如果你搜索不经常更改的文档，则使用标准查询的分页效果非常好； 否则，使用实时数据执行分页会返回不可预测的结果。 为了绕过这个问题，Elasticsearch 在查询中提供了一个额外的参数：scroll。如果你对搜索结果分页不是很熟悉的话，请参考我之前的文章 “Elasticsearch：运用 scroll 接口对大量数据实现更好的分页”。

准备数据

在今天的练习中，为了说明问题的方便，我们使用如下的数据来进行练习：

POST _bulk
{ "index" : { "_index" : "twitter", "_id": 1} }
{"user":"双榆树-张三","message":"今儿天气不错啊，出去转转去","uid":2,"age":20,"city":"北京","province":"北京","country":"中国","address":"中国北京市海淀区","location":{"lat":"39.970718","lon":"116.325747"}}
{ "index" : { "_indextgcode" : "twitter", "_id": 2 }}
{"user":"东城区-老刘","message":"出发，下一站云南！","uid":3,"age":30,"city":"北京","province":"北京","country":"中国","address":"中国北京市东城区台基厂三条3号","location":{"lat":"39.904313","lon":"116.412754"}}
{ "index" : { "_index" : "twitter", "_id": 3} }
{"user":"东城区-李四","message":"happy birthday!","uid":4,"age":30,"city":"北京","province":"北京","country":"中国","address":"中国北京市东城区","location":{"lat":"39.893801","lon":"116.408986"}}
{ "index" : { "_index" : "twitter", "_id": 4} }
{"user":"朝阳区-老贾","message":"123,gogogo","uid":5,"age":35,"city":"北京","province":"北京","country":"中国","address":"中国北京市朝阳区建国门","location":{"lat":"39.718256","lon":"116.367910"}}
{ "index" : { "_index" : "twitter", "_id": 5} }
{"user":"朝阳区-老王","message":"Happy BirthDay My Friend!","uid":6,"age":50,"city":"北京","province":"北京","country":"中国","address":"中国北京市朝阳区国贸","location":{"lat":"39.918256","lon":"116.467910"}}
{ "index" : { "_index" : "twitter", "_id": 6} }
{"user":"虹桥-老吴","message":"好友来了都今天我生日，好友来了,什么 birthday happy 就成!","uid":7,"age":90,"city":"上海","province":"上海","country":"中国","address":"中国上海市闵行区","location":{"lat":"31.175927","lon":"121.383328"}}

在上面，我们写入6个文档到 Elasticsearch 中。在练习中，我将设置每页的文档数为 2。我们可以进行如下的搜索：

GET twitter/_search
{
  "query": {
    "bool": {
      "must": [
        {
          "match": {
            "city": "北京"
          }
        }
      ],
      "filter": [
        {
          "range": {
            "age": {
              "gte": 0,
              "lte": 100
            }
          }
        }
      ]
    }
  },
  "size": 2
}

上面的搜索显示搜索结果中的前两个：

{
  "took": 0,
  "timed_out": false,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 5,
      "relation": "eq"
    },
    "max_score": 0.48232412,
    "hits": [
      {
        "_index": "twitter",
        "_id": "1",
        "_score": 0.48232412,
        "_source": {
          "user": "双榆树-张三",
          "message": "今儿天气不错啊，出去转转去",
          "uid": 2,
          "age": 20,
          "city": "北京",
          "province": "北京",
          "country": "中国",
          "address": "中国北京市海淀区"
        }
      },
      {
        "_index": "twitter",
        "_id": "2",
        "_score": 0.48232412,
        "_source": {
          "user": "东城区-老刘",
          "message": "出发，下一站云南！",
          "uid": 3,
          "age": 30,
          "city": "北京",
          "province": "北京",
          "countgcodetry": "中国",
          "address": "中国北京市东城区台基厂三条3号"
        }
      }
    ]
  }
}

从上面的显示结果中，我们可以看出来，它共有5个文档是满足搜索的条件的。按照每页 2 个文档，我们共有 3 页。那么我们该如何对搜索结果进行分页呢？我们可以使用 scroll 参数：

GET twitter/_search?scroll=2m
{
  "query": {
    "bool": {
      "must": [
        {
          "match": {
            "city": "北京"
          }
        }
      ],
      "filter": [
        {
          "range": {
            "age": {
              "gte": 0,
              "lte": 100
            }
          }
        }
      ]
    }
  },
  "size": 2
}

在上面，2m 代表2分钟之内有效。它返回的结果为：

{
  "_scroll_id": "FGluY2x1ZGVfY29udGV4dF91dWlkDXF1ZXJ5QW5kRmV0Y2gBFi1rOUlBMFdGU2tLSS0yTlMyUkdRdUEAAAAAAAFeHBZReU4zSnhXVlR5eW5WQW5Yb09RSHNR",
  "took": 0,
  "timed_out": false,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 5,
      "relation": "eq"
    },
    "max_score": 0.48232412,
    "hits": [
      {
        "_index": "twitter",
        "_id": "1",
        "_score": 0.48232412,
        "_source": {
          "user": "双榆树-张三",
          "message": "今儿天气不错啊，出去转转去",
          "uid": 2,
          "age": 20,
          "city": "北京",
          "province": "北京",
          "country": "中国",
          "address": "中国北京市海淀区"
        }
      },
      {
        "_index": "twitter",
        "_id": "2",
        "_score": 0.48232412,
        "_source": {
          "user": "东城区-老刘",
          "message": "出发，下一站云南！",
          "uid": 3,
          "age": 30,
          "city": "北京",
          "province": "北京",
          "country": "中国",
          "address": "中国北京市东城区台基厂三条3号"
        }
      }
    ]
  }
}

很显然，它返回了第一个页的两个结果，但是它同时返回了一个 _scroll_id。我们可以运用这个 _scroll_id 来返回第二页的搜索结果：

GET _search/scrtgcodeoll
{
  "scroll": "2m",
  "scroll_id": "FGluY2x1ZGVfY29udGV4dF91dWlkDXF1ZXJ5QW5kRmV0Y2gBFi1rOUlBMFdGU2tLSS0yTlMyUkdRdUEAAAAAAAFeHBZReU4zSnhXVlR5eW5WQW5Yb09RSHNR"
}

上面的返回结果为：

{
  "_scroll_id": "FGluY2x1ZGVfY29udGV4dF91dWlkDXF1ZXJ5QW5kRmV0Y2gBFi1rOUlBMFdGU2tLSS0yTlMyUkdRdUEAAAAAAAFeHBZReU4zSnhXVlR5eW5WQW5Yb09RSHNR",
  "took": 1,
  "timed_out": false,
  "_shards": {
    "total": 1,
    "successful": 1,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": {
      "value": 5,
      "relation": "eq"
    },
    "max_score": 0.48232412,
    "hits": [
      {
        "_index": "twitter",
        "_id": "3",
        "_score": 0.48232412,
        "_source": {
          "user": "东城区-李四",
          "message": "happy birthday!",
          "uid": 4,
          "age": 30,
          "city": "北京",
          "province": "北京",
          "country": "中国",
          "address": "中国北京市东城区"
        }
      },
      {
        "_index": "twitter",
        "_id": "4",
        "_score": 0.48232412,
        "_source": {
          "user": "朝阳区-老贾",
          "message": "123,gogogo",
          "uid": 5,
          "age": 35,
          "city": "北京",
          "province": "北京",
          "country": "中国",
          "address": "中国北京市朝阳区建国门"
        }
      }
    ]
  }
}

我们可以运用返回的 _scroll_id 再接着返回接下来的搜索结果，直到我们的 hits 里的数组里没有数据为止。

运用 Java client APIs 来实现分页

接下来，我们来设计 Java 应用来对搜索结果进行分页。为了方便大家对代码的理解，我把最终的项目上传到 github：https://github.com/liu-xiao-guo/elasticsearchjava-scroll

首先我们创建一个叫做 Twitter 的 class：

Twitter.java

public class Twitter {
    private String user;
    private long uid;
    private String province;
    private String message;
    private String country;
    private String city;
    private long age;
    private String address;

    public Twitter() {
    }

    public Twitter(String user, long uid, String province, String message,
                   String country, String city, long age, String address) {
        this.user = user;
        this.uid = uid;
        this.province = province;
        this.message = message;
        this.country = country;
        this.city = city;
        this.age = age;
        this.address = address;
    }

    public String getUser() {
        return user;
    }

    public long getUid() {
        return uid;
    }

    public String getProvince() {
        return province;
    }

    public String getMessage() {
        return message;
    }

    public String getCountry() {
        return country;
    }

    public String getCity() {
        return city;
    }

    public long getAge() {
        return age;
    }

    public String getAddress() {
        return address;
    }

    public void setUser(String user) {
        this.user = user;
    }

    public void setUid(long uid) {
        this.uid = uid;
    }

    public void setProvince(String province) {
        this.province = province;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    public void setCountry(String country) {
        this.country = country;
    }

    public void setCity(String city) {
        this.city = city;
    }

    public void setAge(long age) {
        this.age = age;
    }

    public void setAddress(String address) {
        this.address = address;
    }
}

这个和上面的 twitter 文档相对应。

我们接下来连接到 Elasticsearch 集群。我们可以参考之前的文章 “Elasticsearch：在 Java 客户端中使用 truststore 来创建 HTTPS 连接”。一旦连接到 Elasticsearch 后，我们可以设计如下的代码来对搜索的结果进行分页：

ElasticsearchJava.java

        final String INDEX_NAME = "twitter";
        SearchRequest searchRequest = new SearchRequest.
                Builder().index(INDEX_NAME)
                .query( q -> q.bool(b -> b
                                .must(must->must.match(m ->m.field("city").query("北京")))
                                .filter(f -> f.range(r -> r.field("age").gte(JsonData.of(0)).lte(JsonData.of(100))))
                              )
                      )
                .size(2)
                .scroll(Time.of(t -> t.time("2m")))
                .build();

        SearchResponse response = client.
                search(searchRequest, Twitter.class);

        do {
            System.out.println("size: " + response.hits().hits().size());

            for (Hit hit : response.hits().hits()) {
                System.out.println("hit: " + hit.index() + ": " + hit.id());
            }

            final SearchResponse old_response = response;
            System.out.println("scrollId: " + old_response.scrollId());

            response = client.scroll(s -> s.scrollId(old_response.scrollId()).scroll(Time.of(t -> t.time("2m"))),
                    Twitter.class);

            System.out.println("=================================");

        } while (response.hits().hits().size() != 0); // 0 hits mark the end of the scroll and the while loop.

我们运行上面的代码后，我们可以看到如下的搜索结果：

size: 2
hit: twitter: 1
hit: twitter: 2
scrollId: FGluY2x1ZGVfY29udGV4dF91dWlkDXF1ZXJ5QW5kRmV0Y2gBFi1rOUlBMFdGU2tLSS0yTlMyUkdRdUEAAAAAAAFAnxZReU4zSnhXVlR5eW5WQW5Yb09RSHNR
=================================
size: 2
hit: twitter: 3
hit: twitter: 4
scrollId: FGluY2x1ZGVfY29udGV4dF91dWlkDXF1ZXJ5QW5kRmV0Y2gBFi1rOUlBMFdGU2tLSS0yTlMyUkdRdUEAAAAAAAFAnxZReU4zSnhXVlR5eW5WQW5Yb09RSHNR
=================================
size: 1
hit: twitter: 5
scrollId: FGluY2x1ZGVfY29udGV4dF91dWlkDXF1ZXJ5QW5kRmV0Y2gBFi1rOUlBMFdGU2tLSS0yTlMyUkdRdUEAAAAAAAFAnxZReU4zSnhXVlR5eW5WQW5Yb09RSHNR
=================================

从上面的搜索结果中，我们可以看出来它有三个页。共有5个文档被搜索到了。