How Event Sourcing in Akka Persistent Actors Works

by Jannik Arndt

While the Akka documentation is incredibly well written, it has surprisingly few images. Since I visualize concepts to remember them, here is my take on how Event Sourcing in Akka Persistence works:

The text I took this from is

A persistent actor receives a (non-persistent) command which is first validated if it can be applied to the current state. Here validation can mean anything, from simple inspection of a command message’s fields up to a conversation with several external services, for example. If validation succeeds, events are generated from the command, representing the effect of the command. These events are then persisted and, after successful persistence, used to change the actor’s state. When the persistent actor needs to be recovered, only the persisted events are replayed of which we know that they can be successfully applied. In other words, events cannot fail when being replayed to a persistent actor, in contrast to commands. Event sourced actors may also process commands that do not change application state such as query commands for example., emphasis mine


I recently created a wonderful bug.

This is a basic example how to implement oAuth2 using Akka HTTP and Scala. It provides three endpoints. From the clients point of view:

  • / — publicly accessible, returns “Welcome!”,
  • /auth — provide your username and password, receive an access_token in return,
  • /api — secured by oAuth, send the access_token in a header to gain access.

From the server’s point of view:

  • / — publicly accessible, do nothing,
  • /auth — receive basic auth credentials, verify they’re in the list of known credentials, create an access_token, return it,
  • /api — receive authorization header, check if access_token is in list of valid tokens.

Since oAuth tokens are short lived, the server also has to invalidate expired tokens.

Getting a Akka HTTP-based backend up and running on Heroku for free can be done in less then 30 minutes — if you know the tricks.