ElasticSearch - 为什么通过id进行简单查询会导致超时异常？

有时，生产环境中存在以下例外情况，

2020-01-29 17:10:46.085 ERROR 2852 --- [o-8022-exec-258] c.c.p.common.dao.SearchDao               : Search person by id failed

java.net.SocketTimeoutException: 30,000 milliseconds timeout on connection http-outgoing-832 [ACTIVE]
        at org.elasticsearch.client.RestClient.extractAndWrapCause(RestClient.java:789) ~[elasticsearch-rest-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestClient.performRequest(RestClient.java:225) ~[elasticsearch-rest-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestClient.performRequest(RestClient.java:212) ~[elasticsearch-rest-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestHighLevelClient.internalPerformRequest(RestHighLevelClient.java:1433) ~[elasticsearch-rest-high-level-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestHighLevelClient.performRequest(RestHighLevelClient.java:1403) ~[elasticsearch-rest-high-level-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestHighLevelClient.performRequestAndParseEntity(RestHighLevelClient.java:1373) ~[elasticsearch-rest-high-level-client-7.1.1.jar!/:7.1.1]
        at org.elasticsearch.client.RestHighLevelClient.get(RestHighLevelClient.java:699) ~[elasticsearch-rest-high-level-client-7.1.1.jar!/:7.1.1]

Caused by: java.net.SocketTimeoutException: 30,000 milliseconds timeout on connection http-outgoing-832 [ACTIVE]
        at org.apache.http.nio.protocol.HttpAsyncRequestExecutor.timeout(HttpAsyncRequestExecutor.java:387) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.client.InternalIODispatch.onTimeout(InternalIODispatch.java:92) ~[httpasyncclient-4.1.4.jar!/:4.1.4]
        at org.apache.http.impl.nio.client.InternalIODispatch.onTimeout(InternalIODispatch.java:39) ~[httpasyncclient-4.1.4.jar!/:4.1.4]
        at org.apache.http.impl.nio.reactor.AbstractIODispatch.timeout(AbstractIODispatch.java:175) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.BaseIOReactor.sessionTimedOut(BaseIOReactor.java:263) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.AbstractIOReactor.timeoutCheck(AbstractIOReactor.java:492) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.BaseIOReactor.validate(BaseIOReactor.java:213) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.AbstractIOReactor.execute(AbstractIOReactor.java:280) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.BaseIOReactor.execute(BaseIOReactor.java:104) ~[httpcore-nio-4.4.11.jar!/:4.4.11]
        at org.apache.http.impl.nio.reactor.AbstractMultiworkerIOReactor$Worker.run(AbstractMultiworkerIOReactor.java:591) ~[httpcore-nio-4.4.11.jar!/:4.4.11]

但这只是一个简单的查询，而不是一个复杂的查询

 curl 'http://localhost:9201/person/_doc/30154410564?pretty'

这时候负载很低

那么为什么会存在这些超时异常？并且有很多搜索查询，但是为什么只有这种简单的查询query by id才能导致此异常？

人员索引是从Oracle DB同步的，并且有一个计划的任务，每10分钟它将同步更改的人员到人员索引，如果此时访问人员索引，则会导致30,000 milliseconds timeout。那么如何解决呢？而且看来Java客户端访问将存在此现象，但curl在命令行访问不存在该现象。

PS：

health status index               uuid                   pri rep docs.count docs.deleted store.size pri.store.size
green  open   person              jb3msRw5S9ixgXN5SLd6bw   1   0  140754205     19239587     19.8gb         19.8gb

并且在这个时候有索引写人索引

RestClient配置：

private final RestHighLevelClient restHighLevelClient;
restHighLevelClient = new RestHighLevelClient(RestClient.builder(new HttpHost(host, port)));