Apache Pinot

does anyone know why i may get the following error from the broker `"message": "2 servers [pinot-server-1_O, pinot-server-0_O] not responded"`  I just changed the version of my broker trying to downgrade the entire system to `0.9.3`

anyone familiar with this error?

```"message": "QueryExecutionError:\njava.lang.RuntimeException: Caught exception while running CombinePlanNode.\n\tat org.apache.pinot.core.plan.CombinePlanNode.run(CombinePlanNode.java:146)\n\tat org.apache.pinot.core.plan.InstanceResponsePlanNode.run(InstanceResponsePlanNode.java:41)\n\tat org.apache.pinot.core.plan.GlobalPlanImplV0.execute(GlobalPlanImplV0.java:45)\n\tat org.apache.pinot.core.query.executor.ServerQueryExecutorV1Impl.processQuery(ServerQueryExecutorV1Impl.java:296)\n...\nCaused by: java.util.concurrent.ExecutionException: java.lang.NullPointerException\n\tat java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122)\n\tat java.base/java.util.concurrent.FutureTask.get(FutureTask.java:205)\n\tat org.apache.pinot.core.plan.CombinePlanNode.run(CombinePlanNode.java:135)\n\t... 15 more\n...\nCaused by: java.lang.NullPointerException\n\tat org.apache.pinot.core.util.trace.TraceContext.registerThreadToRequest(TraceContext.java:140)\n\tat org.apache.pinot.core.util.trace.TraceCallable.call(TraceCallable.java:41)\n\t... 8 more",```

btw a restart of servers made this go away but i’m curious why it happened on the first place lol

So you were running old broker and new server version?

server was on `0.9.3`  and broker was newer, things were working fine then i tried to downgrade broker to `0.9.3` and that happened ^

So recommended upgrade order is controller - broker - server. And downgrade is reverse. 

servers was `0.9.3` and then i was downgrading the broker from a later version