search（2）- elasticsearch scala终端：elastic4s

   上篇谈到：elasticsearch本身是一个完整的后台系统，对其的操作使用是通过终端api进行的。elasticsearch本身提供了多种编程语言的api，包括java的esjava。而elastic4s是一套基于esjava之上的scala api。

先看看scala 终端 ElasticClient的构建过程：

 import com.sksamuel.elastic4s.ElasticDsl._ val esjava =  JavaClient(ElasticProperties("http://localhost:9200")) val client = ElasticClient(esjava)

先构建JavaClient，JavaClient包嵌了个esjava的RestClient进行具体的操作：

class JavaClient(client: RestClient) extends HttpClient { ... //send request to elasticsearch
override def send(req: ElasticRequest, callback: Either[Throwable, HttpResponse] => Unit): Unit = { if (logger.isDebugEnabled) { logger.debug("Executing elastic request {}", Show[ElasticRequest].show(req)) } val l = new ResponseListener { override def onSuccess(r: org.elasticsearch.client.Response): Unit = callback(Right(fromResponse(r))) override def onFailure(e: Exception): Unit = e match { case re: ResponseException => callback(Right(fromResponse(re.getResponse))) case t => callback(Left(JavaClientExceptionWrapper(t))) } } val request = new Request(req.method, req.endpoint) req.params.foreach { case (key, value) => request.addParameter(key, value) } req.entity.map(apacheEntity).foreach(request.setEntity) //perform actual request sending
 client.performRequestAsync(request, l) } ... }

上面这个RestClient即是elasticsearch提供的javaClient。而elastic4s的具体操作是通过RestClient.performRequestAsync进行的，如下：

 

public class RestClient implements Closeable { ... /** * Sends a request to the Elasticsearch cluster that the client points to. * The request is executed asynchronously and the provided * {@link ResponseListener} gets notified upon request completion or * failure. Selects a host out of the provided ones in a round-robin * fashion. Failing hosts are marked dead and retried after a certain * amount of time (minimum 1 minute, maximum 30 minutes), depending on how * many times they previously failed (the more failures, the later they * will be retried). In case of failures all of the alive nodes (or dead * nodes that deserve a retry) are retried until one responds or none of * them does, in which case an {@link IOException} will be thrown. * * @param request the request to perform * @param responseListener the {@link ResponseListener} to notify when the * request is completed or fails */
    public void performRequestAsync(Request request, ResponseListener responseListener) { try { FailureTrackingResponseListener failureTrackingResponseListener = new FailureTrackingResponseListener(responseListener); InternalRequest internalRequest = new InternalRequest(request); performRequestAsync(nextNodes(), internalRequest, failureTrackingResponseListener); } catch (Exception e) { responseListener.onFailure(e); } } ... }

另外，ElasticProperties是一个javaClient与ES连接的参数结构，包括IP地址：

/** * Contains the endpoints of the nodes to connect to, as well as connection properties. */
case class ElasticProperties(endpoints: Seq[ElasticNodeEndpoint], options: Map[String, String] = Map.empty)

ElasticProperties包含了ES地址ElasticNodeEndPoint及其它连接参数（如果需要的话），如下：

 it should "support prefix path with trailing slash" in { ElasticProperties("https://host1:1234,host2:2345/prefix/path/") shouldBe ElasticProperties(Seq(ElasticNodeEndpoint("https", "host1", 1234, Some("/prefix/path")), ElasticNodeEndpoint("https", "host2", 2345, Some("/prefix/path")))) }

当elastic4s完成了与elasticsearch的连接之后，就可以把按ES要求组合的Json指令发送到后台ES去执行了。elastic4s提供了一套DSL, 一种嵌入式语言，可以帮助用户更方便的用编程模式来组合ES的指令Json。当然，用户也可以直接把字符类的Json直接通过ElasticClient发送到后台ES。下面是一个简单可以运行的elastic4s示范：

import com.sksamuel.elastic4s.http.JavaClient import com.sksamuel.elastic4s.requests.common.RefreshPolicy import com.sksamuel.elastic4s.{ElasticClient, ElasticProperties} object HttpClientExampleApp extends App { // you must import the DSL to use the syntax helpers
 import com.sksamuel.elastic4s.ElasticDsl._ val esjava =  JavaClient(ElasticProperties("http://localhost:9200")) val client = ElasticClient(esjava) client.execute { bulk( 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"), indexInto("books" ).fields("title" -> "中国火锅大全", "content" -> "本书是全国中式烹饪中有关火锅的各种介绍") ).refresh(RefreshPolicy.WaitFor) }.await val json =
    """       |{ |  "query" : { |    "match" : {"title" : "火锅"} | } |} |""".stripMargin
  val response = client.execute { search("books").source(json)   // .matchQuery("title", "火锅")
  }.await

  // prints out the original json
 println(response.result.hits.hits.head.sourceAsString) client.close() }