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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Akka Persistence - CQRS-ES</title>
<meta name="description" content="Presentation for Trivento about Akka Persistence and how to use it for CQRS-ES and why I created the process framework.">
<meta name="author" content="Jeroen Gordijn">
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<body>
<div class="reveal">
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<div class="slides">
<section>
<h1>Akka Persistence</h1>
<h3>CQRS-Event Sourcing</h3>
<p>
<small>Why I created <a href="https://github.com/jgordijn/process">Process Framework</a></small>
</p>
</section>
<section>
<h2>Agenda</h2>
<ul>
<li>Refresh CQRS & Event Sourcing (ES)</li>
<li>Akka Persistence</li>
<li>Modelling a process</li>
<li>Process Framework</li>
</ul>
</section>
<!-- Example of nested vertical slides -->
<section>
<section>
<h2>CQRS</h2>
<p>Command Query Responsibility Segregation.</p>
<img src="/images/cqrs.png" height="460px"/>
<p style="position: absolute; bottom: 0px; left:0px; font-size: x-small">source: <a hlink="http://www.udidahan.com/2009/12/09/clarified-cqrs/">http://www.udidahan.com/2009/12/09/clarified-cqrs</a></p>
</section>
<section>
<h2>Event Sourcing</h2>
<img src="images/eventsourcing.png" height="480px"/>
<p style="position: absolute; bottom: 0px; left:0px; font-size: x-small">source: <a hlink="https://msdn.microsoft.com/en-us/library/jj591559.aspx">https://msdn.microsoft.com/en-us/library/jj591559.aspx</a></p>
</section>
<section>
<h2>Event Sourcing</h2>
<p align="left">
Remember everything
</p>
<ol>
<li>Restore state by loading events (if needed)</li>
<li>Do work</li>
<li>Save event</li>
<li>Update internal state with <em><strong><u>saved event</u></strong></em><br/>
Ensures written events can be read</li>
</ol>
<aside class="notes" data-markdown>
Crucial written events are used in update.
1. Forces enough info in events
2. Forces events can be read
</aside>
</section>
<section>
<h2>Pseudo code</h2>
<pre><code class="scala" data-trim contenteditable>
val result = doSomething()
val event = Done(result)
persist(event).andThen {
updateState(event)
}
def updateState = {
case evt: Done => globalState.something = evt.thing}
}
</code></pre>
</section>
<section data-markdown>
<script type="text/template">
## CQRS-ES: Perfect match?
1. Commands lead to events
2. ES just stores these events
3. View can use stored events
</script>
</section>
<section>
<h2>CQRS & ES</h2>
<img src="images/cqrs_es.png" height="520px"/>
<p style="position: absolute; bottom: 0px; left:0px; font-size: x-small">source: <a hlink="https://msdn.microsoft.com/en-us/library/jj591559.aspx">https://msdn.microsoft.com/en-us/library/jj591559.aspx</a></p>
</section>
</section>
<section data-markdown data-separator="^\n---\n$" data-separator-vertical="^\n--\n$" data-separator-notes="^Note:">
<script type="text/template">
## Akka Persistence
1. No longer Experimental in 2.4
2. PersistentActor
3. PersistentView
4. AtLeastOnceDelivery
--
## PersistentActor
* Command Processor backed by Event Store
* <span style="color:rgb(255, 198, 109);">`persistenceId`</span> (key in the store)
* <span style="color:rgb(255, 198, 109);">`reveiveCommand`</span> process incoming commands
* <span style="color:rgb(255, 198, 109);">`receiveRecover`</span> process events on recovery
* <span style="color:rgb(255, 198, 109);">`persist(`</span>`evt`<span style="color:rgb(255, 198, 109);">`){`</span> `fn: evt => Unit` <span style="color:rgb(255, 198, 109);">`}`</span>
Save event to eventstore and call `fn` when persisted
--
## PersistentActor
Note:
Geef duidelijk aan volgorde, Send, Persist, Update
Functionaliteit van recovery end receiveCommand
--
## PersistentView
* <span style="color:rgb(255, 198, 109);">`persistenceId`</span> key in the store of events
* <span style="color:rgb(255, 198, 109);">`viewId`</span> view id to persist snapshots
* <span style="color:rgb(255, 198, 109);">`receive`</span> process events (either persistent or not)
* Polls event store !!
--
## PersistentView
---
## Generic Akka usage
Note:
Stimulus from outside
--
## (Long) Running Process
* Actor is started with initial data
* Resond to sender
* Actor autonomously performs tasks to complete
* Process flow is important
--
## Simple Process
--
###Traditional Akka
Note:
Open Intellij
--
## Problems
* Business logic part of recovery
* Hard to see flow
* Huge file (Order flow in OHM 993 lines)
--
## Advanced Process
---
# Process Framework
--
## What is a process?
- Flow <!-- .element: class="fragment" data-fragment-index="1" -->
One step after another <!-- .element: class="fragment" data-fragment-index="1" -->
- Steps <!-- .element: class="fragment" data-fragment-index="2" -->
The logic of a process state <!-- .element: class="fragment" data-fragment-index="2" -->
--
## Step
1. Action
2. Result ⇒ Event
3. Update state
<strong>Event Sourcing</strong>: First persist (2.), then update state (3.)
--
## Process Flow
- Chain of steps
--
## Features
* Retry step until successful
* Execute steps in parallel
* Control Flow with `IfThenElse`
* Callback on steps (`onComplete`)
--
--
## Problems
* One big mutable state
This gives many `Option[T]` in state
* Isolation Recovery does not completely follow flow
Note:
Steps are not completely isolated. Playback is done in flow
--
## Future features
* Make more typesafe/functional
Steps should be a function from `State ⇒ State'`
* Better isolation of steps
* Flow visualizer (both compile- & runtime)
Note:
More typesafety will make compiler help with flow.
--
---
</script>
</section>
</div>
</div>
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