How Akka.Net handles system falts during message processing

Question

Suppose that one of cluster nodes received a message and one of actors started to process it. Somewhere in the middle this node died for some reason. What will happen with message, I mean will it be processed by another available node or will be lost?

Answer 1

By default akka (and every other actor model framework) offers at-most-once delivery. This means that messages are send to actors using best effort guarantees - if they won't reach the target they won't be redelivered. This also means, that if message reached the target, but the process associated with it was interrupted before finishing, it won't be retried.

That being said, there are numerous ways to offer a redelivery between actors with various guarantees.

1. The simplest and most unreliable is to use Ask pattern in combination with i.e. Polly library. This however won't help if a node, on which sender lives, will die - simply because message are still stored only in memory.
2. The more reliable pattern is to use some event log/queue in front of your cluster (i.e. Azure Service Bus, RabbitMQ or Kafka). In this approach clients are sending requests via bus/queue, while the first actor in process pipeline is responsible for picking it up. If some actor or node in pipeline dies, the whole pipeline for that message is being retried.
3. Another idea is to use at-least-once delivery found in Akka.Peristence module. It allows you to use eventsourcing capabilities of persistent actors to persist messages. However IMO it requires a bit of exerience with Akka.

All of these approaches present at-least-once delivery guarantees, which means that it's possible to send the same message to its destination more than once. This also means, that your processing logic needs to acknowledge that by either an idempotent behavior or by recognizing and removing duplicates on the receiver side.