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.
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
