Rails: Shadow Facets of Concurrency

Eugene Kalenkovich


audience.except(&:can_see_this).each do |person|
  # Please!
  person.move(:closer)
end

An Ode to 6+ Concurrency Facets (~10 minutes each) in ??? Minutes

You may not worry about concurrency if:

You use Rails
Your application is single-threaded
You have a very low traffic

Do you worry?

When?

RAILS_ENV=development

RAILS_ENV=production

Shared resources
Unpredictable request processing
- Who
- When

Part 1

Rails and Concurrency

RAILS_ENV=test

database.yml:


test:
  adapter: postgresql  # mysql2
  database: your_db_test

test_helper.rb (spec_helper.rb):


def concurrently processes = 10
  processes.times do
    fork do
      yield
    end
  end
  assert Process.waitall.map(&:last).all? &:success?
end


def concurrently processes = 10
  processes.times do
    fork do
      yield
    end
  end
  assert Process.waitall.map(&:last).all? &:success?
end


def concurrently processes = 10
  processes.times do
    fork do
      yield
    end
  end
  assert Process.waitall.map(&:last).all? &:success?
end

RAILS_ENV=test


def concurrently processes = 10
  ActiveRecord::Base.remove_connection
  processes.times do
    fork do
      begin
        ActiveRecord::Base.establish_connection
        yield
      rescue => e
        puts "#{e.class.name}: #{e}" && exit 1
      ensure
        ActiveRecord::Base.remove_connection
      end
    end
  end
  ActiveRecord::Base.establish_connection
  assert Process.waitall.map(&:last).all? &:success?
end


def concurrently processes = 10
  ActiveRecord::Base.remove_connection
  processes.times do
    fork do
      begin
        ActiveRecord::Base.establish_connection
        yield
      rescue => e
        puts "#{e.class.name}: #{e}" && exit 1
      ensure
        ActiveRecord::Base.remove_connection
      end
    end
  end
  ActiveRecord::Base.establish_connection
  assert Process.waitall.map(&:last).all? &:success?
end


def concurrently processes = 10
  ActiveRecord::Base.remove_connection
  processes.times do
    fork do
      begin
        ActiveRecord::Base.establish_connection
        yield
      rescue => e
        puts "#{e.class.name}: #{e}" && exit 1
      ensure
        ActiveRecord::Base.remove_connection
      end
    end
  end
  ActiveRecord::Base.establish_connection
  assert Process.waitall.map(&:last).all? &:success?
end

Facet #1: One and Only

first_or_create


class Number < ActiveRecord::Base
  # t.integer :value
end


describe Number do
  it 'should have unique values' do
    concurrently do
      10.times do
        Number.where(value: Number.count).first_or_create
      end
    end
    Number.count.must_equal Number.select('distinct value').count
  end
end


describe Number do
  it 'should have unique values' do
    concurrently do
      10.times do
        Number.where(value: Number.count).first_or_create
      end
    end
    Number.count.must_equal Number.select('distinct value').count
  end
end


describe Number do
  it 'should have unique values' do
    concurrently do
      10.times do
        Number.where(value: Number.count).first_or_create
      end
    end
    Number.count.must_equal Number.select('distinct value').count
  end
end


1) Failure:
test_0001_should_have_unique_values(Number) :
Expected: 49
  Actual: 99

first_or_create.inspect


> Number.where(value: 3).first_or_create
  Number Load (16.2ms)  SELECT "numbers".* FROM "numbers" WHERE "numbers"."value" = 3 LIMIT 1
   (0.2ms)  BEGIN
  SQL (9.0ms)  INSERT INTO "numbers" ("value") VALUES ($1) RETURNING "id"  [["value", 3]]
   (0.4ms)  COMMIT


> Number.where(value: 3).first_or_create
  Number Load (16.2ms)  SELECT "numbers".* FROM "numbers" WHERE "numbers"."value" = 3 LIMIT 1
   (0.2ms)  BEGIN
  SQL (9.0ms)  INSERT INTO "numbers" ("value") VALUES ($1) RETURNING "id"  [["value", 3]]
   (0.4ms)  COMMIT


> Number.where(value: 3).first_or_create
  Number Load (16.2ms)  SELECT "numbers".* FROM "numbers" WHERE "numbers"."value" = 3 LIMIT 1
   (0.2ms)  BEGIN
  SQL (9.0ms)  INSERT INTO "numbers" ("value") VALUES ($1) RETURNING "id"  [["value", 3]]
   (0.4ms)  COMMIT


> Number.where(value: 3).first_or_create
  Number Load (0.4ms)  SELECT "numbers".* FROM "numbers" WHERE "numbers"."value" = 3 LIMIT 1


> Number.where(value: 3).first_or_create
  Number Load (0.4ms)  SELECT "numbers".* FROM "numbers" WHERE "numbers"."value" = 3 LIMIT 1


> Number.where(value: 3).first_or_create
  Number Load (16.2ms)  SELECT "numbers".* FROM "numbers" WHERE "numbers"."value" = 3 LIMIT 1
   (0.2ms)  BEGIN
  SQL (9.0ms)  INSERT INTO "numbers" ("value") VALUES ($1) RETURNING "id"  [["value", 3]]
   (0.4ms)  COMMIT


> Number.where(value: 3).first_or_create
  Number Load (16.2ms)  SELECT "numbers".* FROM "numbers" WHERE "numbers"."value" = 3 LIMIT 1
   (0.2ms)  BEGIN
  SQL (9.0ms)  INSERT INTO "numbers" ("value") VALUES ($1) RETURNING "id"  [["value", 3]]
   (0.4ms)  COMMIT


> Number.where(value: 3).first_or_create
  Number Load (16.2ms)  SELECT "numbers".* FROM "numbers" WHERE "numbers"."value" = 3 LIMIT 1
   (0.2ms)  BEGIN
  SQL (9.0ms)  INSERT INTO "numbers" ("value") VALUES ($1) RETURNING "id"  [["value", 3]]
   (0.4ms)  COMMIT


> Number.where(value: 3).first_or_create
  Number Load (0.4ms)  SELECT "numbers".* FROM "numbers" WHERE "numbers"."value" = 3 LIMIT 1


> Number.where(value: 3).first_or_create
  Number Load (0.4ms)  SELECT "numbers".* FROM "numbers" WHERE "numbers"."value" = 3 LIMIT 1


> Number.where(value: 3).first_or_create
  Number Load (0.4ms)  SELECT "numbers".* FROM "numbers" WHERE "numbers"."value" = 3 LIMIT 1
   (0.2ms)  BEGIN
  SQL (0.3ms)  INSERT INTO "numbers" ("value") VALUES ($1) RETURNING "id"  [["value", 3]]
   (0.5ms)  COMMIT

validates :uniquiness


class ValidatedNumber < ActiveRecord::Base
  validates :value, uniqueness: true
end


describe ValidatedNumber do
  it 'should have unique values' do
    concurrently do
      10.times do
        ValidatedNumber.create{|r| r.value = ValidatedNumber.count}
      end
    end
    unique = ValidatedNumber.select("distinct value").count
    ValidatedNumber.count.must_equal unique
  end
end


1) Failure:
test_0001_should_have_unique_values(ValidatedNumber):
Expected: 37
  Actual: 96


> ValidatedNumber.create(value: 1)
   (0.1ms)  BEGIN
  ValidatedNumber Exists (25.2ms)  SELECT 1 AS one FROM "validated_numbers" WHERE "validated_numbers"."value" = 1 LIMIT 1
  SQL (14.7ms)  INSERT INTO "validated_numbers" ("value") VALUES ($1) RETURNING "id"  [["value", 1]]
   (0.7ms)  COMMIT


> ValidatedNumber.create(value: 1)
   (0.1ms)  BEGIN
  ValidatedNumber Exists (25.2ms)  SELECT 1 AS one FROM "validated_numbers" WHERE "validated_numbers"."value" = 1 LIMIT 1
  SQL (14.7ms)  INSERT INTO "validated_numbers" ("value") VALUES ($1) RETURNING "id"  [["value", 1]]
   (0.7ms)  COMMIT

unique.fix :db


class CreateConstrainedNumbers < ActiveRecord::Migration
  def change
    create_table :constrained_numbers do |t|
      t.integer :value
    end
    add_index :constrained_numbers, :value, unique: true
  end
end


class CreateConstrainedNumbers < ActiveRecord::Migration
  def change
    create_table :constrained_numbers do |t|
      t.integer :value
    end
    add_index :constrained_numbers, :value, unique: true
  end
end


# Running tests:

ActiveRecord::RecordNotUnique: PG::Error: ERROR:  duplicate key value violates unique constraint "index_constrained_numbers_on_value"
DETAIL:  Key (value)=(0) already exists.
: INSERT INTO "constrained_numbers" ("value") VALUES ($1) RETURNING "id"
ActiveRecord::RecordNotUnique: PG::Error: ERROR:  duplicate key value violates unique constraint "index_constrained_numbers_on_value"
DETAIL:  Key (value)=(0) already exists.
: INSERT INTO "constrained_numbers" ("value") VALUES ($1) RETURNING "id"
ActiveRecord::RecordNotUnique: PG::Error: ERROR:  duplicate key value violates unique constraint "index_constrained_numbers_on_value"
DETAIL:  Key (value)=(0) already exists.
: INSERT INTO "constrained_numbers" ("value") VALUES ($1) RETURNING "id"
ActiveRecord::RecordNotUnique: PG::Error: ERROR:  duplicate key value violates unique constraint "index_constrained_numbers_on_value"
DETAIL:  Key (value)=(0) already exists.
: INSERT INTO "constrained_numbers" ("value") VALUES ($1) RETURNING "id"
ActiveRecord::RecordNotUnique: PG::Error: ERROR:  duplicate key value violates unique constraint "index_constrained_numbers_on_value"
DETAIL:  Key (value)=(0) already exists.
: INSERT INTO "constrained_numbers" ("value") VALUES ($1) RETURNING "id"
[...]


# Running tests:

ActiveRecord::RecordNotUnique: PG::Error: ERROR:  duplicate key value violates unique constraint "index_constrained_numbers_on_value"
DETAIL:  Key (value)=(0) already exists.
: INSERT INTO "constrained_numbers" ("value") VALUES ($1) RETURNING "id"
ActiveRecord::RecordNotUnique: PG::Error: ERROR:  duplicate key value violates unique constraint "index_constrained_numbers_on_value"
DETAIL:  Key (value)=(0) already exists.
: INSERT INTO "constrained_numbers" ("value") VALUES ($1) RETURNING "id"
ActiveRecord::RecordNotUnique: PG::Error: ERROR:  duplicate key value violates unique constraint "index_constrained_numbers_on_value"
DETAIL:  Key (value)=(0) already exists.
: INSERT INTO "constrained_numbers" ("value") VALUES ($1) RETURNING "id"
ActiveRecord::RecordNotUnique: PG::Error: ERROR:  duplicate key value violates unique constraint "index_constrained_numbers_on_value"
DETAIL:  Key (value)=(0) already exists.
: INSERT INTO "constrained_numbers" ("value") VALUES ($1) RETURNING "id"
ActiveRecord::RecordNotUnique: PG::Error: ERROR:  duplicate key value violates unique constraint "index_constrained_numbers_on_value"
DETAIL:  Key (value)=(0) already exists.
: INSERT INTO "constrained_numbers" ("value") VALUES ($1) RETURNING "id"
[...]

unique.fix :db, diy: true


class SafeNumber < ActiveRecord::Base
  def self.first_or_create_where(*args)
    where(*args).first_or_create
  rescue ActiveRecord::RecordNotUnique
    retry
  end
end


class SafeNumber < ActiveRecord::Base
  def self.first_or_create_where(*args)
    where(*args).first_or_create
  rescue ActiveRecord::RecordNotUnique
    retry
  end
end


# Running tests:

.

Finished tests in 8.337332s, 0.1199 tests/s, 0.2399 assertions/s.

1 tests, 2 assertions, 0 failures, 0 errors, 0 skips


ActiveRecord::StatementInvalid: Mysql2::Error: Deadlock found when trying to get lock; try restarting transaction: INSERT INTO `safe_numbers` (`value`) VALUES (132)
ActiveRecord::StatementInvalid: Mysql2::Error: Deadlock found when trying to get lock; try restarting transaction: INSERT INTO `safe_numbers` (`value`) VALUES (132)
F


ActiveRecord::StatementInvalid: Mysql2::Error: Deadlock found when trying to get lock; try restarting transaction: INSERT INTO `safe_numbers` (`value`) VALUES (132)
ActiveRecord::StatementInvalid: Mysql2::Error: Deadlock found when trying to get lock; try restarting transaction: INSERT INTO `safe_numbers` (`value`) VALUES (132)
F

unique.fix :db, diy: true, mysql: true


def self.first_or_create_where(*args)
  where(*args).first_or_create
rescue ActiveRecord::RecordNotUnique
  retry
rescue ActiveRecord::StatementInvalid => e
  retry if e.message =~ /Deadlock/
  # Thank you, MySQL and Mysql2!
  raise
end


def self.first_or_create_where(*args)
  where(*args).first_or_create
rescue ActiveRecord::RecordNotUnique
  retry
rescue ActiveRecord::StatementInvalid => e
  retry if e.message =~ /Deadlock/
  # Thank you, MySQL and Mysql2!
  raise
end


# Running tests:

.

Finished tests in 3.234423s, 0.3092 tests/s, 0.6183 assertions/s.

1 tests, 2 assertions, 0 failures, 0 errors, 0 skips

Facet #2:
I Can Haz One or Many?

or Guilty by Association

class Dog


class Dog < ActiveRecord::Base
  has_one :head
  has_many :legs
end


class Head < ActiveRecord::Base
  belongs_to :dog
end

class Leg < ActiveRecord::Base
  belongs_to :dog
end

Dog.build


20.times{ Dog.create }

concurrently do
  begin
    headless = Dog.includes(:head).where(heads: {dog_id: nil}).first
    headless && headless.create_head

    legless = Dog.includes(:legs).where(legs: {dog_id: nil}).first
    legless && legless.legs = 4.times.map{ Leg.create }
  end while headless || legless
end


20.times{ Dog.create }

concurrently do
  begin
    headless = Dog.includes(:head).where(heads: {dog_id: nil}).first
    headless && headless.create_head

    legless = Dog.includes(:legs).where(legs: {dog_id: nil}).first
    legless && legless.legs = 4.times.map{ Leg.create }
  end while headless || legless
end


20.times{ Dog.create }

concurrently do
  begin
    headless = Dog.includes(:head).where(heads: {dog_id: nil}).first
    headless && headless.create_head

    legless = Dog.includes(:legs).where(legs: {dog_id: nil}).first
    legless && legless.legs = 4.times.map{ Leg.create }
  end while headless || legless
end


20.times{ Dog.create }

concurrently do
  begin
    headless = Dog.includes(:head).where(heads: {dog_id: nil}).first
    headless && headless.create_head

    legless = Dog.includes(:legs).where(legs: {dog_id: nil}).first
    legless && legless.legs = 4.times.map{ Leg.create }
  end while headless || legless
end

assert_sanity


Dog.all_str.must_equal "20 dogs with 1 head and 4 legs"


--- expected
+++ actual
@@ -1 +1 @@
-"20 dogs with 1 head and 4 legs"
+"3 dogs with 1 head and 8 legs, 2 dogs with 2 heads and 28 legs, 2 dogs with 1 head and 12 legs, 1 dog with 1 head and 4 legs, 1 dog with 4 heads and 24 legs, 1 dog with 4 heads and 32 legs, 1 dog with 3 heads and 16 legs, 1 dog with 2 heads and 32 legs, 1 dog with 1 head and 16 legs, 1 dog with 6 heads and 8 legs, 1 dog with 1 head and 20 legs, 1 dog with 2 heads and 16 legs, 1 dog with 4 heads and 36 legs, 1 dog with 5 heads and 24 legs, 1 dog with 8 heads and 20 legs, 1 dog with 2 heads and 40 legs"


--- expected
+++ actual
@@ -1 +1 @@
-"20 dogs with 1 head and 4 legs"
+"3 dogs with 1 head and 8 legs, 2 dogs with 2 heads and 28 legs, 2 dogs with 1 head and 12 legs, 1 dog with 1 head and 4 legs, 1 dog with 4 heads and 24 legs, 1 dog with 4 heads and 32 legs, 1 dog with 3 heads and 16 legs, 1 dog with 2 heads and 32 legs, 1 dog with 1 head and 16 legs, 1 dog with 6 heads and 8 legs, 1 dog with 1 head and 20 legs, 1 dog with 2 heads and 16 legs, 1 dog with 4 heads and 36 legs, 1 dog with 5 heads and 24 legs, 1 dog with 8 heads and 20 legs, 1 dog with 2 heads and 40 legs"

dog.head.inspect


> Dog.first.create_head
  Dog Load (0.3ms)  SELECT "dogs".* FROM "dogs" LIMIT 1
   (0.1ms)  BEGIN
  SQL (12.0ms)  INSERT INTO "heads" ("dog_id") VALUES ($1) RETURNING "id"  [["dog_id", 1]]
   (0.9ms)  COMMIT
  Head Load (0.2ms)  SELECT "heads".* FROM "heads" WHERE "heads"."dog_id" = 1 LIMIT 1
   (0.1ms)  BEGIN
   (0.2ms)  UPDATE "heads" SET "dog_id" = NULL WHERE "heads"."id" = 1
   (0.4ms)  COMMIT


> Dog.first.create_head
  Dog Load (0.3ms)  SELECT "dogs".* FROM "dogs" LIMIT 1
   (0.1ms)  BEGIN
  SQL (12.0ms)  INSERT INTO "heads" ("dog_id") VALUES ($1) RETURNING "id"  [["dog_id", 1]]
   (0.9ms)  COMMIT
  Head Load (0.2ms)  SELECT "heads".* FROM "heads" WHERE "heads"."dog_id" = 1 LIMIT 1
   (0.1ms)  BEGIN
   (0.2ms)  UPDATE "heads" SET "dog_id" = NULL WHERE "heads"."id" = 1
   (0.4ms)  COMMIT


> Dog.first.create_head
  Dog Load (0.3ms)  SELECT "dogs".* FROM "dogs" LIMIT 1
   (0.1ms)  BEGIN
  SQL (12.0ms)  INSERT INTO "heads" ("dog_id") VALUES ($1) RETURNING "id"  [["dog_id", 1]]
   (0.9ms)  COMMIT
  Head Load (0.2ms)  SELECT "heads".* FROM "heads" WHERE "heads"."dog_id" = 1 LIMIT 1
   (0.1ms)  BEGIN
   (0.2ms)  UPDATE "heads" SET "dog_id" = NULL WHERE "heads"."id" = 1
   (0.4ms)  COMMIT


> Dog.first.create_head
  Dog Load (0.3ms)  SELECT "dogs".* FROM "dogs" LIMIT 1
   (0.1ms)  BEGIN
  SQL (12.0ms)  INSERT INTO "heads" ("dog_id") VALUES ($1) RETURNING "id"  [["dog_id", 1]]
   (0.9ms)  COMMIT
  Head Load (0.2ms)  SELECT "heads".* FROM "heads" WHERE "heads"."dog_id" = 1 LIMIT 1
   (0.1ms)  BEGIN
   (0.2ms)  UPDATE "heads" SET "dog_id" = NULL WHERE "heads"."id" = 1
   (0.4ms)  COMMIT


> Dog.first.create_head
  Dog Load (0.3ms)  SELECT "dogs".* FROM "dogs" LIMIT 1
   (0.1ms)  BEGIN
  SQL (12.0ms)  INSERT INTO "heads" ("dog_id") VALUES ($1) RETURNING "id"  [["dog_id", 1]]
   (0.9ms)  COMMIT
  Head Load (0.2ms)  SELECT "heads".* FROM "heads" WHERE "heads"."dog_id" = 1 LIMIT 1
   (0.1ms)  BEGIN
   (0.2ms)  UPDATE "heads" SET "dog_id" = NULL WHERE "heads"."id" = 1
   (0.4ms)  COMMIT

dog.legs.inspect


>   Dog.first.legs = Leg.all.sample(4)
  Dog Load (0.3ms)  SELECT "dogs".* FROM "dogs" LIMIT 1
  Leg Load (0.2ms)  SELECT "legs".* FROM "legs"
  Leg Load (0.3ms)  SELECT "legs".* FROM "legs" WHERE "legs"."dog_id" = 1
   (0.1ms)  BEGIN
   (0.2ms)  UPDATE "legs" SET "dog_id" = 1 WHERE "legs"."id" = 6
   (0.2ms)  UPDATE "legs" SET "dog_id" = 1 WHERE "legs"."id" = 3
   (0.2ms)  UPDATE "legs" SET "dog_id" = 1 WHERE "legs"."id" = 10
   (0.1ms)  UPDATE "legs" SET "dog_id" = 1 WHERE "legs"."id" = 5
   (0.5ms)  COMMIT


> Dog.first.legs = Leg.all.sample(4)
  Dog Load (0.4ms)  SELECT "dogs".* FROM "dogs" LIMIT 1
  Leg Load (0.3ms)  SELECT "legs".* FROM "legs"
  Leg Load (0.3ms)  SELECT "legs".* FROM "legs" WHERE "legs"."dog_id" = 1
   (0.1ms)  BEGIN
  SQL (0.3ms)  UPDATE "legs" SET "dog_id" = NULL WHERE "legs"."dog_id" = 1 AND "legs"."id" IN (10, 5)
   (0.2ms)  UPDATE "legs" SET "dog_id" = 1 WHERE "legs"."id" = 2
   (0.2ms)  UPDATE "legs" SET "dog_id" = 1 WHERE "legs"."id" = 9
   (0.6ms)  COMMIT


> Dog.first.legs = Leg.all.sample(4)
  Dog Load (0.4ms)  SELECT "dogs".* FROM "dogs" LIMIT 1
  Leg Load (0.3ms)  SELECT "legs".* FROM "legs"
  Leg Load (0.3ms)  SELECT "legs".* FROM "legs" WHERE "legs"."dog_id" = 1
   (0.1ms)  BEGIN
  SQL (0.3ms)  UPDATE "legs" SET "dog_id" = NULL WHERE "legs"."dog_id" = 1 AND "legs"."id" IN (10, 5)
   (0.2ms)  UPDATE "legs" SET "dog_id" = 1 WHERE "legs"."id" = 2
   (0.2ms)  UPDATE "legs" SET "dog_id" = 1 WHERE "legs"."id" = 9
   (0.6ms)  COMMIT

dog.head.fix

unique.fix :db, diy: true, mysql: mysql?

dog.fix :db


headless = Dog.includes(:head).where(heads: {dog_id: nil}).first
headless && Dog.transaction do
  headless.lock!
  headless.create_head
end
legless = Dog.includes(:legs).where(legs: {dog_id: nil}).first
legless && Dog.transaction do
  legless.lock!
  legless.legs = 4.times.map{ Leg.create }
end


headless = Dog.includes(:head).where(heads: {dog_id: nil}).first
headless && Dog.transaction do
  headless.lock!
  headless.create_head
end
legless = Dog.includes(:legs).where(legs: {dog_id: nil}).first
legless && Dog.transaction do
  legless.lock!
  legless.legs = 4.times.map{ Leg.create }
end


1 tests, 2 assertions, 0 failures, 0 errors, 0 skips


--- expected
+++ actual
@@ -1 +1 @@
-"50 dogs with 1 head and 4 legs"
+"30 dogs with 1 head and 4 legs, 11 dogs with 2 heads and 4 legs, 4 dogs with 3 heads and 4 legs, 4 dogs with 4 heads and 4 legs, 1 dog with 5 heads and 4 legs"

dog.refix :db


> Dog.first.create_head
  Dog Load (0.3ms)  SELECT "dogs".* FROM "dogs" LIMIT 1
   (0.1ms)  BEGIN
  SQL (12.0ms)  INSERT INTO "heads" ("dog_id") VALUES ($1) RETURNING "id"  [["dog_id", 1]]
   (0.9ms)  COMMIT
  Head Load (0.2ms)  SELECT "heads".* FROM "heads" WHERE "heads"."dog_id" = 1 LIMIT 1
   (0.1ms)  BEGIN
   (0.2ms)  UPDATE "heads" SET "dog_id" = NULL WHERE "heads"."id" = 1
   (0.4ms)  COMMIT


> Dog.first.head = Head.create
  Dog Load (39.8ms)  SELECT "dogs".* FROM "dogs" LIMIT 1
   (0.1ms)  BEGIN
  SQL (49.1ms)  INSERT INTO "heads" ("dog_id") VALUES ($1) RETURNING "id"  [["dog_id", nil]]
   (0.6ms)  COMMIT
"replace"
  Head Load (0.4ms)  SELECT "heads".* FROM "heads" WHERE "heads"."dog_id" = 1 LIMIT 1
   (0.1ms)  BEGIN
   (0.4ms)  UPDATE "heads" SET "dog_id" = NULL WHERE "heads"."id" = 2
   (0.2ms)  UPDATE "heads" SET "dog_id" = 1 WHERE "heads"."id" = 3
   (0.5ms)  COMMIT


headless = Dog.includes(:head).where(heads: {dog_id: nil}).first
headless && Dog.transaction do
  headless.lock!
  headless.head = Head.create
end
legless = Dog.includes(:legs).where(legs: {dog_id: nil}).first
legless && Dog.transaction do
  legless.lock!
  legless.legs = 4.times.map{ Leg.create }
end


# Running tests:

.

Finished tests in 2.527906s, 0.3956 tests/s, 0.7912 assertions/s.

1 tests, 2 assertions, 0 failures, 0 errors, 0 skips

dog.fix :diy


headless = Dog.includes(:head).where(heads: {dog_id: nil}).first
if headless
  headless.create_head
  Head.update_all({ dog_id: nil },
    { id: Head.where(dog_id: headless.id).order(:id).pluck(:id)[0..-2] })
end
legless = Dog.includes(:legs).where(legs: {dog_id: nil}).first
if legless
  legless.legs = 4.times.map{ Leg.create }
  Leg.update_all({ dog_id: nil },
    { id: Leg.where(dog_id: legless.id).order(:id).pluck(:id)[0..-5] })
end


headless = Dog.includes(:head).where(heads: {dog_id: nil}).first
if headless
  headless.create_head
  Head.update_all({ dog_id: nil },
    { id: Head.where(dog_id: headless.id).order(:id).pluck(:id)[0..-2] })
end
legless = Dog.includes(:legs).where(legs: {dog_id: nil}).first
if legless
  legless.legs = 4.times.map{ Leg.create }
  Leg.update_all({ dog_id: nil },
    { id: Leg.where(dog_id: legless.id).order(:id).pluck(:id)[0..-5] })
end


headless = Dog.includes(:head).where(heads: {dog_id: nil}).first
if headless
  headless.create_head
  Head.update_all({ dog_id: nil },
    { id: Head.where(dog_id: headless.id).order(:id).pluck(:id)[0..-2] })
end
legless = Dog.includes(:legs).where(legs: {dog_id: nil}).first
if legless
  legless.legs = 4.times.map{ Leg.create }
  Leg.update_all({ dog_id: nil },
    { id: Leg.where(dog_id: legless.id).order(:id).pluck(:id)[0..-5] })
end


# Running tests:

.

Finished tests in 2.957711s, 0.3956 tests/s, 0.7912 assertions/s.

1 tests, 2 assertions, 0 failures, 0 errors, 0 skips

Facet #3: Emigration

db:migrate

Add table - OK
Add column, add/remove index - so-so
Remove column - oops (before 4.0)

try :remove_column


remove_column :extra_columns, :extra


> ExtraColumn.create
   (0.2ms)  BEGIN
  SQL (34.9ms)  INSERT INTO "extra_columns" ("extra", "value") VALUES ($1, $2) RETURNING "id"  [["extra", nil], ["value", nil]]
   (0.2ms)  ROLLBACK
ActiveRecord::StatementInvalid: PG::Error: ERROR:  column "extra" of relation "extra_columns" does not exist at character 30
: INSERT INTO "extra_columns" ("extra", "value") VALUES ($1, $2) RETURNING "id"


> ExtraColumn.create
   (0.2ms)  BEGIN
  SQL (34.9ms)  INSERT INTO "extra_columns" ("extra", "value") VALUES ($1, $2) RETURNING "id"  [["extra", nil], ["value", nil]]
   (0.2ms)  ROLLBACK
ActiveRecord::StatementInvalid: PG::Error: ERROR:  column "extra" of relation "extra_columns" does not exist at character 30
: INSERT INTO "extra_columns" ("extra", "value") VALUES ($1, $2) RETURNING "id"

remove_column.inspect


def columns
  @columns ||= connection.schema_cache.columns[table_name].map do |col|
    col = col.dup
    col.primary = (col.name == primary_key)
    col
  end
end

All operations are defined in terms of klass.columns (directly or indirectly)
True for Rails 2.x, 3.0 3.2

remove_column.fix


class ExtraColumn < ActiveRecord::Base
  def self.columns
    @my_columns ||= super.reject{ |c|
      c.name == 'extra'
    }
  end
end

Deploy


remove_column :extra_columns, :extra

Do cleanup and re-deploy

remove_column.fix :rails_3_1

Facet #4: Assets & Liabilities

deploy :shutdown

deploy :rolling_swap

deploy :rolling_shutdown

rake assets:fix


config.action_controller.asset_host = "//your.super.asset.host"

For S3 you can use gem 'asset_sync'
... or roll out your own solution to upload precompiled assets before deployment
Do not delete old assets too early


config.assets.digest = true

Part 2

Off Rails

Facet #5. Development and Deployment

Concurrency of development with deployment . . . has almost always proven counterproductive

Harold Brown

Development and Deployment

I Can Haz Four Volunteers?

changed_feature.deploy!

Teach old dog new tricks (but do not show them)
Teach new dog old tricks
Swap your dogs
Wait...
Now you can forget old tricks

Your website is an API server and a client at the same time (but client is behind!)
Treat changing features as a new API version

You may not worry about concurrency if:

You do not care
But if you do – worry, and you will (almost) always find your way

Rails is omakase. A team of chefs picked out the ingredients, designed the APIs, and arranged the order of consumption on your behalf according to their idea of what would make for a tasty full-stack framework.

DHH

NOTICE: Consuming raw or undercooked meats, poultry, seafood, shellfish, or eggs may increase your risk of foodborne illness

Facets #6 - #N. Homework

Design a Presentation Web App

User can add a slide at an arbitrary place
User can easily reorder slides
All edits are saved "behind the scenes"

Consider:

Synchronous vs. asynchronous messaging
Lost slide reordering messages
Reordering messages coming out-of-order
User adding, editing and/or reordering on a stale page
Extra bonus: collaborative editing

Please talk to me when you are done...

Credits

Hakim El Hattab - Reveal.js, http://lab.hakim.se/reveal-js/#/
"Concurrency." Merriam-Webster.com. Merriam-Webster, n.d. Web. 23 Aug. 2013. [http://www.merriam-webster.com/dictionary/concurrency]
Wikipedia - http://en.wikipedia.org/wiki/Concurrency_(computer_science)
David Heinemeier Hansson - http://david.heinemeierhansson.com/2012/rails-is-omakase.html
Harold Brown - http://www.foreignaffairs.com/articles/40540/harold-brown/is-sdi-technically-feasible
Images:

By Garrett Fitzgerald [CC-BY-SA-2.5] - http://commons.wikimedia.org/wiki/File:SeattleMonorailAccident.jpg
By SoRah42 [CC BY-SA 3.0] - http://sorah42.deviantart.com/art/Unique-187278898
By Photographer not identified [Public domain], via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:GuateQuake1976BentRailsA.jpg
By Studio Lévy and Sons (Studio Lévy & fils) [2] ([1]) [Public domain], via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Train_wreck_at_Montparnasse_1895.jpg
By John McNab [CC BY-NC-SA 2.0] - http://www.flickr.com/photos/johnmcnab/6511819727/
http://commons.wikimedia.org/wiki/File:Loket_vid_j%C3%A4rnv%C3%A4gsolyckan_i_Get%C3%A5_1918.jpg
By A.J. Russell (American, 1830 - 1902) (1830 - 1902) (American) (photographer, Details of artist on Google Art Project) (Google Art Project: Home - pic) [Public domain], via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:A.J._Russell_(American_-_Railroad_Accident_Caused_by_Rebels_-_Google_Art_Project.jpg
By SCFiasco [CC BY-NC-SA 2.0] - http://www.flickr.com/photos/scfiasco/4490322916/

Q & A

Code: https://github.com/UncleGene/concur

Slides: http://203.softover.com/concur/rency

Rails: Shadow Facets of Concurrency

An Ode to 6+ Concurrency Facets (~10 minutes each) in ??? Minutes

You may not worry about concurrency if:

Do you worry?

When?

RAILS_ENV=development

RAILS_ENV=production

Part 1

Rails and Concurrency

RAILS_ENV=test

RAILS_ENV=test

Facet #1: One and Only

first_or_create

first_or_create.inspect

validates :uniquiness

unique.fix :db

unique.fix :db, diy: true

unique.fix :db, diy: true, mysql: true

Facet #2:I Can Haz One or Many?

or Guilty by Association

class Dog

Dog.build

assert_sanity

dog.head.inspect

dog.legs.inspect

dog.head.fix

dog.fix :db

dog.refix :db

dog.fix :diy

Facet #3: Emigration

db:migrate

try :remove_column

remove_column.inspect

remove_column.fix

remove_column.fix :rails_3_1

Facet #4: Assets & Liabilities

deploy :shutdown

deploy :rolling_swap

deploy :rolling_shutdown

rake assets:fix

Part 2

Off Rails

Facet #5. Development and Deployment

Development and Deployment

changed_feature.deploy!

You may not worry about concurrency if:

Facets #6 - #N. Homework

Design a Presentation Web App

Consider:

Please talk to me when you are done...

Credits

Q & A

Facet #2:
I Can Haz One or Many?