Event sourcing: making it functional (7)

TL;DR

It’s the seventh chapter of making event sourcing functional. In the last post we introduced the base class for aggregates capturing events and applying them on the state. In this entry we question this choice and make the final step to make our code more functional. Let’s go!

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Question your choices

Let’s again take a look at the method responsible for receiving the gateway response

paymentstate

This is a void method. It’s not a pure function though. It “returns” its result by passing emitted events to the apply function. We could rewrite it in the following manner:

paymentstate

Now you can probably see, that there are two methods in there. One that emits, the event, possibly taking the state into consideration and the second that applies the event. We can make the first one even more generic and make it return IEnumerable of events.

PaymentState.png

Final revelation

Now you can see, that ReceiveImpl is the true implementation of the aggregate action, but it does not require the aggregate class at all! It gets the state, the action parameter and returns events! The ReceiveGatewayResponse is now just an infrastructure code that applies events, which is totally unneeded! We no longer have the Payment aggregate! All we have is just a set of functions that acts on the state basis, accepts some parameters and returns events. We can make it even an extension to call it in an easier way.

paymentstate

This pure function is so easy to test. You can test it using Given, When, Then with events, but you can test it with regular unit testing as well.

Now you can see that we were able to split the Payment aggregate into set of functions, that accept a state and other needed parameters returning a enumerable of events. Is there anything easier to test and to work with? For sure you need to pay some tax by introducing storing and applying events on the infrastructure side, but still it’s worth it, as we change aggregates, from being classes to just sets of simple functions.

Summary

I hope you enjoyed this series and that I was able to encourage you to see aggregates and event sourcing from a bit more functional point of view. All the commenters, thank you for providing valuable feedback. See you soon!

Event sourcing: making it functional (6)

TL;DR

In the last entry we changed the Payment aggregate to modify state by raising events. The events weren’t captured though. In this post, we’ll change the aggregate to enable recording of these changes.

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Capturing events

The easiest way to capture events is to make them pass one additional method. We could provide one Apply method in the aggregate itself that beside applying the event it would. To make it usable in all the aggregates we could create a base class that would have this method. Let’s apply it to the Payment first

paymentstate

Now, we need the aggregate base class to hold the capture the applied events.

The base aggregate

We use the base class to track the changes and provide one point of entry to applying events. The dynamic is used to implement it fast. If you wanted to optimize, you could remove it by writing a custom dispatcher that calls directly a method from the derived aggregate.

paymentissued.png

When the action ends, a handler executing it calls GetEvents and gets all the events that were raised during this session. This enables you to write simple tests with a Given-When-Then approach and easily extract the changes for persistence purposes. On the other hand, we introduced the base class for the aggregate. What could be done more?

Summary

By introducing a base AggregateRoot class we enabled capturing all raised events. This simplified a little bit Payment aggregate itself, but introduced the base class. It’s time to move to the final part where we make it functional removing some code we’ve written so far.

Event sourcing: making it functional (5)

TL;DR

After defining events of the Payment aggregate it’s time to move on and work on applying these events.

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State

To apply events, the state of Payment will be extracted to a separate class. This should be a familiar pattern to all event sourcing practitioners, but let me show the code for clear understanding of our approach

paymentstate

How to construct the state

  1. There are no public setters. The state has readonly properties.
  2. All events are applied with an Apply method.
  3. The only way to change the state is to apply an event.
  4. No Apply throws an exception. The event has already happened and the state is just an accumulator of the changes.

New Payment Aggregate

The Payment aggregate now can be transformed to use this state, by removing all the state changes and raising/applying events in these places:

paymentstate

Summary

We know how to extract a state from the aggregate and apply all the events. Although Payment uses events, it does not store them in any form nor allows accessing them for processing. In the current shape the Payment isn’t much different from the previous version. We extracted a few event classes and the state, but we still can’t treat events as the first class citizen. we need to move one step further and we will do it in the next blog post.

Zapraszam na szkolenie z EventSourcingu: Warszawa, 16-03-2017

Zapraszam Was na moje szkolenie otwarte dotyczące EventSourcingu w .NET!

Tematem zdarzeń zajmuję się od dawna i nastał czas aby podzielić się tą wiedzą z Wami w formie szkolenia. Rejestracja odbywa się przez system Evenea na tej stronie i tam też znajdziesz wszystkie informacje.

Strona szkolenia

Jeżeli wahasz się i chciałbyś zobaczyć mnie w boju przed szkoleniem, zapraszam na moją jutrzejszą prezentację na Warszawskiej Grupie .NET, pt.

Keep Its Storage Simple Stupid,
an opinionated guide to NoSQL with Windows Azure Storage and Event Sourcing

Podczas niej omówię pewne aspekty projektu w modelu SaaS, w którym zastosowałem zdarzenia i Event Sourcing do obsłużenia złożonej domeny biznesowej. Dla uczestników spotkania przygotowałem też mały prezent związany ze szkoleniem 😉

Do zobaczenia!

Event sourcing: making it functional (4)

TL;DR

In the last entry we defined the aggregate implementation that we’ll work on. Let’s move forward and make it an event sourced aggregate.

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Events

The first and the most important event, is the fact of issuing the payment itself. Let’s define it in the following way:

paymentissued

The event contains all the data provided to the constructor of the payment aggregate in the past.

The next one is raised and applied when the payment is processed successfully. Let’s make it a class without any members. We just want to record a notion of success.

paymentissued

The last but not least is the one raised in case of the error. We make the errors explicit in here as we’d like to react to payments failure. One could model it in a different way, providing one event class, but then again, just follow this take on the payment problem.

paymentissued

Summary

We discovered three meaningful events:

  1. PaymentIssued
  2. PaymentProcessedSuccessfully
  3. PaymentProcessedWithFailure

In the next entry we will start rewriting the aggregate to use these.

Event sourcing: making it functional (3)

TL;DR

We’re on our journey to move from event sourcing oriented on the aggregates to a more functional approach. In the first entry, we went through some of the DDD building blocks. In the second, we defined an interface of the aggregate that we’ll work with. Before, going to the event sourced approached, let’s go through the aggregate’s implementation, somehow related to the Blue Book approach, without event sourcing.

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Payment aggregate!

Let’s dive straight into the code

paymentapi

We can see that a payment is issued for a user, with a specified payment method and a given amount of money. Yes, it’s simplistic, but bear with me, and follow its implementation for a while.

The other method is responsible for receiving the gateway response and applying it onto the aggregate. The status and the description are updated accordingly.

So far so good? In the next entry we’ll make this aggregate event sourced!

Event sourcing: making it functional (2)

TL;DR

In the last post we covered the basic concept of DDD, the aggregate and its root. Before we move on with the non-functional approach, let’s try to define at least one aggregate, that could be used as an example for future posts and code samples.

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A payment

For sake of providing an aggregate example let me reuse the payment example I introduced for the service kata purposes. The aggregate boundary is handling a single payment. This includes:

  • selecting payment method
  • registering an answer from the payment gateway
  • storing status

Having this we can easily imagine that the API of this aggregate could be similar to:

paymentapi

As you can see, there are no dummy setters or meaningless primitive properties. We exposed meaningful methods and properties for showing the part of the state we want to show. Additionally, the payment constructor shows all the parameters that are required to issue a payment.

With this example we can move forward and provide more insight into its initial implementation and refactoring it towards more functional design.