Hello! I've recently been experimenting with architectures that use CQRS / Cluster Sharding / Event Sourcing / DDD, and I have some beginner questions.
1) What is the best way to handle changing the structure of "events" as business requirements (or refactorings) change over time? As a brief example: // v1 case class ChangeName(id: String, first:String) extends Cmd case class NameChanged(id: String, first: String) extends Evt // v2 // now biz req. changes and we want to be able to change both first and last name // i can freely change the 'Cmd' because we don't persist it, however if I change the 'Evt' // i will get 'RecoveryFailure was caused by: java.io.InvalidClassException' during recoveries case class ChangeName(id: String, first:String, last: String) extends Cmd case class NameChanged(id: String, first: String, last: String) extends Evt It seems I could always just maintain 2 different events, e.g. NameChangedV1, and NameChangedV2, but that doesn't seem maintainable in the long run. Another approach I thought of is to save a snapshot before introducing any breaking changes to an "Event" interface before bringing down the app for upgrading. This seems hard - is it even possible with the 'ClusterSharding' extension to send a message (e.g. Snap) to all of its known id's in the system? How does changing events affect the "read" side of things in CQRS? ============ 2) When a node crashes, how can I make sure the "entry" gets restarted and completes processing when the node is brought back up? Here's a very brief example that may help clarify what I'm asking: // sent in the beginning of the flow from a front-end case class ProcessMessage(id: String, message: Message) extends Cmd case class MessageToBeProcessed(id: String, message: Message) extends Evt // sent at the end of the flow from another processor actor case class MessageProcessed(id: String, message: Message) extends Cmd case class MessageComplete(id: String, message: Message) extends Evt Let's say the node crashes right after I persist the event 'MessageToBeProcessed' and before I send it to a processor actor to complete processing. Will the 'ClusterSharding' extension re-activate this entry on another available node right away, or will it wait until that entry receives another "Cmd"? If it doesn't re-activate, is there a recommended approach to solve this? Thanks! -Chris -- >>>>>>>>>> Read the docs: http://akka.io/docs/ >>>>>>>>>> Check the FAQ: >>>>>>>>>> http://doc.akka.io/docs/akka/current/additional/faq.html >>>>>>>>>> Search the archives: https://groups.google.com/group/akka-user --- You received this message because you are subscribed to the Google Groups "Akka User List" group. To unsubscribe from this group and stop receiving emails from it, send an email to akka-user+unsubscr...@googlegroups.com. To post to this group, send email to akka-user@googlegroups.com. Visit this group at http://groups.google.com/group/akka-user. For more options, visit https://groups.google.com/d/optout.
