On Sep 18, 12:55 am, Howard Yeh <[email protected]> wrote:
> Hi,
>
> I understand that ActiveRecord is threadsafe, but because the C-
> adapters for both postgresql and mysql use blocking calls, it's
> surprisingly easy to cause deadlock.
Does it eventually undeadlock ? when mysql blocks because there's a
deadlock it should eventually give up (although of course with your
test script this would soon happen again). Deadlocks are just a risk
of life when it comes to transactions. MRI's threading model does
indeed mean that a threadsafe rails is much less useful than on
something like jruby
Fred
>
> assume Balance is a model that tracks the balance of a bank account,
> and we don't use optimistic locking.
>
> b = Balance.find_or_create_by_id(1)
> while true
> Thread.new {
> b = Balance.find(1) # assumes account is there
> b.balance = b.balance + 1
> b.save!
> puts "."
> }
> end
>
> this would reliably lock a process. it only happens when transaction
> is used (which it is, when balance.save!). The steps leading to the
> deadlock seems to be this
>
> thread1 => begin
> thread1 => update => mysql
> thread1 preempted
> thread2 scheduled
> thread2 => begin
> thread2 => update => mysql
> thread2 => C read()
> thread2 blocks whole process on read()
> thread1 doesn't get to commit
>
> process blocks
>
> you can try running the test script,
>
> > ruby thread-test 1 # should be ok, because no transaction is used
> > ruby thread-test 2 # locks
> > ruby thread-test 3 # locks
>
> File: 'thread-test.rb':
>
> require 'rubygems'
> require 'thread'
> require 'activerecord'
>
> DB = ENV["DB"] || "mysql"
> `rm sql.log` if File.exist?("sql.log")
> ActiveRecord::Base.logger = ::Logger.new("sql.log")
> ActiveRecord::Base.establish_connection \
> :adapter => DB,
> #:adapter => "mysql"
> #:adapter => "postgresql"
> #:adapter => "jdbcmysql"
> :database => "thread_test",
> :username => "vp",
> :password => "vp",
> :socket => "/var/run/mysqld/mysqld.sock",
> :host => "localhost",
> :port => 5432,
> :pool => 50
>
> ActiveRecord::Schema.define(:version => 1) do
> create_table "balances", :force => true do |t|
> t.integer "balance", :default => 0
> end
> end
>
> class Balance < ActiveRecord::Base
> end
>
> b = Balance.find_or_create_by_id(2)
> puts b.balance
>
> which_test = ARGV[0].to_i || 1
>
> case which_test
> when 1
> puts "Raw update, no transaction"
> while true
> Thread.new {
> sql = <<-SQL
> UPDATE balances SET balance = balance+1 WHERE id = 2
> SQL
> ActiveRecord::Base.connection.execute(sql)
>
> puts "."
> }
> end
> when 2
> puts "Update through ActiveRecord"
> while true
> Thread.new {
> b = Balance.find(2)
> b.balance = b.balance + 1
> b.save!
> puts "."
> }
> end
> when 3
> puts "Raw update in transaction"
> while true
> Thread.new {
> sql = <<-SQL
> UPDATE balances SET balance = balance+1 WHERE id = 2
> SQL
> ActiveRecord::Base.transaction do
> ActiveRecord::Base.connection.execute(sql)
> end
> puts "."
> }
> end
> end
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