Rails: Shadow Facets of Concurrency
Eugene Kalenkovich
audience.except(&:can_see_this).each do |person|
# Please!
person.move(:closer)
endAn 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?
enddef concurrently processes = 10
processes.times do
fork do
yield
end
end
assert Process.waitall.map(&:last).all? &:success?
enddef concurrently processes = 10
processes.times do
fork do
yield
end
end
assert Process.waitall.map(&:last).all? &:success?
endRAILS_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?
enddef 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?
endFacet #1: One and Only
first_or_create
class Number < ActiveRecord::Base
# t.integer :value
enddescribe 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
enddescribe 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
enddescribe 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
end1) Failure:
test_0001_should_have_unique_values(Number) :
Expected: 49
Actual: 99first_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
enddescribe 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
endclass 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
endclass 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 skipsActiveRecord::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)
FActiveRecord::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)
Funique.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
enddef 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 skipsFacet #2:
I Can Haz One or Many?
or Guilty by Association
class Dog
class Dog < ActiveRecord::Base
has_one :head
has_many :legs
endclass Head < ActiveRecord::Base
belongs_to :dog
end
class Leg < ActiveRecord::Base
belongs_to :dog
endDog.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
end20.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
end20.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
end20.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
endassert_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 }
endheadless = 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 }
end1 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) COMMITheadless = 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 skipsdog.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] })
endheadless = 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] })
endheadless = 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 skipsFacet #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
endremove_column :extra_columns, :extraremove_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 = truePart 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
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/