Kalix applications follow the Command Query Responsibility Segregation (CQRS) pattern (see https://developer.lightbend.com/docs/akka-guide/concepts/cqrs.html).

Kalix' Entities represent the command side where you change the state of your model in a strictly consistent and isolated manner. Kalix' Views represent the query side of your application. Views are optimized for reads and allow you to query your model by fields other than the entity identifier.

When implementing a View, you define what will be its internal state (your view model) and how you want to query it. The Query string defines which fields will be indexed and how the query will be executed.

The query is executed by the Kalix when a request is made to the View.

Views are updated in response to Event Sourced Entity events, Value Entity state changes or messages from a Topic.

Each incoming change is handled by a command handler. Command handlers are methods returning an kalix.scalasdk.view.View.UpdateEffect. The command handler is responsible for updating the View state.