If the state of the entities are persistent you may stop entities that are not used to
reduce memory consumption. This is done by the application specific implementation of
the entity actors for example by defining receive timeout (
context.setReceiveTimeout).
If a message is already enqueued to the entity when it stops itself the enqueued message
in the mailbox will be dropped. To support graceful passivation without losing such
messages the entity actor can send this
Passivate message to its parent
ShardRegion.
The specified wrapped
stopMessage will be sent back to the entity, which is
then supposed to stop itself. Incoming messages will be buffered by the
ShardRegion
between reception of
Passivate and termination of the entity. Such buffered messages
are thereafter delivered to a new incarnation of the entity.
PoisonPill is a perfectly fine stopMessage.