What can be done with Java, but should better be done with Erlang (@pavlobaron)

What can be done with

Java

but should better be done with

Erlang

Pavlo Baron
Geek‘s
Guide pavlo.baron@codecentric.de
To The Working Life @pavlobaron

Hey, dude.

What sort of application
should be optimally implemented
using your language?

Clojure, Groovy, Scala dude:

java*

Putonghua dude:

谢谢

Erlang dude:

fit → do();

_ → badarg.

This dude in front of you

is very picky.

So, let' assume that:

=

=

List<Integer> l =
Arrays.asList(1, 2, 3, 4, 5);
List<Integer> r =
new ArrayList<Integer>();
for (int i : l) {
r.add(i * 2);
}

List<Integer> l =
Arrays.asList(1, 2, 3, 4, 5);
Iterable<Integer> t =
Iterables.transform(l,
new Function<Integer, Integer>() {
public Integer apply(Integer i) {
return I * 2;
}
});

And this is just a simple map.
It gets even worse
with filter and fold

[X * 2 || X <- lists:seq(1, 5)].

List<Integer> l = Arrays.asList(1, 2, 3, 4, 5); Iterable<Integer> t = Iterables.transform(l, new Function<Integer, Integer>() {public Integer apply(Integer i) {return I * 2;}});

What can be done with Java, but should better be done with Erlang (@pavlobaron)

synchronized (this) {
if (!crawledSites.contains(site)) {
linkedSites.add(site);
}
}

…

public class CountingSemaphore {
private int signals = 0;
public synchronized void take() {
this.signals++;
this.notify();
}

public synchronized void release()
throws InterruptedException {
while (this.signals == 0) wait();
this.signals--;
}
}

1> A = 5.
5
2> A = 10.
** exception error: no match of
right hand side value 10
3>

The simplest way to avoid problems
with concurrency is to share only
immutable data between threads.
Immutable data is data which can
not be changed.

(from a Java concurrency tutorial)

Immutable class:

- all its fields are final
- class declared as final
- “this” reference is not allowed to
escape during construction

Any fields which refer to
mutable data objects:

- are private
- have no setter method
- are never directly returned of
otherwise exposed to a caller
- if they are changed internally
in the class this change is not
visible and has no effect
outside of the class

1> F = fun(F) -> F(F) end.
#Fun<erl_eval.6.80247286>
2> F(F).

…..................hours later................

BREAK: (a)bort (c)ontinue (p)roc info
(i)nfo (l)oaded (v)ersion (k)ill (D)b-tables
(d)istribution

a

...
for (paramFiber = recursion1.doit__1(paramEProc, paramEObject);
paramFiber == EProc.TAIL_MARKER;
paramFiber = (EFun)localFiber.getCallee())
{
S_O localS_O;
switch (localFiber.up())
{
case 2:
paramEProc.tail.go(paramEProc, localFiber.down());
localS_O = new S_O();
localS_O.f0 = paramEProc;
localFiber.setState(localS_O, this, 1);
return null;
case 3:
null;
return null;
case 1:
localS_O = (S_O)localFiber.curState;
paramEProc = (EProc)localS_O.f0;
case 0:
}
}
...

1> F = fun([_|_]) ->
1> io:format("string~n");
1> (B) ->
1> io:format("integer~n")
1> end.
#Fun<erl_eval.6.80247286>
2> F(15).
integer
ok
3> F("whatever").
string
ok
4>

try {
// Create a new class loader with the directory
ClassLoader cl = new URLClassLoader(urls);

// Load in the class
Class cls = cl.loadClass("MyReloadableClassImpl");

// Create a new instance of the new class
myObj = (MyReloadableClass)cls.newInstance();
} catch (IllegalAccessException e) {
} catch (InstantiationException e) {
} catch (ClassNotFoundException e) {
}

Excessive class-reloading using
ClassLoader hierarchies is
expensive and leads to JVM
instance fatigue

That's why it's strongly
recommended to do hot
deployment in app servers

% hot_swap:sum(Num)
% adds 1 to Num
1> c(hot_swap).
{ok,hot_swap}
2> hot_swap:sum(1).
2
...
% hot_swap.erl has changed.
% now it adds 2 to Num
…
3> c(hot_swap).
{ok,hot_swap}
4> hot_swap:sum(1).
3
5>

code_change(OldVsn, State, Extra) ->
...code to convert state (and more) during
code change...
{ok, NewState}.

JVM HotSwap is limited to
method bodies.

OSGi is invasive and
state-unaware

your managers will love *Rebel
in production

your ops will love *Rebel
in production

1> A = 20.
20
2> self().
<0.32.0>
3> 5 / 0.
** exception error: bad argument in an
arithmetic expression
in operator '/'/2
called as 5 / 0
4> self().
<0.36.0>
5> A.
20
6>

QueueConnectionFactory connFactory =
new QueueConnectionFactory();
QueueConnection conn = connFactory.createQueueConnection();
QueueSession session = conn.createQueueSession(false,
Session.AUTO_ACKNOWLEDGE);
Queue q = new Queue("world");
QueueSender sender = session.createSender(q);
TextMessage msg = session.createTextMessage();
msg.setText("Hello there!");
sender.send(msg);

QueueReceiver receiver = session.createReceiver(q);
conn.start();
Message m = receiver.receive();
if (m instanceof TextMessage) {
TextMessage txt = (TextMessage) m;
}

session.close();
conn.close();

register(serv, spawn(?MODULE, loop, [])),
serv ! {self(), “Hello there!”},
receive
{_Pid, Msg} ->
…
end.

…

loop() ->
receive
{From, Txt} ->
…
loop();

$ erl +P 134217727

Erlang R14B04 (erts-5.8.5) [source] [64-bit]
[smp:8:8] [rq:8] [async-threads:0] [hipe]
[kernel-poll:false]

Eshell V5.8.5 (abort with ^G)
1> erlang:system_info(process_limit).
134217727
2>

import kilim.Mailbox;
import kilim.Pausable;
import kilim.Task;

public class SimpleTask extends Task {
static Mailbox<String> mb = new Mailbox<String>();

public static void main(String[] args) throws Exception {
new SimpleTask().start();
Thread.sleep(10);
mb.putnb("Hello ");
mb.putnb("Worldn");
mb.putnb("done");
}

public void execute() throws Pausable {
while (true) {
String s = mb.get();
if (s.equals("done")) break;
System.out.print(s);
}
System.exit(0);
}
}

your ops will love this
in production

{serv, 'serv@pc'} ! {self(), “Hello there!”},
receive
{_Pid, Msg} ->
…
end.

…

loop() ->
receive
{From, Txt} ->
…
loop();

import org.gridgain.grid.*;
import org.gridgain.grid.gridify.*;
import org.gridgain.grid.gridify.aop.spring.*;

public final class GridifyHelloWorldSessionExample {
private GridifyHelloWorldSessionExample() { //ensure singleton }

@Gridify(taskClass = GridifyHelloWorldSessionTask.class, timeout = 3000)
public static int sayIt(String phrase) {
System.out.println(phrase);
return phrase.length();
}
public static void main(String[] args) throws GridException {
if (args.length == 0) {
GridFactory.start();
}
else {
GridFactory.start(args[0]);
}
try {
int phraseLen = sayIt("Hello World");
System.out.println(„number of characters is '" + phraseLen + "'.");
} finally {
GridFactory.stop(true);
}
}
}

One GC per OS process.
Complex generation management.
Different GC stategies.
Stop the world situations possible

One GC per Erlang process.
Simple generation management.
(Mostly) one GC stategy.
Stop the world situations unlikely.

Hey, dude.

So do you imply Erlang is better
than Java for everything?

Erlang dude:

body() -> [
#h1 { text="My Simple Application" },
#label { text="What is your name?" },
#textbox { },
#button { text="Submit" }
].

Java dude:

LOL
at
you

Ruby on rails dude:

ROFL
at
y'all

Erlang dude:

calc_month(S) ->
L = ["Jan", "Feb", "Mar", "Apr", "May",
"Jun", "Jul", "Aug", "Sep", "Oct",
"Nov", "Dec"],
find_ix(L, S, 1).

Erlang dude:

Doing number crunching, you would
completely utilize the available cores with
few (as many) threads.

Erlang is for time sharing and doesn't
like long blocking processes.

C dude:

ROFL
at
y'all

Some code examples were
taken from public blogs / sites

Most images originate from
istockphoto.com

except few ones taken
from Wikipedia and product pages
or generated through public
online generators

What can be done with Java, but should better be done with Erlang (@pavlobaron)

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