I want to know how can we send a message to a particular actor in cluster without knowing the actual path..
For Example...
I have 5 nodes all have same actorsytem and formed a cluster.
Each node will have 2 actors in it all actors across cluster have unique name.
I have an actor system outside the cluster.which not part of this cluster.
I have to send message from this actor system to cluster actors.
How can i map respective message to respective actor in cluster each time.Without knowing the actual path of the actor.
Without cluster sharding how actors inside cluster will accessed by outside cluster.
There is a direct answer to your question (at the end), but I think you need to go back to the docs and rethink your design. One of the primary reasons for this is that while what you are asking is possible, it goes against all of the best practices and many of the features involved are deprecated or not recommended. The best practice is that clusters should be self-contained and clusters should expose via well defined APIs and not actor internals. To quote from the remoting docs:
When building an Akka application, you would usually not use the Remoting concepts directly, but instead use the more high-level Akka Cluster utilities or technology-agnostic protocols such as HTTP, gRPC etc.
Essentially the docs are tell you that what you are trying to do is a bad idea. If two actors need to be able to discover each other, they should be in the same cluster. If for some reason you can't have them in the same cluster, for maintainability reasons you should expose the actor in the cluster via REST/gRPC or some other well defined API rather than trying to allow direct access to the actors.
Similarly, in the section about cluster client the docs say:
Cluster Client is deprecated in favor of using Akka gRPC. It is not advised to build new applications with Cluster Client, and existing users should migrate.
But, here's an attempt to answer the question directly. To quote the first sentence of the Actor discovery docs, "There are two general ways to obtain Actor references: by creating actors and by discovery using the Receptionist."
Unrelated side note: (That is a little misleading though, because there are some other ways in untyped Actors, and you also can obviously just be given a ActorRef. It's not the the docs are wrong, I'm just taking them a little out of context.).
Anyway, that leads to the direct answer to your question: ClientClusterReceptionist. You can find details on how to use it in the Classic Cluster Client docs. It's going to require some untyped actors, but since you are using deprecated approaches that probably isn't an issue.
But, moreso, you really should rethink this because based on your last question, it seems like you really aren't understanding how either clustering or sharding work.