Rolling update steps for Classic -> Typed Sharding

[patriknw]

DO NOT MERGE

This example illustrates how a rolling update from Classic to Typed Sharding can be performed.
Full cluster shutdown is of course easier but if zero downtime is needed it is possible with these steps.

See the commit diffs to see what needs to be done in this 3 phase rolling update.

roll 1 - Scala: 359e08d, Java: 47bf16a

• update to Akka 2.6.x first
• and add dependeny akka-cluster-sharding-typed although the actors remain untyped in first rollout
• define akka.cluster.sharding.number-of-shards config
  • same value as you have used previously for number of shards
  • same value as you will use for Typed
  • otherwise Typed node will not be able to join in roll 2
• add akka.cluster.sharding.typed.ShardingEnvelope to message extractor
• sender and ask
  • add replyTo ActorRef to the messages that are using sender for the reply
  • change from akka.pattern.ask to akka.pattern.extended.ask to populate
    the replyTo field in the messages
  • use the replyTo field instead of sender() unless it is null

roll 2 - Scala: 0ceac95, Java: b415647

• convert actors to Typed
• use custom ShardingMessageExtractor in ClusterSharding.init
• use same akka.cluster.sharding.number-of-shards config as was in roll 1

roll 3 - Scala: b27976b, Java: dc25a53

• custom ShardingMessageExtractor can be removed

Refs akka/akka#26718

[patriknw]

Another thing I will try is to have response messages that originally were sent to sender().

[patriknw]

I have also tried messages with replies, and also ask. That also works in both directions if doing the preparations in the untyped version as in roll 1 commit. Note that it's using the akka.pattern.extended.ask which supports the replyTo in the message pattern. In the destination actor one can check if the replyTo field is populated or not and then reply to that or the sender().