1. Erlang vs. Java
January 19, 2011
Artan Cami

2. (i) Erlang history
(ii) Erlang resources
(iii) Java history
(iv) Compare philosophies
(v) Problem definition
(vi) JEE approach & principles
(vii) JEE solution details
(viii) Erlang/OTP approach & principles
(ix) Erlang solution details
(x) Erlang language basics
(xi) Speculations

3. erlang history
http://www.erlang.org/course/history.html
1982-1985 telecom experiments with about 20 languages => need high symbolic language
1985-1986 experiments with Lisp, Prolog, Parlog... => need concurrency primitives and
error recovery => need new language
1987 first experiments with Erlang
1988 “Erlang escapes from the lab!”
1990 Erlang presented in ISS’90 => new users introduced
1992 Erlang ported to VxWorks, PC, Mac ...
1993 Distribution is added to Erlang => run homogeneous Erlang to heterogeneous
hardware
1997 OTP (open telecom platform) started
1998 Erlang banned => open source Erlang
2002 Concurrency Oriented Programming
2006 SMP (symmetric multiprocessing machine) Erlang - Multicore.

4. erlang resources
๏ Programming Erlang: Software for a Concurrent World - Joe Armstrong - Jul 2007
๏ ERLANG Programming - Francesco Cesarini, Simon Thompson - Jun 2009
๏ Erlang and OTP in Action - Martin Logan, Eric Merritt, Richard Carlson - Nov 2010
๏ www.erlang.org - Erlang Programming Language, Official Site
๏ www.trapexit.org - Erlang Community Site
๏ erlyweb.org - The Erlang Twist on Web Frameworks
๏ erlide.sourceforge.net - The Erlang IDE. Powered by Eclipse.
๏ www.infoq.com/interviews - interviews taken in Erlang Factory events.

5. java history
http://www.java.com/en/javahistory
1991 The Green Project begins
1992 “Oak”
1994 WebRunner - first browser for moving objects and executable content
1995 Oak becomes Java. Java technology released to a select group on wicked.neato.org
1996 JDKtm 1.0 released; first JavaOne conference
1997 Over 220,000 downloads of JDK 1.1 in just three weeks; JavaOne worlds largest
developer conference with over 8,000 attendees
1998 JDK 1.1 2 million downloads; Visa Java Smart Card; JCP formalized
1999 J2EE beta released; Java 2 source code released; over 20,000 people in JavaOne
2001 Over 2 million downloads of J2EE
2003 About 75% of professional developers use Java as their primary language
2004 Mars-Rover touches down on Mars; J2SE 5 (Tiger) released
2005 4.5 million java developers; 2.5 billion Java-enabled devices

6. philosophies/principles
• Java :
• “the computer is the network”
• write once, run anywhere
• simple
• concurrency multithreading
• object orientated programming
• Erlang :
• the world is parallel
• concurrent processes belong to the language - not OS
• very light weight concurrency
• share nothing; no mutable state; no locks
• pure asynchronous message passing
• “let it crash” - no defensive programming
• functional programming

7. bookstore problem
typical online bookstore
- books: find books, display list of books, display the book details
• book details: book summary information, editorial reviews, product details,
authors details, customer reviews, related books: frequently bought together,
other books on this topic, other books by this author(s)
- customer: create and maintain profile; store, find, and display transaction history.
- orders: shopping cart capability; payment and shipment types; order placement.
- non-functional requirements
➡ S - store app, B - books apps, C - customer apps; O - order apps

8. jee approach and patterns
• MVC
• Define layers: presentation, model, and data
• Model: EJB3 - stateless, stateful, singleton session beans;
message-driven beans;
• container management, session facade, entity access object;
• ORM layer: JPA implementation
• EntityManager, entity POJOs;
• Pick a presentation framework: struts, jsf, templating, or
portal

9. jee - deployment strategy
collocated JVM 1
Applications
S B C O
JVM 2
Applications
Load Balancer
S B C O
JVM 3
Applications
S B C O

10. jee - deployment strategy
distributed
JVM 2
EJB Applications
BEJB
JVM 1
Web Applications
JVM 3
SWEB BWEB CWEB OWEB
EJB Applications
CEJB OEJB

11. jee - books applications
J V M 1 J V M 2
W e b C o n t a i n e r E J B C o n t a i n e r ORM
BookEntity
BookEAO EJB
RelatedBooksEntity
BooksController
Books EJB BookContentEntity
AuthorEntity
BooksView RelatedBookslEAO
EJB
BookDetailView
...View
PublisherEntity
BookContentView

12. jee -customer applications
J V M 1 J V M 3
W e b C o n t a i n e r E J B C o n t a i n e r ORM
ProfileEntity
ProfileEAO EJB
CustomerController PaymentTypesEntity
Customer EJB
OrdersHistoryEntity
CustomerProfileView
OrdersHistoryEAO
...View EJB
OrdersHisotryView

13. jee - orders applications
J V M 1 J V M 3
W e b C o n t a i n e r E J B C o n t a i n e r ORM
ShoppingCartContr ShoppingCartEAO ShoppingCartEntity
ShoppingCart EJB
EJB
OrdersController PaymentEntity
PaymentsEAO
ShippingEntity
EJB
ShoppingCartView Orders EJB
...View
PaymentView OrdersEntity
OrdersEAO
EJB

14. jee - sequence diagram
display book details
BooksController BooksEJB BooksEAO RelatedBooksEAO
1
2
3
4
5
7 BookView
6
BookDetailView
8
9
RelatedBooksView
10
11
12
13
14

15. erlang/otp approach and
patterns
• Use large number of processes in order to distribute/
parallelize as much as possible;
• process is the main building block with its own stack, garbage collection, message
queue, security model.
• message sending/receiving simplicity made possible by pattern matching and simple
data structures;
• Avoid side effects - no shared resources among processes;
• variables that don’t vary - single assignment. No mutable state, no shared memory, no
locks;
• Small messages, big computations
• Distributed database Mnesia
• OTP patterns:
• gen_server, event_handler, supervisor, application, ...

16. erlang - sequence & concurrency diagram
display book details
BooksController BooksDetailModel
1 BooksDetailController
2
3
BookDetailView
4
5
6
7
RelatedBooksModel
RelatedBooksController
2’
3’
RelatedBooksView
4’
5’
6’
7’
8

17. erlang - books applications
Node 2
BooksDetails BooksDetails
Model Model
Node 1
BooksDetails RelatedBooks RelatedBooks
Controller Controller View
BooksDetail
View
Books
Controller

18. erlang - deployment strategy
Distributed
Node 2 Node 3
B O C
processes
processes processes
Node 1
OWEB
BWEB processes CWEB
processes
processes
SWEB
processes

19. erlang language basics
• Modules: .erl files ~ similar to .java files
‣ .beam is the compiled module extension ~ similar to .class in java
‣ -module(module_name). ~ similar to public class in java
‣ -export([list of public functions]). ~ similar to public
method modifier in java
‣ -import(module,[list of public functions]). ~ similar to
import in java
‣ other module attributes:
-behaviour(otp behavior pattern name).
-record(record_name, tuple).
-compile(options).

20. erlang language basics
• Functions ~ similar to methods in java
‣ function_name( arguments ) -> function body.
‣ function body consists of instructions separated by commas
‣ one of the instructions results to the function output
‣ BIFs - build in functions
‣ function is called as module_name:function_name(arguments)

21. erlang language basics
• Processes
‣ the building blocks of concurrency
‣ simple to create;
erlang:system_info(process_limit) 32,767
‣ simple primitives: spawn, send, receive:
Pid = spawn(Node,Fun).
Pid ! Message.
receive ... end.
‣ Links, Exit signals, System processes
spawn_link(Node,Fun) -> Pid
process_flag(trap_exit,true).
exit(Pid, Why)->true.
erlang:monitor(process,Item)->MonitorRef
‣ Registered processes
‣ Independent stack, message queue, garbage collection

22. erlang language basics
• Messages
‣ asynchronous
‣ pass by copying
‣ lightweight and simple due to simple data structures
‣ selective receive due to pattern matching:
loop(Name,Mod,OldState)->
receive
{From,{swap_code,NewMod}}->
From!{Name,acknowledged},
loop(Name,NewMod,OldState);
{From,Request}->
{Response,NewState}=Mod:handle(Request,OldState),
From!{Name,Response},
loop(Name,Mod,NewState)
end.

23. erlang language basics
• Terms
‣ Lists:
Books = [Book1,Book2,Book3]
Books = [Book1 | RestOfBooks]
Book1 = {{isbn,”1-9343560-0-X”},{title,”Programming Erlang”},{authors,{“Joe
Armstrong”}},{pages,515},{publisher,”The Pragmatic Programmer”}}
Book2 = {{isbn,”9781933988788”},{title,”Erlang and OTP in Action”},
{authors,{“Martin Logan”,”Eric Merritt”,”Richard Carlson”}},{pages,397},
{publisher,”Manning”}}
lists:map(fun(X) ->2*X end, L).
[2*X || X<-L]. -list comprehensions
‣ Tuples:
Book1 = {{isbn,”1-9343560-0-X”},{title,”Programming Erlang”},{authors,{“Joe
Armstrong”}},{pages,515},{publisher,”The Pragmatic Programmer”}}
{_,{_,Book1_title},_,_,_}=Book1.
‣ Funs:
Double = fun(X)->2*X end.

24. erlang language basics
• Terms
‣ Atoms, represent the static constants; lowercase letters-and-numbers-and-@ or any
characters within single-quotes
Book1 = {{isbn,”1-9343560-0-X”},{title,”Programming Erlang”},{authors,{“Joe
Armstrong”}},{pages,515},{publisher,”The Pragmatic Programmer”}}
‣ Numbers: integers - limited by available memory, and floats - 64-bit format
‣ Strings: list of integers, within double-quotes
‣ Binaries: efficient to store large quantity of data, in form of sequence of unsigned 8-
bit integers; BIFs that can manipulate binaries; Bit syntax that can pattern match
against binaries.
<<0,1,...,255>>
‣ Term comparisons: number<atom<reference<fun<port<pid<tuple<list<binary

25. erlang language basics
• Variables
‣ single assignment, immutable, name must start with uppercase letter
‣ X=X+1 does not work in Erlang
‣ scope is the lexical unit in which it is defined

26. erlang language basics
• Recursion
‣ looping
‣ tail recursion efficiency
(a) sum(0) -> 0;
sum(N) -> sum(N-1)+N.
(b) do_sum(0, Total) -> Total;
do_sum(N, Total) -> do_sum(N-1, Total+N).

27. erlang language basics
• Case expressions
case Expression of
Pattern1 [when Guard1] -> Expr_seq1;
Pattern2 [when Guard2] -> Expr_seq2;
...
end
• If expressions
if
Guard1 -> Expr_seq1;
Guard2 -> Expr_seq2;
...
end
• Exceptions
try {
block
} catch (exception) {
block
} catch (exception) {
block
} ...
finally {
block
}

28. erlang language basics
• OTP patterns
‣ library of solutions for common problems ~ similar to an App Server container in
JEE
‣ it is part of Erlang standard download
‣ can be utilized by using -behaviour(pattern_name)
‣ common patterns:
-behaviour(gen_server).
-behaviour(gen_event).
-behaviour(supervisor).
-behaviour(application).

29. erlang language basics
• Mnesia
‣ distributed dbms that comes incorporated in the standard Erlang download
‣ can store any type of Erlang data structures
‣ can be configured to have both memory and disk storage
‣ examples:
-record(books,{id, book}).
B=#books{id={“1-9343560-0-X”},
book = {{isbn, “1-9343560-0-X”},{title, “Programming Erlang”},
{authors,{“Joe Armstrong”}},{pages,515},{publisher, “The
Pragmatic Programmer”}}
}

30. erlang language basics
• Jinterface
‣ Java simulation of Erlang
‣ a mapping implementation of Erlang language elements into Java
‣ java erlang-simulated nodes can talk to the erlang nodes using erlang protocol

31. speculations
• sequential and parallel programming - two
dimensions equally important in our problem
solving
• concurrency might no longer remain an attribute
of special problems
• spiral of change might be introducing some new
forces on the OO paradigm