2-akka-persistence.md

We now have a basic chat program, but for a "real" application we usually need to manage persistent data. Today we will do just that, using the akka-persistence module to save our data to Cassandra. Along the way we'll explore the idea of event-sourcing and see how it differs from saving state in a DB like we're used to. We will also learn about Akka's configuration library and see how we can use it to configure Akka or for our own custom configuration.

Task:

Add some necessary new features to your chat application:

• Add user management. The server should manage and persist usernames, passwords and nicknames.
• Save the channels users are subscribed to, to avoid the need to rejoin the same channels every time a user logs in.
• Save the channel's history and send it to clients when they join the channel.

Solution Walk-through:

• Add the appropriate configuration in application.conf:
  • Set both the journal and snapshot-store to use the Cassandra plugin (you can start with the in-memory persistence plugin for now if it's easier, just remember it doesn't actually persist anything).
  • Configure the journal and snapshot-store with the appropriate contact points(localhost), port(9042) and keyspace.
  • Set the journal and snapshot-store to auto-start.
• Run the Cassandra Docker (or use the provided docker-compose).
• Run the server to allow the persistence plugin to create the keyspace in Cassandra.
• Make your UserDetailsActor persistent, and persist user registration events.
• Make your ChannelActor persistent, and persist users' join/leave events.
• Make your ChannelActor persist all messages sent to the channel. Whenever a user joins the channel for the first time replay the channel history for them.
• Make your ChannelManagerActor persist channel creation events, and create the channels when recovering. What would happen if we don't do this?