From: "Simon Kröger" <SimonKroeger@...>
Date: 2006-07-11T06:03:44+09:00
Subject: Re: Best way to synchronize access to a method?

Wes Gamble wrote:
> Wes Gamble wrote:
>> All,
>>
>> I'm reading through the Pickaxe book on synchronization and threading. 
>> I want to have a method that will basically put a MAX() value from a 
>> table and I want to ensure that two threads don't execute this code at 
>> the same time.
>>
>> I'm thinking I can just have my object descend from Monitor (< Monitor) 
>> and then put "synchronize" around the relevant code, like so:
>>
>> class myClass < Monitor
>> ...
>>
>> def get_max_value
>>   synchronize { max_value = do_select_to_get_max_value }
>>   max_value
>> end
>>
>> This should work just fine - correct?
>>
>> Thanks,
>> Wes
> 
> There is an admonition in the Pickaxe book that says "In both the class 
> form and when including MonitorMixin in an existing class it is 
> essential to invoke super in the class's initialize method."
> 
> I want my sychronization to occur in a class method.  So that would seem 
> to obviate the need for the super in the "initialize" block.  Am I 
> correct?
> 
> Thanks,
> Wes

You have several problems in your code you may want to look at before thinking
about Threads:

* myClass has to be spelled MyClass (see Constants in Ruby)

* get_max_value isn't a class method (use self.get_max_value if you want such)

* max_value is only defined inside the block, so you get
  "undefined local variable or method `max_value' for MyClass:Class"

* if you define it as class method the interpreter will show you
  "undefined method `synchronize' for MyClass:Class" because synchronize
  isn't a class method

* most of the time when the documentation is saying "it is essential to..."
  it is there for a reason

I don't want to be harsh, but it would be nice to let the interpreter find
the obvious bugs it is able to find.

maybe that is what you want:

require 'thread'

class MyClass
  @@mutex = Mutex.new

  def self.do_select_to_get_max_value;
    42
  end

  def self.get_max_value
    @@mutex.synchronize do
      return do_select_to_get_max_value
    end
  end	
end

p MyClass::get_max_value

cheers

Simon