Is there a way to achieve service downgrade in akka cluster sharding?

Question

I'm trying to build up an Akka cluster ShardRegion that might need to be downgraded in the production environment when a bug occurs. However, instead of unregistering it by calling

ClusterClientReceptionist.get(nodeActorSystem).unregisterService(shardRegion)

which will terminate the ShardRegion and its child actors after all messages are consumed before PoisonPill, my sharding child actors have their internal state and purposes that need to be accomplished. I need an elegant way to slowly downgrade the process with the ShardRegion to let any session in-between finish, e.g. any new message with a different EntityId will be sent elsewhere.

I haven't yet found any means to downgrade it or just simply stop any new sharding AkkaActor to prop up on the ShardRegion.Is this even achievable in Akka Cluster ShardRegion?

Answer 1

You can accomplish part of this by specifying a custom stopMessage. The shard region will send this command to the entity actors when they are to be passivated or rebalanced. The default is PoisonPill, but a custom one allows the entity actors to do whatever they need to do to shut down (they do need to eventually stop themselves in this scenario).

If you're triggering a rebalance, the messages to the shard will be buffered until all the active entities in that shard have stopped, which may qualify as "any new message with a different entity ID will be sent elsewhere". Note that messages which are being sent outside of cluster sharding (i.e. directly between entity actors) will still be delivered normally (until said entity actors stop).