Introduction to Erlang/(Elixir) at a Webilea Hands-On Session

Erlang
Elixir Webilea Hands-On Session
at Obst & Gemüse, 19.3.2014
By André Graf
@der_graf / @erlio_basel

Preliminaries
● Erlang and Elixir Programs are compiled for
the BEAM*
● Similar datatypes and runtime behavior
● Different syntax, language features, stdlib
*Bogdan/Björn's Erlang Abstract Machine

1986
Erlang has been developed
by Ericsson, for building
highly available telco
systems
Massively concurrent
server systems
Server systems
that run forever
Hot-Code
Upgrades
Inherently parallel
problem domain
High availability
through redundancy
Debugging
Live Systems

● Functional
● Compiled
● Dynamically typed
● VM & Shell
● Very lightweight processes
● Processes don't share state
● Message Passing Concurrency
● Process Supervision (Let it crash)
● Built-in Distribution
● Hot-Code Upgrades

Erlang OTP
● Architecture Patterns
● Middlewares
● Libraries
● Tools
● You could use Erlang witout OTP (nobody does)
● OTP is included in the Erlang distribution
● OTP is closely tied to the Erlang VM

1998
Erlang has been
opensourced and other
companies started using it.
Ericsson went for C++
Massively concurrent
server systems
Server systems
that run forever
Hot-Code
Upgrades
Inherently parallel
problem domain
High availability
through redundancy
Debugging
Live Systems

Introduction to Erlang/(Elixir) at a Webilea Hands-On Session

Erlang vs. Elixir
very subjective
+ Elixir has a more familiar Syntax.
+ Elixir fixes several language design mistakes
from the past. *
+ Elixir will bring more people to the Beam.
+ Elixir has better meta-programming support.
- Elixir has (currently) no own OTP framework.
> killer for production use.
* http://joearms.github.io/2013/05/31/a-week-with-elixir.html

Erlang Hands-On
1. Installation Party
2. Language introduction http://www.tryerlang.org/
3. My first Erlang module
4. My first Erlang process
5. Let it crash
6. OTP behaviours gen_server / supervisor
7. Recap / Discussion

Language Introduction
Goto http://www.tryerlang.org/

My first Erlang Module
● Module-Name == Filename.erl
– e.g. my_first_module.erl
-module(my_first_module).
-export([my_first_function/1]).
% this is a comment for my first function
% this is another line of the comment
my_first_function(Param) ->
io:format(“wow my first function got called with param: ~p~n”, [Param]).
1. Compile with erlc my_first_module.erl
2. Start Erlang shell with: erl
3. Call function my_first_module:my_first_function(“hello world”).

My first Erlang Process
-export([my_first_function/0,
start_my_first_process/0]).
my_first_function() ->
receive
Msg ->
io:format(“wow my first process got a msg: ~p~n”, [Msg])
end.
start_my_first_process() ->
spawn(?MODULE, my_first_function, []).
1. Compile with erlc my_first_module.erl
or if you are inside the shell with:
c(my_first_module).
l(my_first_module).
2. Start your first process:
Pid = my_first_module:start_my_first_process().
3. Send your first message:
Pid ! yeah_this_is_my_first_message.
4. How can you reuse the Process???

Let it Crash
start_my_first_process/0]).
receive
Msg ->
dispatch_my_msg(Msg),
my_first_function() %% yay, recursion!
end.
start_my_first_process() ->
spawn(?MODULE, my_first_function, []).
dispatch_my_msg({hello, world}) ->
io:format(“what a great hello world~n”);
dispatch_my_msg({A, B}) ->
io:format(“tuple with ~p and ~p~n”, [A,B]).
1. kinda stupid dispatcher, right???

Let it Crash
start_my_first_monitored_process/1,
my_monitor/1]).
receive
Msg ->
dispatch_my_msg(Msg),
my_first_function()
end.
my_monitor(ProcName) when is_atom(ProcName) ->
Pid = spawn(?MODULE, my_first_function, []),
true = register(ProcName, Pid),
MonitorRef = monitor(process, Pid),
receive
{'DOWN', MonitorRef, _Type, Pid, _Info} ->
my_monitor(ProcName);
stop ->
demonitor(MonitorRef)
end.
start_my_first_monitored_process(ProcName) ->
spawn(?MODULE, my_monitor, [ProcName]).
...

OTP Behaviours
● gen_server
– Generic way to write servers in Erlang OTP Style
● Supervisor
– A Supervisor that supervises other OTP compliant
processes (e.g. gen_servers, gen_fsms)

gen_server
-module(sample_gen_server).
-behaviour(gen_server).
-export([start_link/0]).
%% gen_server callbacks
-export([init/1,
handle_call/3,
handle_cast/2,
handle_info/2,
terminate/2,
code_change/3]).
%%% API
start_link() ->
gen_server:start_link(?MODULE, [], []).
%%% gen_server callbacks
init([]) ->
{ok, []}.
handle_call(_Request, _From, State) ->
Reply = ok,
{reply, Reply, State}.
handle_cast(_Msg, State) ->
{noreply, State}.
handle_info(_Info, State) ->
{noreply, State}.
terminate(_Reason, _State) ->
ok.
code_change(_OldVsn, State, _Extra) ->
{ok, State}.

supervisor -module(sample_supervisor).
-behaviour(supervisor).
-export([start_link/0]).
%% Supervisor callbacks
-export([init/1]).
%% ===================================================================
%% API functions
%% ===================================================================
start_link() ->
supervisor:start_link({local, ?MODULE}, ?MODULE, []).
%% ===================================================================
%% Supervisor callbacks
%% ===================================================================
init([]) ->
{ok, { {one_for_one, 5, 10}, [ %% 5 restarts within 10 seconds are allowed
{sample_gen_server, {
sample_gen_server, start_link, []
}, permanent, 5000, worker, [sample_gen_server]}
]}}.

Introduction to Erlang/(Elixir) at a Webilea Hands-On Session

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