一、scroll说明和使用场景

scroll的使用场景：大数据量的检索和操作

scroll顾名思义，就是游标的意思，核心的应用场景就是遍历 elasticsearch中的数据；

通常我们遍历数据采用的是分页，elastcisearch还支持from size的方式进行分页查询，使用 from and size 的深度分页，比如说 ?size=10&from=10000，因为 100,000 排序的结果必须从每个分片上取出并重新排序最后返回 10 条。这个过程需要对每个请求页重新进行提取+排序，效率很低，消耗很大，所以默认的最大可分页的数据是10000，超过10000是不建议的；

使用

通过在url末尾带上scroll=1m表示开启一个游标，1m表示游标的有效期为1分钟

POST /record/_search?scroll=1m
{"from": 0,"size": 20
}

返回结果中会把scroll的id带上，再次查询的时候，直接用scroll id查询即可

POST /_search/scroll
{"scroll" : "1m", "scroll_id" : "FGluY2x1ZGVfY29udGV4dF91dWlkDnF1ZXJ5VGhlbkZldGNoAhZuYmpMbVpwWFRUMnNFMUFFSHlSMHB3AAAAAALBy_0WUWxrNTRTaWNUcy1sOHQ0VUo5dzF6dxZoemFkZTlMeFQ4MmoyOW5SUG8ybE53AAAAAAN6ip8WMmk5TWZlQ21RQnFsNURwaXRzSGhCdw==" 
}

二、基于ElasticsearchRestTemplate的实现

这里我们定义了一个template如下，主要作用就是实现一个基于scroll的数据遍历模板，屏蔽开启scroll 以及 scroll遍历所有数据，通过Consumer<T>钩子函数进行数据处理

import lombok.extern.slf4j.Slf4j;
import org.elasticsearch.index.query.BoolQueryBuilder;
import org.elasticsearch.index.query.QueryBuilders;
import org.springframework.data.domain.PageRequest;
import org.springframework.data.elasticsearch.core.ElasticsearchRestTemplate;
import org.springframework.data.elasticsearch.core.SearchHit;
import org.springframework.data.elasticsearch.core.SearchScrollHits;
import org.springframework.data.elasticsearch.core.mapping.IndexCoordinates;
import org.springframework.data.elasticsearch.core.query.NativeSearchQueryBuilder;import java.util.List;
import java.util.concurrent.*;/*** scrollTemplate 模板，用于遍历整个Index的数据* @author xiuzhu* @Date 2023/7/28 13:12*/
@Slf4j
public class ElasticSearchScrollTemplate<T> {ExecutorService executorService = new ThreadPoolExecutor(1, 4,30,TimeUnit.SECONDS,new LinkedBlockingQueue<Runnable>(5),Executors.defaultThreadFactory(),new ThreadPoolExecutor.CallerRunsPolicy());ElasticsearchRestTemplate elasticSearchRestTemplate;Class<T> cls;String indexName;public ElasticSearchScrollTemplate(ElasticsearchRestTemplate template,Class<T> cls,String indexName) {this.elasticSearchRestTemplate = template;this.cls = cls;this.indexName = indexName;}@FunctionalInterfacepublic interface Consumer<T> {public void accept(List<T> objects);}public void execute(Consumer<T> consumer) {//构建查询条件NativeSearchQueryBuilder query = new NativeSearchQueryBuilder();BoolQueryBuilder queryBuilder = QueryBuilders.boolQuery();query.withPageable(PageRequest.of(0, 300));query.withQuery(queryBuilder);//保留0.5分钟long scrollTimeInMillis = 30*1000;IndexCoordinates recordIndex = IndexCoordinates.of(indexName);SearchScrollHits<T> hits = elasticSearchRestTemplate.searchScrollStart(scrollTimeInMillis, query.build(), cls, recordIndex);// scrollIdString scrollId = hits.getScrollId();List<T> recordEntityList = hits.stream().map(SearchHit::getContent).toList();long total = 0L;log.info("================ began scroll index={} ====================", indexName);executorService.submit(()->{consumer.accept(recordEntityList);});total = total + recordEntityList.size();log.info("================  has scroll index={} total={} ====================", indexName, total);while (!hits.isEmpty()) {hits = elasticSearchRestTemplate.searchScrollContinue(scrollId, scrollTimeInMillis, cls, recordIndex);List<T> entities = hits.stream().map(SearchHit::getContent).toList();executorService.submit(()->{consumer.accept(entities);});total = total + entities.size();try {//给系统留GC时间，不然容易内存溢出Thread.sleep(300);} catch (InterruptedException e) {log.error("sleep error", e);}log.info("================  has scroll index={} total={} ====================", indexName, total);}log.info("================ end scroll index={} ====================", indexName);}
}

使用参考：

@Resource(name = "elasticSearchRestTemplate")ElasticsearchRestTemplate elasticsearchRestTemplate;new ElasticSearchScrollTemplate<>(elasticsearchRestTemplate,RecordEntity.class,"record")).execute((entities)->{entities.forEach(item->{//这里进行数据的处理，比如修改数据recordEntityService.save(item);log.info("tag update success record={} api={}", item.getId());});});

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