SOLR 8.9 MLT查询解析器报错：无法获取ID为[XXXXX]的有效索引文档

错误触发场景 #

3分片SolrCloud环境开启基础身份验证，其余功能运行正常，调用MLT查询解析器时，传入已存在的有效文档ID仍报错，报错信息如下：

Error completing MLT request. Could not fetch document with id [XXXXX]
at org.apache.solr.search.mlt.CloudMLTQParser.parse(CloudMLTQParser.java:68)
at org.apache.solr.search.QParser.getQuery(QParser.java:174)
at org.apache.solr.handler.component.QueryComponent.prepare(QueryComponent.java:162)
at org.apache.solr.handler.component.SearchHandler.handleRequestBody(SearchHandler.java:334)
at org.apache.solr.handler.RequestHandlerBase.handleRequest(RequestHandlerBase.java:216)
at org.apache.solr.core.SolrCore.execute(SolrCore.java:2637)
at org.apache.solr.servlet.HttpSolrCall.execute(HttpSolrCall.java:794)
at org.apache.solr.servlet.HttpSolrCall.call(HttpSolrCall.java:567)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:427)
at org.apache.solr.servlet.SolrDispatchFilter.doFilter(SolrDispatchFilter.java:357)
at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:201)
at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1601)
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:548)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143)
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:602)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:235)
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1624)
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:233)
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1435)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:188)
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:501)
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1594)
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:186)
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1350)
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141)
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:191)
at org.eclipse.jetty.server.handler.InetAccessHandler.handle(InetAccessHandler.java:177)
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:146)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127)
at org.eclipse.jetty.rewrite.handler.RewriteHandler.handle(RewriteHandler.java:322)
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127)
at org.eclipse.jetty.server.Server.handle(Server.java:516)
at org.eclipse.jetty.server.HttpChannel.lambda$handle$1(HttpChannel.java:388)
at org.eclipse.jetty.server.HttpChannel.dispatch(HttpChannel.java:633)
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:380)
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:277)
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:311)
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:105)
at org.eclipse.jetty.io.ChannelEndPoint$1.run(ChannelEndPoint.java:104)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.runTask(EatWhatYouKill.java:336)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:313)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:171)
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:129)
at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:383)
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:882)
at org.eclipse.jetty.util.thread.QueuedThreadPool$Runner.run(QueuedThreadPool.java:1036)
at java.lang.Thread.run(Thread.java:748)

使用的查询语句为：
http://localhost:8983/solr/my_collection/select?q={!mlt+qf%3Dtext}XXXXX&start=0&rows=7&fl=id,text

根因 #

最核心的触发原因是Solr内置的CloudMLTQParser执行内部请求拉取源文档时，默认不会携带当前请求的身份认证信息，开启身份验证的环境中该内部请求会被权限拦截，最终被识别为文档不存在抛出错误。
少数非认证场景的触发原因包括：使用了非id的自定义主键字段、自定义分片路由规则导致MLT默认从单分片查找文档失败。

修复方案 #

1. 直接追加查询参数（快速生效） #

在原有查询URL后追加身份凭证参数，MLT内部请求会自动携带该凭证：
http://localhost:8983/solr/my_collection/select?q={!mlt+qf%3Dtext}XXXXX&start=0&rows=7&fl=id,text&username=你的认证账号&password=你的认证密码
替换为你环境的有效认证账号密码即可正常执行。

2. 配置文件预存凭证（更安全） #

修改集合的solrconfig.xml，给MLT解析器预先配置认证信息，无需每次请求传参：

<queryParser name="mlt" class="org.apache.solr.search.mlt.CloudMLTQParserPlugin">
  <str name="username">你的认证账号</str>
  <str name="password">你的认证密码</str>
  <!-- 可保留原有其他MLT配置项 -->
</queryParser>

更新配置后重载集合，原有查询无需修改即可正常使用。

3. 非认证场景修复 #

如果确认未开身份验证仍出现该错误，追加以下参数即可：
&mlt.uniqueKeyField=你的主键字段名&mlt.searchFromAllShards=true

内容的提问来源于stack exchange，提问作者Nishkarsh Dixit