Write You An Actor (System) For Great Good!

Write You an
Actor (System) for
Great Good!
(with JBang, JDK 19, records, pattern
matching and virtual threads!)

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Andrea
Peruffo
@and_prf
Edoardo
Vacchi
@evacchi

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This Is Not A Talk
About Actor
Systems

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This Is Not Only A Talk
About Actor
Systems

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evacchi.github.io

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@evacchi
About Me
Edoardo Vacchi @evacchi
● PL/DSL Research @ UniMi
● R&D @ UniCredit Bank
● Drools, Kogito @ Red Hat

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gist.github.com/viktorklang/2362563

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Has Java reached
feature parity with
10-years ago Scala?

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package io.github.evacchi;
import java.util.concurrent.*;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.function.Function;
import static java.lang.System.out;
public interface Actor {
interface Behavior extends Function {}
interface Effect extends Function {}
interface Address { Address tell(Object msg); }
static Effect Become(Behavior like) { return old -> like; }
static Effect Stay = old -> old;
static Effect Die = Become(msg -> { out.println("Dropping msg [" + msg + "] due to severe case of death."); return Stay; });
record System(ExecutorService executorService) {
public Address actorOf(Function initial) {
abstract class AtomicRunnableAddress implements Address, Runnable
{ final AtomicInteger on = new AtomicInteger(0); }
var addr = new AtomicRunnableAddress() {
final ConcurrentLinkedQueue mb = new ConcurrentLinkedQueue<>();
Behavior behavior = m -> (m instanceof Address self) ? Become(initial.apply(self)) : Stay;
public Address tell(Object msg) { mb.offer(msg); async(); return this; }
public void run() {
try { if (on.get() == 1) { var m = mb.poll(); if (m!=null) { behavior = behavior.apply(m).apply(behavior); } }}
finally { on.set(0); async(); }}
void async() {
if (!mb.isEmpty() && on.compareAndSet(0, 1)) {
try { executorService.execute(this); }
catch (Throwable t) { on.set(0); throw t; }}}
};
return addr.tell(addr);
}
}
}
evacchi.github.io

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@evacchi
What You Will Bring Home
● Learn new Java 19 features
● Learn an interesting concurrency model
● Learn how to run code snippets with JBang

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@evacchi
What You Will Bring Home
● Learn new Java 19 features
● Learn an interesting concurrency model
● Learn how to run code snippets with JBang
● Learn how to write concise Java program
with tricks that will make your coworkers mad

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Agenda

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@evacchi
Agenda
● What is an actor?
● Comparison between actor runtimes
● Features of Java we will be using
☕
● Examples of actors (live coding)
● An actor runtime (live coding)
● A typed actor runtime (live coding)
☕
● An actor-based chat (live coding)
● Virtual Threads and Project Loom
● A Loom-based chat (live coding)
Live coding with

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Actor Definition

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@evacchi
Actor Definition
An actor is a computational entity that, in response to a message it
receives, can concurrently:
● send a finite number of messages to other actors;
● create a finite number of new actors;
● designate the behavior to be used for the next message it receives.
"Everything" is an actor.

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@evacchi
Actor Model
● Modelling stateful systems
● Distributed systems (e.g. leveraging location transparency)
● IoT (e.g. Digital twins)

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@evacchi
Actor Model
● Concurrency Model
● Simple
● "Single-Thread" reasoning
● Message-based, "Mailbox"

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@evacchi
Implementation
Function + State + Message Queue

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@evacchi
🖂 ➡ 🖂 🖂 🖂 🖂 🖂 🖂
Implementation
Private State
Behavior
mailbox

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@evacchi
Implementation
🖂 🖂 🖂 🖂 🖂 🖂 🖂
Private State
Behavior

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@evacchi
Implementation
🖂 🖂 🖂 🖂 🖂 🖂
Private State
Behavior
🖂

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@evacchi
Implementation
🖂 🖂 🖂 🖂 🖂 🖂
Private State
Behavior
🖂
📤

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@evacchi
Implementation
🖂 🖂 🖂 🖂 🖂 🖂
Private State
Behavior
🖂 🖂 ➡

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@evacchi
Messages vs Methods
● you send a message to an actor
● you invoke a method on an object
● however in Smalltalk you send "messages"
to objects

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@evacchi
Messages vs Methods
● you send a message to an actor
● you invoke a method on an object
● however in Smalltalk you send "messages"
to objects
● in fact, some people think that actors
are "proper" OOP as envisioned by
Alan Kay

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https://twitter.com/joeerl/status/1107348549132533760
@evacchi

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@evacchi

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@evacchi
Spawning Actors

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@evacchi
Vending Machines (FSM)

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Actor Runtimes

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@evacchi
When Scala Roamed The World
● Java 8 in 2014
● Context: JDK 5/6/7
○ Mainstream statically typed OOP programming languages
were verbose (C++/Java/C#)
○ Dynamic languages were more concise (Python/Ruby/Groovy)
● Enter Scala
○ Static typing
○ Dynamic feel / conciseness
○ Actor library

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@evacchi
When Scala Roamed The World
● Context: JDK 5/6/7
○ Mainstream statically typed OOP programming languages
were verbose (C++/Java/C#)
○ Dynamic languages were more concise Python/Ruby/Groovy
● Enter Scala
○ Static typing
○ Dynamic feel / conciseness
○ Actor library

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@evacchi
Akka (Scala)
case object Ping
class Pingponger extends Actor {
var count = 10
def receive: Any => Unit = {
case Ping =>
println(s"${self.path} received ping, count down $count")
if (this.count > 0) {
this.count = this.count - 1
sender() ! Ping
}
}
}
val system = ActorSystem("pingpong")
val pinger = system.actorOf(Props[Pingponger](), "pinger")
val ponger = system.actorOf(Props[Pingponger](), "ponger")
pinger.tell(Ping, ponger)

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@evacchi
Akka (Scala)
case class Ping(count: Int)
def receive: Any => Unit = {
case Ping(count) =>
if (count > 0) {
sender() ! Ping(count - 1)
}
}
}
pinger.tell(Ping(10), ponger)

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@evacchi
Akka (Scala)
case class Ping(count: Int)
def receive(msg: Any): Unit =
msg match {
case Ping(count) =>
if (count > 0) {
sender() ! Ping(count - 1)
}
}
}
pinger.tell(Ping(10), ponger)

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@evacchi
Akka + Java
public class MyUntypedActor extends UntypedActor {
LoggingAdapter log = Logging.getLogger(getContext().system(), this);
public void onReceive(Object message) throws Exception {
if (message instanceof Ping) {
Ping p = (Ping) message;
if (p.count() > 0) {
log.info("Received ping : " + p.count());
getSender().tell(new Ping(p.count() - 1), getSelf());
}
}
}

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@evacchi
Scala
Groovy
Ruby
Python
Java

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@evacchi
Erlang
pingponger(Name, Main) ->
receive
{ping, Count, Ping_PID} ->
io:format("~s received ping, count down ~w~n", [Name, Count]),
if Count > 0 ->
Ping_PID ! {ping, Count - 1, self()},
pingponger(Name, Main);
true ->
exit ("done")
end
end.
main(_) ->
Ping1_PID = spawn(fun() -> pingponger("pinger", self()) end),
Ping2_PID = spawn(fun() -> pingponger("ponger", self()) end),
Ping1_PID ! {ping, 10, Ping2_PID}.

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@evacchi
Elixir
defmodule Main do
def pingponger(name) do
receive do
{:ping, count, sender} ->
IO.puts "#{name} received ping, count down #{count}"
if count > 0 do
send sender, {:ping, count - 1, self()}
pingponger(name)
else
:ok
end
end
end
end
pid1 = spawn fn -> Main.pingponger("pinger") end
pid2 = spawn fn -> Main.pingponger("ponger") end
send pid1, {:ping, 10, pid2}

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@evacchi
Scala
Groovy
Ruby
Python
Java
Erlang
Elixir

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@evacchi
Akka + Java
if (message instanceof Ping) {
Ping p = (Ping) message;
if (p.count() > 0) {
log.info("Received ping : " + p.count());
getSender().tell(new Ping(p.count() - 1), getSelf());
}
}
} Can we improve over this?

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Tools Of The Trade

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@evacchi
(Ab)using Language Defaults To Reduce Noise
public class Main {
public static final String HELLO = "hello world";
public static void main(String[] args) {
System.out.println(HELLO);
}
}

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@evacchi
public interface Main {
}
}

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@evacchi
import static java.lang.System.*;
public interface Main {
}
}

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@evacchi
import static java.lang.System.*;
interface Main {
String HELLO = "hello world";
static void main(String... args) { out.println(HELLO); }
}

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https://openjdk.org/projects/amber/design-notes/on-ramp

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public class HelloWorld {
System.out.println("Hello World");
}
}
@evacchi

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public class HelloWorld {
System.out.println("Hello World");
}
}
void main() {
println("Hello World");
}
@evacchi

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Is This
Code-Golfing?
me
recent Java
syntax

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@evacchi
More “Code Golfing”
public class Auth {
private final String login;
private final String token;
public Auth(String login, String token) {
this.login = login;
this.token = token;
}
public String login() { return this.login; }
public String token() { return this.token; }
public boolean equals(Object o) { ... }
public int hashCode() { return Objects.hash(login, token); }
public String toString() { ... }
}
public class Anon {
private final String email;
public Anon(String email) { this.email = email; }
public String email() { return this.email; }
public int hashCode() { ... }
}

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@evacchi
if (message instanceof Auth) {
Auth a = (Auth) message;
check(a.login());
if (validate(a.login(), a.token())) {
// grant full access
} else {
// error
}
} else if (message instanceof Anon) {
Anon a = (Anon) message;
// grant guest access
} else
unhandled(message);
}

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@evacchi
Enhanced instanceof
if (message instanceof Auth a) {
check(a.login());
if (validate(a.login(), a.token())) {
} else {
// error
}
} else if (message instanceof Anon a) {
} else
unhandled(message);
}
pattern matching

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@evacchi
public class Auth {
private final String login;
private final String token;
public Auth(String login, String token) {
this.login = login;
this.token = token;
}
public String login() { return this.login; }
public String token() { return this.token; }
public int hashCode() { return Objects.hash(login, token); }
}
public class Anon {
private final String email;
public Anon(String email) { this.email = email; }
public String email() { return this.email; }
public int hashCode() { ... }
}

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@evacchi
Records
public record Auth(String login, String token) {}
public record Anon(String email) {}

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@evacchi
Enhanced instanceof
if (message instanceof Auth(var login, var token)) {
check(login);
if (validate(login, token)) {
} else {
// error
}
} else if (message instanceof Anon(String email)) {
} else
unhandled(message);
}
deconstruction patterns

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@evacchi
switch (message) {
case Auth(var login, var token) -> {
check(login);
} else {
// error
}
}
case Anon(var email) -> { /* guest access */ };
default -> unhandled(message);
}
}

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@evacchi
switch (message) {
check(login);
} else {
// error
}
}
}
}
Improved Switch

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@evacchi
switch (message) {
check(login);
} else {
// error
}
}
}
}
Records

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@evacchi
switch (message) {
check(login);
} else {
// error
}
}
}
}
deconstruction patterns

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@evacchi
switch (message) {
check(login);
} else {
// error
}
}
}
}
var
pattern

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@evacchi
switch (message) {
check(login);
} else {
// error
}
}
}
}

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@evacchi
Records
public record Auth(String login, String token) {}
public record Anon(String email) {}

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@evacchi
Sealed Hierarchies
sealed interface LoginInfo {}
public record Auth(String login, String token) implements LoginInfo {}
public record Anon(String email) implements LoginInfo {}

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@evacchi
Exhaustive Pattern Matching
public void onReceive(LoginInfo message) throws Exception {
switch (message) {
check(login);
} else {
// error
}
}
}
}
Sealed Type Hierarchy
No Default Case!

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@evacchi
Scala
def onReceive(Any: message): Unit {
message match {
case Auth(login, token) => {
check(login)
} else {
// error
}
}
case Anon(email) => { /* guest access */ };
}
}

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@evacchi
Akka + Java + Pattern Matching
switch (message) {
case String m -> {
log.info("Received String message: {}", message);
getSender().tell(message, getSelf());
}
}
}
}

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@evacchi
Akka + Java
static class Pingponger extends AbstractActor {
@Override
public Receive createReceive() {
return receiveBuilder()
.match(
Ping.class,
ping -> {
out.println(getSelf().path() + " received ping, count down " + ping.getCount());
if (ping.getCount() > 0) {
getSender().tell(new Ping(ping.getCount() - 1), getSelf());
}
})
.build();
}
}
public static void main(String... args) {
ActorSystem system = ActorSystem.apply("pingpong");
ActorRef pinger = system.actorOf(Props.create(Pingponger.class), "pinger");
ActorRef ponger = system.actorOf(Props.create(Pingponger.class), "ponger");
pinger.tell(new Ping(10), ponger);
}

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gist.github.com/viktorklang/2557678

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@evacchi
Agenda
☕
☕
Live coding with

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Live Coding with

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@evacchi
My First Actor

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@evacchi
Vending Machines (FSM)

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Closures vs Classes

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@evacchi
Implementation
Function + State + Message Queue

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@evacchi
Closures vs Classes
The venerable master Qc Na was walking with his student, Anton.
Hoping to prompt the master into a discussion, Anton said
“Master, I have heard that objects are a very good thing - is this true?”
Qc Na looked pityingly at his student and replied…

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@evacchi
Closures vs Classes
“Foolish pupil – objects are merely a poor man’s closures.”

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@evacchi
Closures vs Classes
Chastised, Anton took his leave from his master and returned to his cell,
intent on studying closures. He carefully read the entire "Lambda: The
Ultimate..." series of papers and its cousins, and implemented a small
Scheme interpreter with a closure-based object system. He learned much, and
looked forward to informing his master of his progress.

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@evacchi
Closures vs Classes
On his next walk with Qc Na, Anton attempted to impress his master by
saying “Master, I have diligently studied the matter, and now understand
that objects are truly a poor man’s closures.” Qc Na responded by hitting
Anton with his stick, saying…

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@evacchi
Closures vs Classes
“When will you learn? Closures are a poor man's object.”

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@evacchi
Closures vs Classes
At that moment, Anton became enlightened.
http://people.csail.mit.edu/gregs/ll1-discuss-archive-html/msg03277.html

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@evacchi
Agenda
☕
☕
Live coding with

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Write You a Chat

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@evacchi
Server

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@evacchi
Client Manager

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@evacchi
Thou Shall Not Block!

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Loom

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@evacchi
Green Threads
● in the Java 1.0 days, some JVMs "green" (user-mode) threads.
● The green threads of the 90s still had large, monolithic stacks.
● A product of their time
○ systems were single-core
○ OSes did not have thread support at all.
https://www.infoq.com/articles/java-virtual-threads/

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@evacchi
Virtual Threads
● Virtual threads: only superficial similarity to Green Threads
● More in common with user-mode threads in Go or Erlang
● However: semantically identical to java.lang.Thread

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@evacchi
Green Threads
● Solaris 2.6 software
● the green threads library was user-level
● the Solaris system could process only one green thread at a time,
● Solaris handled the Java runtime as a many-to-one threading implementation
● Java applications could not interoperate with existing MT applications in the
Solaris environment.
● Java threads could not run in parallel on multiprocessors.
● An MT Java application could not harness true OS concurrency for faster
applications on either uniprocessors or multiprocessors.
Green threads library was replaced with native Solaris threads for Java on the Solaris 2.6
platform; this is carried forward on the Solaris 7 and Solaris 8 platforms.
https://docs.oracle.com/cd/E19455-01/806-3461/6jck06gqe/

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@evacchi
Loom
https://wiki.openjdk.org/display/loom/Main
“Project Loom is intended to explore, incubate and
deliver Java VM features and APIs built on top of them
for the purpose of supporting easy-to-use,
high-throughput lightweight concurrency and new
programming models on the Java platform.”

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@evacchi
Loom
● Virtual Threads are mounted on platform threads
(carrier threads)
● Yield in std library

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https://www.youtube.com/watch?v=449j7oKQVkc

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@evacchi
Thou Shall Not Block!

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Write You
a Loom Chat

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Game of Loom:
implementation patterns and
performance implications
playing with virtual threads
by Mario Fusco
@mariofusco Learn More
This Thursday!

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@evacchi
bit.ly/actor19

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Write You An Actor (System) For Great Good!

Write You An Actor (System) For Great Good!

Edoardo Vacchi

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