Tour of Akka Typed: Event Sourcing

Published on 7 October 2019 , last updated on 7 October 2019

Welcome to the fourth part of the Akka Typed series. In the first part we had a look at the the raison d’être of Akka Typed and what advantages it has over the classic, untyped API. In the second part we introduced the concepts of message adapters, the ask pattern and actor discovery. In the third part, we covered one of the core concepts of Actor systems: supervision and failure recovery. In this part of the series we’ll explore one of the most popular use-cases for Akka: event sourcing. We’ll assume that you are already familiar with the concept of event sourcing – if not, first read about it to get some context.

In Akka Classic, event sourcing are implemented using persistent actors, which work as follows:

Persistent Actor in Akka Classic – Source: Reactive Web Applications

The flow is quite straightforward:

Whilst the concepts of Commands and Events were already present in Akka Classic (for example, the command handler is called receiveCommand), the protocol was not enforcing this distinction per-se.

With the Akka Typed API, this has now changed: many of the best practices and patterns used in combination with persistent actors are now offered directly through the API, which makes it easier to implement event-sourced actors. Let’s take a closer look!

EventSourcedBehavior

Since in Akka Typed, everything is a behavior, it makes sense that persistence too is represented via a special type of behavior.

In the previous article of the series, we used a dummy storage for credit cards (that was crashing all the time) to demonstrate supervision. Let’s now implement a real version using the new EventSourcedBehavior:

As we can see from the code above, the concepts of Command and Event are now formalized. They of course need not be called this way, but the API will expect two types to point to these concepts in your code. Additionally, the type of the state (and an “empty” state from which to begin with) needs to be provided too – as we will see, event handlers need to return a new version of the in-memory state that represents the modified state after an event has been applied to it.

Next to formalizing the protocol of persistent actors, the Akka Typed API also formalizes the command and event handlers in order to specify what they can, or rather should, do.

Commands producing effects

A Command handler takes in the current state and a new Command, and returns an Effect:

An Effect can be any of the following:

Say for example that we are making it possible to add credit cards to our storage using a new AddCreditCard command and the corresponding CreditCardAdded:

In the event of a credit card being added, the corresponding event handler is rather straight-forward:

“But wait”, I hear you object, “how can I now reply to the sender to tell them that the credit card was successfully added, or log a warning if it was already there?”. That’s now also supported by the API as SideEffect-s.

Commands producing side-effects

Next to persisting events (or not), command handlers have the ability to specify one or more actions to be executed if the initial effect is executed successfully. Taking our example above, we could decide to log a warning if an already existing card was asked to be added again:

It is possible to chain side-effects if need be:

These chained side-effects will then be executed in sequence.

Arguably, the most common side-effect in persistent actors is to reply to the actor that initiated the command so as to enable at-least-once delivery and exactly-once effect semantics. The Akka Team thought of this and promoted the concept of a reply to a top-level concern in the Akka Typed API.

Commands producing enforced replies

In the spirit of Akka Typed leveraging the compiler in order to prevent programming errors, there’s now a way to ensure that actors with persistent behavior reply to their clients after an event has been persisted (or in other cases, such as validation errors).

In order to be able to use this feature, we’ll need to extend our top-level protocol definition so as to include the concept of replies into it:

Command-s must now specify a reply type. Typically there will be a handful of replies for each operation a persistent actor supports, such as in our case:

At first sight, the protocol may now seem to get a little bit verbose, but then again chances are that you’ll require this amount of messages anyway in a real-world system – so it’s even the better if they are organised and checked by the compiler.

In order to use the feature we’ll need to use the withEnforcedReplies factory of EventSourcedBehavior:

So for example, if we now omit to reply to the sender that the credit card has been successfully added, we’ll get the following compile-time error:

(granted, the compilation error may be a little bit cryptic – but the mention of ReplyEffect might be a good hint as to what is going on)

Replies (or the omission thereof) can be produced via the Effect.reply, Effect.noReply, Effect.thenReply and Effect.thenNoReply methods:

Writing large persistent behaviors

Our example so far is pretty simple: the state is “flat” in the sense that there is only really one state of the credit card storage with a more or less filled map of known cards. Adding / removing or querying this state is not all too complicated, and for the time being the amount of supported operations on the storage is rather limited.

In real life, however, persistent entities tend to have many subtleties and to grow in terms of supported operations. It is therefore a good idea to stick to a few simple rules that make the code more readable. The Akka Typed documentation describes this with an accordingly involved example. I’d definitely recommend you checking it out in order to get a grasp of how to deal with a more complex domain. Whilst the recommendations are a matter of taste, it turns out that organizing the code as proposed has its advantages insofar as they follow the teachings of domain driven design, where the state represents the core domain object.

We’ll be exploring the two most impactful recommendations by applying them to our example, which we’ll enrich with the capability of looking up credit cards by id – as said, make sure to check out the style guide to get an idea of what a more advanced example looks like.

Defining event handlers in the state

Event handlers are not side-effecting and therefore the only thing they do is to alter the in-memory state based on an event. As such, keeping the handlers in the state is something that has a quite natural feel to it:

Defining command handlers in the state

Taking this idea one step further, it is also possible to push the command handlers inside of the state:

The result of this style is that the technical behavior definition is now quite straight-forward:

All the business logic is contained in the Storage state object, which, in combination with the actor protocol, is really the core of what a persistent entity should all be about.

This is it for this article of the series. Note that we haven’t touched all aspects of persistent behaviors as that would be too much to cover. If you are just getting started with Akka, make sure to think twice about serialization and schema evolution as these concerns are in my experience some of the more important aspects of developing production applications with Akka Persistence.

As usual, here’s the concept comparison table between Akka Classic and Akka Typed:

extends AbstractActorEventSourcedBehavior[Command, Event, State](...)
def receiveCommand: ReceivecommandHandler = (state, cmd) => // returns Effect
def receiveRecover: ReceiveeventHandler = (state, evt) => // returns State
persist(event) { event => // callback ... }- Effect.persist(event)
- eventHandler = (state, evt) => // returns State

- Effect.andThen() side-effects
RecoveryCompleted messageRecoveryCompleted signal
EventSourcedBehavior.withEnforcedReplies

The code for this article can be found on GitHub.


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