Hi!

We recently made a mistake by having two separate clusters use the same 
journal DB which obviously doesn't work but it lead to some questions about 
how the cluster sharding works.

We detected our problem by seeing a stack trace containing lines of code 
that didn't exist anywhere in the code deployed in that cluster and after 
tripple checking that we were sure we did run the correct code in all 
places in the cluster we finally found out that it in fact must be talking 
with an actor outside of the cluster. Looking at what the cluster sharding 
coordinator persists in the journal we found an IP address of a node in the 
other cluster.

Granted the data in the journal table is pretty invalid but this still 
raises the question why the shard coordinator could start using nodes that 
aren't part of the cluster just based on some historical persisted info in 
the journal table. For example what happens if you run sharding over two 
nodes and decide to move the second node to another cluster. You take down 
the system and change the config files and then start the two systems 
again, could the sharding coordinator on the first node under any 
circumstances still use the second node based on the info in the journal 
table even though it's now not part of the same cluster anymore?

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