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@josevalim / Plataformatec
Concurrency
Ruby
in
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About me
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•Started with Ruby in 2006
About me
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•Started with Ruby in 2006
•Open Source Contributor
About me
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•Started with Ruby in 2006
•Open Source Contributor
•Joined Rails Core in 2010
About me
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•Started with Ruby in 2006
•Open Source Contributor
•Joined Rails Core in 2010
•Author of elixir-lang.org
About me
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No content
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Topics
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•Why concurrency?
Topics
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•Why concurrency?
•The problem today
Topics
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•Why concurrency?
•The problem today
•Tools of the trade
Topics
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Why concurrency?
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Server Server Server Server
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50 cores
$2600
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It is no longer about
the future, it is
about now
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Server
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In Ruby, we talk
about threads
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We need our
applications to be
thread-safe
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The problem
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class User
def self.current_user=(user)
@@current_user = user
end
def self.current_user
@@current_user
end
end
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post "/do_something" do
User.current_user =
user_from_request
# some work
Mailer.deliver_to(
User.current_user)
end
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1. Assign current_user
2. Do some work
3. Get current_user to
send an e-mail
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thread 1 current_
user thread 2
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thread 1 current_
user thread 2
jose
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thread 1 current_
user thread 2
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thread 1 current_
user thread 2
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thread 1 current_
user thread 2
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thread 1 current_
user thread 2
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matz
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thread 1 current_
user thread 2
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thread 1 current_
user thread 2
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thread 1 current_
user thread 2
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thread 1 current_
user thread 2
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thread 1 current_
user thread 2
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thread 1 current_
user thread 2
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User.current_user is
shared mutable state
(global)
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ActionMailer::Base.from
ActionController::Base.logger
Devise.password_length
SimpleForm.form_options
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What is Ruby?
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Ruby is a programming
language with different
implementations
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Rails 2.2
threadsafe
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Thread-safety means
different things for
different implementations
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hash = {}
10.times do |i|
Thread.new {
hash[i] = "Hello #{rand}"
}
end
Threadsafe?
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Thread-safe
YARV Yes
JRUBY No
RBX 2.0 No
Hashes
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global virtual machine lock (YARV)
@cached[template] =
File.read(template)
static VALUE
env_aset(VALUE obj,
VALUE nm, VALUE val)
{
...
hash.c
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global virtual machine lock (YARV)
@cached[template] =
File.read(template)
static VALUE
env_aset(VALUE obj,
VALUE nm, VALUE val)
{
...
hash.c
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C Code
C2 C3
C4
C1
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C Code
C2 C3
C4
C1
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C Code
C2 C3
C4
C1
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C Code
C2
C3
C4
C1
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C Code
C2
C3
C4
C1
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Without the GVL, other
Ruby implementations
may try to change the
same Hash at the same
time, corrupting it
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• Class definitions
• Instance variables
• Class variables
• Data structures
...
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Tools of the trade
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Thread locals
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class User
def self.current_user=(user)
@@current_user = user
end
def self.current_user
@@current_user
end
end
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class User
def self.current_user=(user)
Thread.current[:cu] = user
end
def self.current_user
Thread.current[:cu]
end
end
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Time.zone
I18n.locale
ActiveSupport::Notifications.
instrumenter
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class Auth
def initialize(callback)
@callback = callback
end
def call(info)
@user = user_from_info(info)
res = @callback.call(info)
update_user(res) # access @user
res
end
end
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auth =
Auth.new -> { puts "Hello" }
auth.call(id: 13)
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class Auth
def initialize(callback)
@callback = callback
end
def call(info)
@user = user_from_info(info)
res = @callback.call(info)
update_user(res) # access @user
res
end
end
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class AuthMiddleware
def initialize(callback)
@callback = callback
end
def call(info)
@user = user_from_info(info)
res = @callback.call(info)
update_user(res) # access @user
res
end
end
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config.middleware.use(
AuthMiddleware
)
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Eager loading
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class MyApp < Sinatra::Base
autoload :Broker, "my_app/broker"
post "/content" do
# ...
MyApp::Broker.push(info)
end
end
Threadsafe?
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module MyApp::Broker
# ...
def push do
# ...
end
# ...
end
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module MyApp::Broker
# ...
def push do
# ...
end
# ...
end
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module MyApp::Broker
# ...
def push do
# ...
end
# ...
end
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Let’s replace all autoloads
by requires!
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module MyApp
extend ActiveSupport::Autoload
eager_autoload do
autoload :Broker
end
end
MyApp.eager_load!
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config.eager_load_namespaces << MyApp
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Mutex
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hash = {}
10.times do |i|
Thread.new {
hash[i] = "Hello #{rand}"
}
end
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No content
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“There are heaps of non
thread-safe usage of
Hashes in Rails and the
surrounding gems”
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Using hashes as a cache
is an extremely common
pattern in Ruby
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@cached[template] =
File.read(template)
Template lookup
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@controller_names[“posts”] =
PostsController
Router
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Helper
def presenter_for(model)
"#{model.class.name}Presenter".
constantize.new(model)
end
presenter_for(Post.new)
#=> PostPresenter<@post=...>
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@@p = Hash.new do |h, k|
h[k] = "#{k}Presenter".constantize
end
def presenter_for(model)
@@p[model.class.name].new(model)
end
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hash = {}
10.times do |i|
Thread.new {
hash[i] = "Hello #{rand}"
}
end
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hash = {}
mutex = Mutex.new
10.times do |i|
Thread.new {
mutex.synchronize {
hash[i] = "Hello #{rand}"
}
}
end
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Hash
C2 C3
C4
C1
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Hash
C2 C3
C4
C1
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Hash
C2 C3
C4
C1
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Hash
C2
C3
C4
C1
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Hash
C2
C3
C4
C1
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Thread safe gem
https://github.com/headius/thread_safe
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ThreadSafe::Hash.new
ThreadSafe::Array.new
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Better tools
of the trade
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Queues
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No content
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live_assets#sse
Manager
SSE
Subscriber
live_assets#sse
SSE
live_assets#sse
SSE
Subscriber
app/assets vendor/assets
lib/assets
Operating System Notifications
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class Manager
def initialize
@subscribers = []
end
def subscribe(s)
@subscribers << s
end
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def unsubscribe(s)
@subscribers.delete(s)
end
def notify(event)
@subscribers.each do |s|
s << event
end
end
end
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live_assets#sse
Manager
SSE
Subscriber
live_assets#sse
SSE
live_assets#sse
SSE
Subscriber
app/assets vendor/assets
lib/assets
Operating System Notifications
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class Manager
attr_reader :queue
def initialize
@subscribers = []
@queue = Queue.new
end
def loop
while message = @queue.pop
event, *args = message
send(event, *args)
end
end
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manager = Manager.new
Thread.new { manager.loop }
manager.queue <<
[:notify, :reloadCSS]
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C2 C3
C4
C1
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C2 C3
C4
C1
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C2 C3
C4
C1
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We need abstractions
that can move us away
from shared mutable state
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github.com/celluloid
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Conclusion
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We have learned about
different tools to tackle
thread safety issues
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thread.rb
• Thread
• Mutex
• ConditionVariable
• Queue
• SizedQueue
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Gems
• atomic
• thread_safe
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The different semantics
existing in different
implementations makes
concurrency in Ruby harder
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We need to push
for better semantics
at the same time we change
how we think about concurrency
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Thank you!
@josevalim / Plataformatec