Hello
I wanted to integrate akka-cluster-sharding with my akka-persistence
application. I read the documentation and browsed through the code in
activator but one thing is still a bit of a mystery to me.
Currently in my application each PersistentActor is identified by UUID
value. All messages that those actors receive inherit custom trait
DomainCommand
which has method aggregateRootId: UUID. Based on this and on documentation
my IdExtractor looks like this:
val domainCommandIdExtractor: ShardRegion.IdExtractor = {
case command: DomainCommand => (command.aggregateRootId.toString,
command)
}
Now what about ShardResolver. According to the documentation it says that:
Try to produce a uniform distribution, i.e. same amount of entries in each
> shard. As a rule of thumb, the number of shards should be a factor ten
> greater than the planned maximum number of cluster nodes.
And in the example there is (id % 12).toString.
I am wondering if it would be wrong to just return string representation of
UUID - that way I will have unique number of entries per shard because
ShardResolver
should return unique strings per PersistentActor.
If I understand the documentation correctly it would result in 1 actor per
shard grouping (and if number of shards > number of nodes I will have
multiple shards on one node) - the drawback (I think) is that using this
scheme the large number of shards may cause problems during
rebalancing/adding new actor when the number of shards is large.
If the scheme of just returning UUID value is not very good idea (be it
because of the reason I mentioned or the other one) should I use some
number of first characters from UUID so it will result in grouping of many
actors per shard? This way I will have 16, 256, ... possible return values
from ShardResolver
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