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<h1><a href="#entity" name="entity" class="anchor"><span class="anchor-link"></span></a>Entity</h1>
<pre class="prettyprint"><code class="language-scala">trait StateReader[F[_], S] {
def read: F[S]
}
trait EventWriter[F[_], E] {
def write(event: E, other: E*): F[Unit]
}
trait Entity[F[_], S, E] extends StateReader[F, S] with EventWriter[F, E]
</code></pre>
<p><a href="api/core/endless/core/entity/Entity.html" title="endless.core.entity.Entity"><code>Entity</code></a> is parametrized with entity state <code>S</code> and events <code>E</code>. It is a typeclass which represents reader-writer capabilities for <code>F</code> with event-sourcing semantics, i.e. the abilities to read current entity state from the context and persist events. <code>Entity</code> is used to describe entity behavior (e.g. <a href="https://github.com/endless4s/endless/tree/v0.31.0/example/src/main/scala/endless/example/logic/BookingEntityBehavior.scala">BookingEntityBehavior</a>). </p>
<h2><a href="#functional-event-sourcing" name="functional-event-sourcing" class="anchor"><span class="anchor-link"></span></a>Functional event sourcing</h2>
<p><em>Reader-writer</em> is a natural fit for describing event sourcing behavior: the monadic chain represents event sequencing and corresponding evolution of the state (see also <a href="https://pavkin.ru/aecor-part-2/">here</a> and <a href="https://www.youtube.com/watch?v=kDkRRkkVlxQ">here</a>).</p>
<p>Advantages of this abstraction are:</p>
<ul>
<li>the command is the call and the reply is simply the final resulting value in the monadic chain, there are no explicit representations</li>
<li>maximal composability since it’s just <code>flatMap</code> all the way down, making it easier to work with a reduced set of events</li>
<li><code>read</code> always provides the up-to-date state and event folding happens transparently behind the scenes</li>
<li>pure & side-effect-free logic that is easy to test</li>
<li>the algebra interpreter does not support asynchronicity or other such effects lower in the hierarchy, which makes it impossible to accidentally introduce them in the command handling logic</li>
</ul><div class="callout warning "><div class="callout-title">Responsivity</div>
<p>When defining event-sourced entities, it is considered best practice to process commands and formulate a reply quickly as it makes the system responsive. Long-running processes should only initiate as the result of events, which also has the added benefit that they can be restored upon recovery or be driven by a projection. See <a href="effector.html">Effector</a> to find out how to describe side effects with <em>endless</em>. </p></div><div class="callout note "><div class="callout-title">About performance</div>
<p>When composing a sequence of computations which has multiple <em>writes</em> with interspersed <em>reads</em>, the state is folded before each read by the interpreter. This is necessary to provide a constant version of the state during interpretation.</p>
<p>This is an operation that the runtime (Pekko/Akka) will also do behind the scenes when evolving the entity state. Redundant invocations of the folding function can therefore occur with the elevated monadic abstraction. </p>
<p>However, in most cases this overhead is insignificant:</p>
<ul>
<li>event application needs to be as fast and simple as possible as it is of course invoked repeatedly for each event during recovery</li>
<li>any possible redundant folds only happens upon reception of a command, not upon recovery</li>
<li>command handling behavior with multiple interspersed reads and writes is less frequent than the more common read-then-write pattern</li>
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