We observe problems with both cluster sharding and cluster singletons.
With sharders - usually problem is corrupted journal that prevents sharding 
coordinator from starting. In our situation easiest thing to do is to 
delete all data from journal and restart it - problem is that I can't find 
a way to detect that situation in a different way than observe logs - I 
can't find any way to detect such failure from the code. It should be 
pretty easy - as usually `akka.cluster.sharding.ShardCoordinator.State` 
throws exception with requirementFailed, but there is no way I can find a 
way to react on that - no easy way to put `supervisorStrategy` for shard 
coordinator or no other way to detect its state.
We can't use `ddata` mode - as current implementation does not work in our 
environment, when we need to scale nodes up and down - as it requires 
majority of nodes to respond, it fails to work even on simplest cases of 
scaling down in a small cluster.

Similar situation applies to cluster singleton - if cluster singleton is 
stuck, there is no way to detect that situation from the code - only by 
observing logs.

Does anybody have experience in handling such situations?
I'm trying to implement some external monitoring for both things, with 
basically sending `Identity` message to actors that are supposed to exist - 
singletons - but it looks like rewriting already existing code inside akka.

Maybe adding such failure detection capabilities to akka (publish event 
bus, adding ability to set supervisor strategy ) is a better approach?

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