute(T)的T代表elastic4s支持的所有ES操作类型。这种方法实现有赖于scala的typeclass模式。我们先看看execute函数定义：

  // Executes the given request type T, and returns an effect of Response[U]
  // where U is particular to the request type.
  // For example a search request will return a Response[SearchResponse].
  def execute[T, U, F[_]](t: T)(implicit
                                executor: Executor[F],
                                functor: Functor[F],
                                handler: Handler[T, U],
                                manifest: Manifest[U]): F[Response[U]] = {
    val request = handler.build(t)
    val f = executor.exec(client, request)
    functor.map(f) { resp =>
      handler.responseHandler.handle(resp) match {
        case Right(u) => RequestSuccess(resp.statusCode, resp.entity.map(_.content), resp.headers, u)
        case Left(error) => RequestFailure(resp.statusCode, resp.entity.map(_.content), resp.headers, error)
      }
    }
  }

这个函数比较重要的功能之一应该是构建服务请求了。这个功能是通过handler.build(t)实现的。handler: Handler[T,U]是个隐式参数，它就是一个typeclass： 

/**
  * A [[Handler]] is a typeclass used by the [[ElasticClient]] in order to
  * create [[ElasticRequest]] instances which are sent to the elasticsearch
  * server, as well as returning a [[ResponseHandler]] which handles the
  * response from the server.
  *
  * @tparam T the type of the request object handled by this handler
  * @tparam U the type of the response object returned by this handler
  */
abstract class Handler[T, U: Manifest] extends Logging {
  def responseHandler: ResponseHandler[U] = ResponseHandler.default[U]
  def build(t: T): ElasticRequest
}

这个抽象类中有两个函数，其中一个就是build(t: T)：ElasticRequest，也是个抽象方法，必须在构建实例时实现。在execute(T)中handler是一个隐式参数，也就是说如果在调用这个函数的可视域内能发现Handler[T,U]实例，则可获取handler，然后可调用handler.build(t)来构建请求。这个T类型是即是调用execute(T)这个T类型了，上面说过T可以是ES的任何操作类型，也就是说如果这些操作类型都继承了Handler[T,U]，那么必须按照要求实现build(t:T)来构建该操作类型所需的服务请求ElasticRequest。下面就是例子里两个操作类型需要的隐式实例：

 implicit object IndexHandler extends Handler[IndexRequest, IndexResponse] {

    override def responseHandler: ResponseHandler[IndexResponse] = new ResponseHandler[IndexResponse] {
      override def handle(response: HttpResponse): Either[ElasticError, IndexResponse] = response.statusCode match {
        case 201 | 200                   => Right(ResponseHandler.fromResponse[IndexResponse](response))
        case 400 | 401 | 403 | 409 | 500 => Left(ElasticError.parse(response))
        case _                           => sys.error(response.toString)
      }
    }

    override def build(request: IndexRequest): ElasticRequest = {

      val (method, endpoint) = request.id match {
        case Some(id) =>
          "PUT" -> s"/${URLEncoder.encode(request.index.name, StandardCharsets.UTF_8.name())}/_doc/${URLEncoder.encode(id.toString, StandardCharsets.UTF_8.name())}"
        case None =>
          "POST" -> s"/${URLEncoder.encode(request.index.name, StandardCharsets.UTF_8.name())}/_doc"
      }

      val params = scala.collection.mutable.Map.empty[String, String]
      request.createOnly.foreach(
        createOnly =>
          if (createOnly)
            params.put("op_type", "create")
      )
      request.routing.foreach(params.put("routing", _))
      request.parent.foreach(params.put("parent", _))
      request.timeout.foreach(params.put("timeout", _))
      request.pipeline.foreach(params.put("pipeline", _))
      request.refresh.map(RefreshPolicyHttpValue.apply).foreach(params.put("refresh", _))
      request.version.map(_.toString).foreach(params.put("version", _))
      request.ifPrimaryTerm.map(_.toString).foreach(params.put("if_primary_term", _))
      request.ifSeqNo.map(_.toString).foreach(params.put("if_seq_no", _))
      request.versionType.map(VersionTypeHttpString.apply).foreach(params.put("version_type", _))

      val body   = IndexContentBuilder(request)
      val entity = ByteArrayEntity(body.getBytes, Some("application/json"))

      logger.debug(s"Endpoint=$endpoint")
      ElasticRequest(method, endpoint, params.toMap, entity)
    }
  }


...

  implicit object SearchHandler extends Handler[SearchRequest, SearchResponse] {

    override def build(request: SearchRequest): ElasticRequest = {

      val endpoint =
        if (request.indexes.values.isEmpty)
          "/_all/_search"
        else
          "/" + request.indexes.values
            .map(URLEncoder.encode(_, "UTF-8"))
            .mkString(",") + "/_search"

      val params = scala.collection.mutable.Map.empty[String, String]
      request.requestCache.map(_.toString).foreach(params.put("request_cache", _))
      request.searchType
        .filter(_ != SearchType.DEFAULT)
        .map(SearchTypeHttpParameters.convert)
        .foreach(params.put("search_type", _))
      request.routing.map(_.toString).foreach(params.put("routing", _))
      request.pref.foreach(params.put("preference", _))
      request.keepAlive.foreach(params.put("scroll", _))
      request.allowPartialSearchResults.map(_.toString).foreach(params.put("allow_partial_search_results", _))
      request.batchedReduceSize.map(_.toString).foreach(params.put("batched_reduce_size", _))

      request.indicesOptions.foreach { opts =>
        IndicesOptionsParams(opts).foreach { case (key, value) => params.put(key, value) }
      }

      request.typedKeys.map(_.toString).foreach(params.put("typed_keys", _))

      val body = request.source.getOrElse(SearchBodyBuilderFn(request).string())
      ElasticRequest("POST", endpoint, params.toMap, HttpEntity(body, "application/json"))
    }
  }

以上IndexHandler, SearchHandler就是针对index,search操作的Handler[T,U]隐式实例。它们的build(t:T)函数分别按传入的T类型参数构建了各自要求格式的服务请求。

我总是觉着：不一定所有类型的服务请求都适合用DSL来构建，比如多层逻辑条件的json，可能不容易用DSL来实现（我个人的顾虑）。那么应该有个接口直接json文本嵌入request-entity。elastic4s在各种操作类型的服务请求类型如IndexRequest, SearchRequest,BulkRequest等提供了source:Option[String]字段接收json文本，如下：

case class IndexRequest(index: Index,
...
                        source: Option[String] = None)
    extends BulkCompatibleRequest {
      ...
      def source(json: String): IndexRequest = copy(source = json.some)

      ...
    }

case class SearchRequest(indexes: Indexes,
                         ...
                         source: Option[String] = None,
                         ...
                         typedKeys: Option[Boolean] = None) {
                         ...
   /**
    * Sets the source of the request as a json string. Note, if you use this method
    * any other body-level settings will be ignored.
    *
    * HTTP query-parameter settings can still be used, eg limit, routing, search type etc.
    *
    * Unlike rawQuery, source is parsed at the "root" level
    * Query must be valid json beginning with '{' and ending with '}'.
    * Field names must be double quoted.
    *
    * NOTE: This method only works with the HTTP client.
    *
    * Example:
    * {{{
    * search in "*" limit 5 source {
    * """{ "query": { "prefix": { "bands": { "prefix": "coldplay", "boost": 5.0, "rewrite": "yes" } } } }"""
    * } searchType SearchType.Scan
    * }}}
    */
  def source(json: String): SearchRequest = copy(source = json.some)
                       
                         ...

                         }

现在，我们可以直接用json文本了：

  val json =
    """
      |{
      |  "query" : {
      |    "match" : {"title" : "火锅"}
      |  }
      |}
      |""".stripMargin
  val response = client.execute {
    search("books").source(json)   //      .matchQuery("title", "火锅")
  }.await