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</div><div id='content' class='page-1'><div class='row'><div class='row-fluid'><div class='col-lg-3'><ul class='nav-sidebar'><li><a href='./'>Blog index</a></li><li class='active'><a href='#doc'>Using Apache Pekka actors and GPars actors with Groovy</a></li><li><a href='#_the_example' class='anchor-link'>The example</a></li><li><a href='#_a_pekka_implementation_in_groovy' class='anchor-link'>A Pekka implementation in Groovy</a></li><li><a href='#_a_gpars_implementation_in_groovy' class='anchor-link'>A GPars implementation in Groovy</a></li><li><a href='#_conclusion' class='anchor-link'>Conclusion</a></li></ul><br/><ul class='nav-sidebar'><li style='padding: 0.35em 0.625em; background-color: #eee'><span>Related posts</span></li><li><a href='./gpars-meets-virtual-threads'>GPars meets Virtual Threads</a></li><li><a href='./groovy-list-processing-cheat-sheet'>Groovy List Processing Cheat Sheet</a></li></ul></div><div class='col-lg-8 col-lg-pull-0'><a name='doc'></a><h1>Using Apache Pekka actors and GPars actors with Groovy</h1><p><span>Author: <i>Paul King</i></span><br/><span>Published: 2022-07-17 11:24PM</span></p><hr/><div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p><span class="image right"><img src="https://pekko.apache.org/assets/images/pekko_logo.png" alt="pekka logo" width="100"></span>
<a href="https://pekko.apache.org/">Apache Pekka</a> is a project undergoing <a href="https://incubator.apache.org/">incubation</a> at the Apache Software Foundation.
It is an Apache licensed fork of the Akka project (based on Akka version 2.6.x) and provides a
framework for building applications that are concurrent, distributed, resilient and elastic.
Pekko provides high-level abstractions for concurrency based on actors,
as well as additional libraries for persistence, streams, HTTP, and more.
It provides Scala and Java APIs/DSLs for writing your applications. We&#8217;ll be using the latter.
We&#8217;ll look at just one example of using Pekka actors.</p>
</div>
<div class="paragraph">
<p><span class="image right"><img src="img/gpars_logo.png" alt="gpars" width="180"></span>
By way of comparison, we&#8217;ll also be looking at <a href="http://www.gpars.org/">GPars</a>,
a concurrency library for Java and Groovy with support for actors, agents,
concurrent &amp; parallel map/reduce, fork/join, asynchronous closures, dataflow, and more.
A <a href="https://groovy.apache.org/blog/gpars-meets-virtual-threads">previous blog post</a>
looks at additional features of GPars and how to use it with virtual threads.
Here, we&#8217;ll just look at the comparable actor features for our Pekka example.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_the_example">The example</h2>
<div class="sectionbody">
<div class="paragraph">
<p>A common first example involving actors involves creating two actors where one actor
sends a message to the second actor which sends a reply back to the first.
We could certainly do that, but we&#8217;ll use a slightly more interesting example involving three
actors. The example comes from the
<a href="https://pekko.apache.org/docs/pekko/current/typed/actors.html#first-example">Pekka documentation</a>
and is illustrated in the following diagram (from the Pekka documentation):</p>
</div>
<div class="paragraph">
<p><span class="image"><img src="https://pekko.apache.org/docs/pekko/current/typed/images/hello-world2.png" alt="actors in our system - from pekka documentation"></span></p>
</div>
<div class="paragraph">
<p>The system consists of the following actors:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>The <code>HelloWorldMain</code> actor creates the other two actors and sends
an initial message to kick off our little system. The initial message
goes to the <code>HelloWorld</code> actor and gives the <code>HelloWorldBot</code> as the reply address.</p>
</li>
<li>
<p>The <code>HelloWorld</code> actor is listening for <code>Greet</code>
messages. When it receives one, it sends a <code>Greeted</code> acknowledgement back to a reply address.</p>
</li>
<li>
<p>The <code>HelloWorldBot</code> is like an echo chamber. It returns any message it receives.
This would potentially be an infinite loop, however, the actor has a parameter
to tell it the maximum number of times to echo the message before stopping.</p>
</li>
</ul>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_a_pekka_implementation_in_groovy">A Pekka implementation in Groovy</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This example uses Groovy 4.0.13, JDK11 and Pekka 1.0.0, but should also
run using later JDK versions.</p>
</div>
<div class="paragraph">
<p>The Pekka documentation gives Java and Scala implementations.
You should notice that the Groovy implementation is similar to the Java one
but just a little shorter. The Groovy code is a little more complex than the
equivalent Scala code. We could certainly use Groovy meta-programming to
simplify the Groovy code in numerous ways but that is a topic for another day.</p>
</div>
<div class="paragraph">
<p>Here is the code for <code>HelloWorld</code>:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">class HelloWorld extends AbstractBehavior&lt;HelloWorld.Greet&gt; {
static record Greet(String whom, ActorRef&lt;Greeted&gt; replyTo) {}
static record Greeted(String whom, ActorRef&lt;Greet&gt; from) {}
static Behavior&lt;Greet&gt; create() {
Behaviors.setup(HelloWorld::new)
}
private HelloWorld(ActorContext&lt;Greet&gt; context) {
super(context)
}
@Override
Receive&lt;Greet&gt; createReceive() {
newReceiveBuilder().onMessage(Greet.class, this::onGreet).build()
}
private Behavior&lt;Greet&gt; onGreet(Greet command) {
context.log.info "Hello $command.whom!"
command.replyTo.tell(new Greeted(command.whom, context.self))
this
}
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>First we define <code>Greet</code> and <code>Greeter</code> records to have strong typing for the messages in our system.
We then define the details of our actor. A fair bit of this is boilerplate. The interesting part
is inside the <code>onGreet</code> method. We log the message details before sending back the <code>Greeted</code> acknowledgement.</p>
</div>
<div class="paragraph">
<p>The <code>HelloWorldBot</code> is similar. You should notice some state variables which keep an
invocation counter and a maximum number of invocations before terminating:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">class HelloWorldBot extends AbstractBehavior&lt;HelloWorld.Greeted&gt; {
static Behavior&lt;HelloWorld.Greeted&gt; create(int max) {
Behaviors.setup(context -&gt; new HelloWorldBot(context, max))
}
private final int max
private int greetingCounter
private HelloWorldBot(ActorContext&lt;HelloWorld.Greeted&gt; context, int max) {
super(context)
this.max = max
}
@Override
Receive&lt;HelloWorld.Greeted&gt; createReceive() {
newReceiveBuilder().onMessage(HelloWorld.Greeted.class, this::onGreeted).build()
}
private Behavior&lt;HelloWorld.Greeted&gt; onGreeted(HelloWorld.Greeted message) {
greetingCounter++
context.log.info "Greeting $greetingCounter for $message.whom"
if (greetingCounter == max) {
return Behaviors.stopped()
} else {
message.from.tell(new HelloWorld.Greet(message.whom, context.self))
return this
}
}
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>The interesting logic is in the <code>onGreeted</code> method. We increment the counter and either stop,
if we have reached the maximum count threshold, or echo back the message contents to the sender.</p>
</div>
<div class="paragraph">
<p>Let&#8217;s have a look at the final actor:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">class HelloWorldMain extends AbstractBehavior&lt;HelloWorldMain.SayHello&gt; {
static record SayHello(String name) { }
static Behavior&lt;SayHello&gt; create() {
Behaviors.setup(HelloWorldMain::new)
}
private final ActorRef&lt;HelloWorld.Greet&gt; greeter
private HelloWorldMain(ActorContext&lt;SayHello&gt; context) {
super(context)
greeter = context.spawn(HelloWorld.create(), 'greeter')
}
@Override
Receive&lt;SayHello&gt; createReceive() {
newReceiveBuilder().onMessage(SayHello.class, this::onStart).build()
}
private Behavior&lt;SayHello&gt; onStart(SayHello command) {
var replyTo = context.spawn(HelloWorldBot.create(3), command.name)
greeter.tell(new HelloWorld.Greet(command.name, replyTo))
this
}
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>There is a <code>SayHello</code> record, to act as a strongly typed incoming message.
The <code>HelloWorldMain</code> actor creates the other actors.
It creates one <code>HelloWorld</code> actor which is the <em>greeter</em> target of subsequent messages.
For each incoming <code>SayHello</code> message, it creates a <em>bot</em>, then sends a message
to the <em>greeter</em> containing the <code>SayHello</code> payload and telling it to reply to the <em>bot</em>.</p>
</div>
<div class="paragraph">
<p>Finally, we need to kick off our system. We create the <code>HelloWorldMain</code> actor and
send it two messages:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var system = ActorSystem.create(HelloWorldMain.create(), 'hello')
system.tell(new HelloWorldMain.SayHello('World'))
system.tell(new HelloWorldMain.SayHello('Pekko'))</code></pre>
</div>
</div>
<div class="paragraph">
<p>The log output from running the script will look similar to this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>[hello-pekko.actor.default-dispatcher-3] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Hello World!
[hello-pekko.actor.default-dispatcher-3] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Hello Pekko!
[hello-pekko.actor.default-dispatcher-5] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Greeting 1 for World
[hello-pekko.actor.default-dispatcher-3] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Greeting 1 for Pekko
[hello-pekko.actor.default-dispatcher-3] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Hello World!
[hello-pekko.actor.default-dispatcher-3] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Hello Pekko!
[hello-pekko.actor.default-dispatcher-5] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Greeting 2 for World
[hello-pekko.actor.default-dispatcher-3] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Hello World!
[hello-pekko.actor.default-dispatcher-3] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Greeting 3 for World
[hello-pekko.actor.default-dispatcher-6] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Greeting 2 for Pekko
[hello-pekko.actor.default-dispatcher-6] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Hello Pekko!
[hello-pekko.actor.default-dispatcher-6] INFO org.codehaus.groovy.vmplugin.v8.IndyInterface - Greeting 3 for Pekko
[hello-pekko.actor.default-dispatcher-6] INFO org.apache.pekko.actor.CoordinatedShutdown - Running CoordinatedShutdown with reason [ActorSystemTerminateReason]</pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_a_gpars_implementation_in_groovy">A GPars implementation in Groovy</h2>
<div class="sectionbody">
<div class="paragraph">
<p>This example uses Groovy 4.0.13, JDK11 and GPars 1.2.1, but should
run with any JDK 8+ version.</p>
</div>
<div class="paragraph">
<p>We&#8217;ll follow the same conventions for strongly typed messages in our GPars example.
Here are our three message containers:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">record Greet(String whom, Actor replyTo) { }
record Greeted(String whom, Actor from) {}
record SayHello(String name) { }</code></pre>
</div>
</div>
<div class="paragraph">
<p>Now we&#8217;ll define our <code>helloWorld</code> actor:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">helloWorld = actor {
loop {
react { Greet command -&gt;
println "Hello $command.whom!"
command.replyTo &lt;&lt; new Greeted(command.whom, helloWorld)
}
}
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Here, we are using GPars Groovy continuation-style DSL for defining actors.
The <code>loop</code> indicates that the actor will loop continually.
When we receive the <code>Greet</code> message, we log the details to stdout and
send the acknowledgement.</p>
</div>
<div class="paragraph">
<p>If we don&#8217;t want to use the DSL syntax, we can use the related classes directly.
Here we&#8217;ll define a <code>BotActor</code> using this slightly more verbose style.
It shows adding the state variables we need for tracking the invocation count:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">class BotActor extends DefaultActor {
int max
private int greetingCounter = 0
@Override
protected void act() {
loop {
react { Greeted message -&gt;
greetingCounter++
println "Greeting $greetingCounter for $message.whom"
if (greetingCounter &lt; max) message.from &lt;&lt; new Greet(message.whom, this)
else terminate()
}
}
}
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Our main actor is very simple. It is waiting for <code>SayHello</code> messages, and when it receives one,
it sends the payload to the helloWorld greeter telling it to reply to a newly created <em>bot</em>.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">var main = actor {
loop {
react { SayHello command -&gt;
helloWorld &lt;&lt; new Greet(command.name, new BotActor(max: 3).start())
}
}
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Finally, we start the system going by sending some initial messages:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="prettyprint highlight"><code data-lang="groovy">main &lt;&lt; new SayHello('World')
main &lt;&lt; new SayHello('GPars')</code></pre>
</div>
</div>
<div class="paragraph">
<p>The output looks like this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre>Hello World!
Hello GPars!
Greeting 1 for World
Greeting 1 for GPars
Hello World!
Hello GPars!
Greeting 2 for World
Hello World!
Greeting 2 for GPars
Hello GPars!
Greeting 3 for World
Greeting 3 for GPars</pre>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_conclusion">Conclusion</h2>
<div class="sectionbody">
<div class="paragraph">
<p>We have had a quick glimpse at using actors with Apache Pekka and GPars.</p>
</div>
<div class="paragraph">
<p>The sample code can be found here:</p>
</div>
<div class="paragraph">
<p><a href="https://github.com/paulk-asert/groovy-pekka-gpars" class="bare">https://github.com/paulk-asert/groovy-pekka-gpars</a></p>
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