Using scrolls in Java

一般搜索请求都是返回一”页”数据，无论数据量多大都一起返回给用户，Scroll API可以允许我们检索大量数据(甚至全部数据)。Scroll API允许我们做一个初始阶段搜索并且持续批量从Elasticsearch里拉取结果直到没有结果剩下。这有点像传统数据库里的cursors(游标)。

Scroll API的创建并不是为了实时的用户响应，而是为了处理大量的数据(Scrolling is not intended for real time user requests, but rather for processing large amounts of data)。从 scroll 请求返回的结果只是反映了 search 发生那一时刻的索引状态，就像一个快照(The results that are returned from a scroll request reflect the state of the index at the time that the initial search request was made, like a snapshot in time)。后续的对文档的改动(索引、更新或者删除)都只会影响后面的搜索请求。

import static org.elasticsearch.index.query.QueryBuilders.*;

QueryBuilder qb = termQuery("multi", "test");

SearchResponse scrollResp = client.prepareSearch(test)

.addSort(FieldSortBuilder.DOC_FIELD_NAME, SortOrder.ASC)

.setScroll(new TimeValue(60000)) //为了使用 scroll，初始搜索请求应该在查询中指定 scroll 参数，告诉 Elasticsearch 需要保持搜索的上下文环境多长时间(滚动时间)

.setQuery(qb)

.setSize(100).get(); //max of 100 hits will be returned for each scroll

//Scroll until no hits are returned

do {

for (SearchHit hit : scrollResp.getHits().getHits()) {

//Handle the hit...

}

scrollResp = client.prepareSearchScroll(scrollResp.getScrollId()).setScroll(new TimeValue(60000)).execute().actionGet();

} while(scrollResp.getHits().getHits().length != 0); // Zero hits mark the end of the scroll and the while loop.

如果超过滚动时间，继续使用该滚动ID搜索数据，则会报错：

Caused by: SearchContextMissingException[No search context found for id [2861]]

at org.elasticsearch.search.SearchService.findContext(SearchService.java:613)

at org.elasticsearch.search.SearchService.executeQueryPhase(SearchService.java:403)

at org.elasticsearch.search.action.SearchServiceTransportAction$SearchQueryScrollTransportHandler.messageReceived(SearchServiceTransportAction.java:384)

at org.elasticsearch.search.action.SearchServiceTransportAction$SearchQueryScrollTransportHandler.messageReceived(SearchServiceTransportAction.java:381)

at org.elasticsearch.transport.TransportRequestHandler.messageReceived(TransportRequestHandler.java:33)

at org.elasticsearch.transport.RequestHandlerRegistry.processMessageReceived(RequestHandlerRegistry.java:75)

at org.elasticsearch.transport.TransportService$4.doRun(TransportService.java:376)

at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)

at java.lang.Thread.run(Thread.java:745)

虽然当滚动有效时间已过，搜索上下文(Search Context)会自动被清除，但是一值保持滚动代价也是很大的，所以当我们不在使用滚动时要尽快使用Clear-Scroll API进行清除。

清除Scroll

/**

* 清除滚动ID

* @param client

* @param scrollIdList

* @return

*/

public static boolean clearScroll(Client client, List scrollIdList){

ClearScrollRequestBuilder clearScrollRequestBuilder = client.prepareClearScroll();

clearScrollRequestBuilder.setScrollIds(scrollIdList);

ClearScrollResponse response = clearScrollRequestBuilder.get();

return response.isSucceeded();

}

/**

* 清除滚动ID

* @param client

* @param scrollId

* @return

*/

public static boolean clearScroll(Client client, String scrollId){

ClearScrollRequestBuilder clearScrollRequestBuilder = client.prepareClearScroll();

clearScrollRequestBuilder.addScrollId(scrollId);

ClearScrollResponse response = clearScrollRequestBuilder.get();

return response.isSucceeded();

}

实例

public class ScrollsAPI extends ElasticsearchClientBase {

private String scrollId;

@Test

public void testScrolls() throws Exception {

SearchResponse scrollResp = client.prepareSearch("twitter")

.addSort(FieldSortBuilder.DOC_FIELD_NAME, SortOrder.ASC)

.setScroll(new TimeValue(60000)) //为了使用 scroll，初始搜索请求应该在查询中指定 scroll 参数，告诉 Elasticsearch 需要保持搜索的上下文环境多长时间(滚动时间)

.setQuery(QueryBuilders.termQuery("user", "kimchy")) // Query 查询条件

.setSize(5).get(); //max of 100 hits will be returned for each scroll

//Scroll until no hits are returned

scrollId = scrollResp.getScrollId();

do {

for (SearchHit hit : scrollResp.getHits().getHits()) {

//Handle the hit...

System.out.println("" + hit.getSource().toString());

}

scrollResp = client.prepareSearchScroll(scrollId).setScroll(new TimeValue(60000)).execute().actionGet();

}

while (scrollResp.getHits().getHits().length != 0); // Zero hits mark the end of the scroll and the while loop.

}

@Override

public void tearDown() throws Exception {

ClearScrollRequestBuilder clearScrollRequestBuilder = client.prepareClearScroll();

clearScrollRequestBuilder.addScrollId(scrollId);

ClearScrollResponse response = clearScrollRequestBuilder.get();

if (response.isSucceeded()) {

System.out.println("成功清除");

}

super.tearDown();

}

}