Not receiving published message in different actor system


I want to communicate actors across the cluster, So I am planning to use pub sub protocal in akka.
And I found sample java code from the below link

If I use same actor system for both publish and subscribe then I can see the published messages.
If it is different actor system then I am unable to see.

I am getting the below error

[INFO] [12/12/2019 19:08:12.785] [] [akka://ReactorCluster/system/distributedPubSubMediator] Message [java.lang.String] from Actor[akka://ReactorCluster/user/publisher#298442915] to Actor[akka://ReactorCluster/system/distributedPubSubMediator#549143944] was not delivered. [4] dead letters encountered. If this is not an expected behavior, then [Actor[akka://ReactorCluster/system/distributedPubSubMediator#549143944]] may have terminated unexpectedly, This logging can be turned off or adjusted with configuration settings ‘akka.log-dead-letters’ and ‘akka.log-dead-letters-during-shutdown’.

Can’t we use different actor system for publish and subscribe ?

And what is the difference between Classic Distributed Publish Subscribe and normal Distributed Publish Subscribe

Please advice me

Could someone clarify me the above question ?

A few things to check:

  • Verify that the mediator is started on each node (debug logs should help)
  • Verify that you use the same config for the mediator on each node
  • Gossip of subscribers for a topic takes a little time, if you publish before a node knows there is a subscriber on the other node it will not be sent there