Rails Concurrency Gotchas

Surpresas no usode Concorrência em by Marcos Toledo Rails

Marcos RenatoBaroni Toledo @mtoledo

Socram O’taner

Socram O’tanerMarcos Renato

O’taner

“I’m trying tounsubscribe butit says email has already been taken”!

validates_uniquen ess_of # id :integer(4) # email :string(255) class User < ActiveRecord::Base validates_uniqueness_of :email end

WTF .. ?

Invalid Records!User.ﬁnd_by_email(“angry_customer@email.com”).valid?# => false

Rails can’t guaranteeuniqueness!

Solutions?

Uniqueness Constraint!class AddUniquenessIndexToUserEmail < ActiveRecord::Migration def self.up add_index :users, :email, :unique => true end def self.down remove_index :users, :email endend

Success!

Another Email

Airbrake!

NoMethodError: undeﬁned method `title for nil:NilClass “nil post”?

:dependent => :destroyclass Comment < ActiveRecord::Base belongs_to :post validates_presence_of :postendclass Post < ActiveRecord::Base has_many :comments, :dependent => :destroyend

WTF .. ?

Orphan Models!Comment.all.reject(&:post).size# => 1

Rails can’t guaranteedependency either!

Solutions?

Database Foreign Keys!create_table :comments do |table| table.integer :post_id, :null => falseendActiveRecord::Base.connection.execute <<-EOS ALTER TABLE `comments` ADD CONSTRAINT FOREIGN KEY (post_id) REFERENCES posts(id) ON DELETE restrict ON UPDATE cascadeEOS

Success!

1 - Noob“Use database constraints”

O’taner

ruby 100% CPU!

Solutions?

ProfessionalDebuggingssh serverkillall ruby

Success!

WET Code

Inﬁnite Loopclass Comment < ActiveRecord::Base after_create :update_comment_count def update_comment_count post = Post.ﬁnd post_id post.comment_count += 1 post.save rescue StaleObjectError retry endend

WTF .. ?

TransactionIsolation Level “I” in ACID

1 - READUNCOMMITTED2 - READCOMMITTED3 - REPEATABLEREAD

PostgreREAD-COMMITTED MySQLREPEATABLE-READ

mysql> begin;Query OK, 0 rows affected (0,00 sec)mysql> select * from posts where id = 1;+----+-------+--------------+| id | title | lock_version |+----+-------+--------------+| 1 | Title | 0|+----+-------+--------------+1 row in set (0,00 sec)mysql> update posts set title = New Title where lock_version = 0;Query OK, 0 rows affected (0,00 sec)Rows matched: 0 Changed: 0 Warnings: 0mysql> select * from posts where id = 1;+----+-------+--------------+| id | title | lock_version |+----+-------+--------------+| 1 | Title | 0|+----+-------+--------------+1 row in set (0,00 sec)

mysql> begin;Query OK, 0 rows affected (0,00 sec)mysql> select * from posts where id = 1;+----+-------+--------------+| id | title | lock_version |+----+-------+--------------+| 1 | Title | 0|+----+-------+--------------+1 row in set (0,00 sec)mysql> update posts set title = New Title where lock_version = 0;Query OK, 0 rows affected (0,00 sec)Rows matched: 0 Changed: 0 Warnings: 0 // raises StaleObjectErrormysql> select * from posts where id = 1;+----+-------+--------------+| id | title | lock_version |+----+-------+--------------+| 1 | Title | 0|+----+-------+--------------+1 row in set (0,00 sec)

Solutions?

READ- COMMITTED! /etc/my.cnf[mysqld]transaction-isolation = READ-COMMITTED Just like postgre ;)

mysql> begin;Query OK, 0 rows affected (0,00 sec)mysql> select * from posts where id = 1;+----+-------+--------------+| id | title | lock_version |+----+-------+--------------+| 1 | Title | 0|+----+-------+--------------+1 row in set (0,00 sec)mysql> update posts set title = New Title where lock_version = 0;Query OK, 0 rows affected (0,00 sec)Rows matched: 0 Changed: 0 Warnings: 0 // raises StaleObjectErrormysql> select * from posts where id = 1;+----+-----------+--------------+| id | title | lock_version |+----+-------+--------------+| 1 | Title | 1|+----+-------+--------------+1 row in set (0,00 sec)

Success!

2 - Beginner“Use READ-COMMITTED”

O’taner

Production Console> u1 = User.ﬁrst=> #<User id: 1, login: "marcos", options: {:active=>false}> u1.update_attributes :options => {:active => true}=> true> u1.reload=> #<User id: 1, login: "marcos", options: {:active=>false}

WTF .. ?

# login :string(255)# options :textclass User < ActiveRecord::Base serialize :options def activate self.options = {:active => true} self.save endend

No ‘:active => false’!

Dirty Checking> u1 = User.ﬁrst=> #<User id: 1, login: "marcos", options: {:active=>false}> u2 = User.ﬁrst=> #<User id: 1, login: "marcos", options: {:active=>false}> u1.update_attributes :options => {:active => true}=> true> u2.update_attributes :login => "mtoledo"=> true> u1.reload=> #<User id: 1, login: "mtoledo", options: {:active=>false}

Serialize isALWAYS dirty!

// u1.update_attributes :options => {:active => true}UPDATE ùsers`SET òptions` = --- n:active: truenWHERE (ùsers`.ìd` = 1)// u2.update_attributes :login => "mtoledo"UPDATE ùsers`SET `login` = mtoledo, òptions` = --- n:active: falsenWHERE (ùsers`.ìd` = 1)

Solutions?

Options Model

# id :integer(4)# user_id :integer(4)# lock_version :integer(4)# options :textclass UserOptions < ActiveRecord::Base serialize :options belongs_to :userend

Success!

3 - Half Mouth“Use an Options Model”

O’taner

production.logERROR 1213 (40001): Deadlock found when trying to get lock;try restarting transaction

DAMN!Pessimistic Locks!

No pessimistic locking!class Comment after_create :update_posts_count def update_posts_count self.posts.increment :comments_count endendclass Posts after_update :update_comments def update_comments comments.each {|c| c.published = self.published; c.save} endend

WTF .. ?

Transac Transaction 1SELECT * FOR tion 2UPDATEFROM `comments`WHERE ìd` = 1 SELECT * FOR UPDATE locked.. FROM `posts` WHERE ìd` = 1SELECT * FORUPDATEFROM `posts` DEADLOCK!WHERE ìd` = 1 SELECT * FOR UPDATE FROM `comments` WHERE ìd` = 1

Transactions lock on update

Transac Transaction 1UPDATE tion 2`comments`SET `title` = “Foo”WHERE ìd` = 1 UPDATE `posts` SET `title` = “bar” locked.. WHERE ìd` = 1UPDATE `posts`SET `title` = “bar”WHERE ìd` = 1 DEADLOCK! UPDATE `comments` SET `title` = “foo” WHERE ìd` = 1

Callbacks =Transactions

Solutions?

Update dependentsbefore parents!

No longer deadlocks!class Comment after_create :update_posts_count def update_posts_count self.posts.increment :comments_count endendclass Posts before_update :update_comments def update_comments comments.each {|c| c.published = self.published; c.save} endend

Avoid StatePropagation

Avoids updating all commentsclass Comment < ActiveRecord::Base def visible? post.published? && self.published? endend

Success!

4 - Advanced“Acquire locks consistently”

O’taner

“There is no safe way to use Thread#kill or Thread#raise.”http://blog.headius.com/2008/02/rubys-threadraise-threadkill-timeoutrb.html

Killing ruby (MRI) threads - bluepill - god - monit - you and me

Simple Transactionclass Order belongs_to :user after_create :mark_user_as_customer def mark_user_as_customer user.customer = true user.save endend

What happened to my transaction?> order = Order.ﬁrst=> #<Order id: 1, user_id: 1>> order.user.customer?=> false> order.user?=> #<User id: 1, login: "marcos", customer: false>

WTF .. ?

If you kill your Ruby(MRI) Threads ..

ActiveRecord will partially commit your transactions!

ActiveRecord transaction (version for kids)def transaction yieldrescue Exception => e rollbackensure commit unless rolledback?end

Main thread is rescueddef print_if_rescued sleep 10rescue Exception puts rescuedensure puts ensuredendprint_if_rescued# $ killall ruby# => rescued# => ensured

Other Threads are NOT!def print_if_rescued sleep 10rescue Exception puts rescuedensure puts ensuredendt = Thread.new { print_if_rescued }t.join# $ killall ruby# => ensured

It commits!def transaction yield # thread killed inside yieldrescue Exception => e rollback # nothing is raisedensure commit unless rolledback?end

Solutions?

DON’T Kill Your Threads! Headius Approv

If you do, don’tuse ActiveRecord+ Threads + MRI

5 Record Certiﬁed Enterprise Active - ARCECA “Don’t use Concurrency ArchitectThread#kill/raise ”

Success!

Recap please!

1 - Noob“Use database constraints”

2 - Beginner“Use READ-COMMITTED”

3 - Half Mouth“Use an Options Model”

4 - Advanced“Acquire locks consistently”

5 Record Certiﬁed Enterprise Active - ARCECA “Don’t use Concurrency ArchitectThread#kill/raise ”

That’s all! Thank You!

Questions?Slides @mtoledo

Rails Concurrency Gotchas

by marcostoledo

Accessibility

Categories

Upload Details

Usage Rights

Statistics

Rails Concurrency Gotchas Presentation Transcript