Akka persistence typed

Introduction

A persistent actor is an event sourced actor. It receives a (non-persistent) command which is first validated.

If validation succeeds, events are generated from the command, representing the effect of the command. These events are then persisted and used to change the actor’s state.

When the event sourced actor needs to be recovered, only the persisted events are replayed as they can be successfully applied to recreate the state of the actor.

Event sourced actors may also process commands that do not change application state (such as query commands).

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Application

This application showcases the usage of a typed persistent actor that fetches top, latest or best news from hacker news website using its API.

As shown in the diagram below, the application consists of two actors; namely,

• NewsApplication that responds to FetchNews message. This actor is the entry point and guardian actor to the actor system. Upon creation, it also creates its child actor EventSourcedActor and sends 5 FetchNews messages to itself.
• EventSourcedActor that responds to commands (FetchTopStories, FetchNewStories and FetchBestStories) and generates events (TopStoriesFetched, NewStoriesFetched and BestStoriesFetched) that gather the news from hacker news, provided the state of the persistent actor does not contain the category of news for that day.

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