From: Michael Neumann <mneumann@...>
Date: 2004-11-04T20:48:18+09:00
Subject: Re: [ANN] Transaction::Simple 1.2.0

On Thu, Nov 04, 2004 at 12:13:20PM +0900, Austin Ziegler wrote:
> Contraindications
> -----------------
> While Transaction::Simple is very useful, it has some severe limitations that
> must be understood. Transaction::Simple:
> 
> * uses Marshal. Thus, any object which cannot be Marshal-ed cannot use
>   Transaction::Simple.
> * does not manage resources. Resources external to the object and its instance
>   variables are not managed at all. However, all instance variables and
>   objects "belonging" to those instance variables are managed. If there are
>   object reference counts to be handled, Transaction::Simple will probably
>   cause problems.
> * is not thread-safe. In the ACID ("atomic, consistent, isolated, durable")
>   test, Transaction::Simple provides C and D, but it is up to the user of
>   Transaction::Simple to provide isolation. Transactions should be considered
>   "critical sections" in multi-threaded applications. Thread safety can be
>   ensured with Transaction::Simple::ThreadSafe.
> * does not maintain Object#__id__ values on rewind or abort. This may change
>   for future versions that will be Ruby 1.8 or better only.

Hm, I've written a similar library for Wee, which saves snapshots of
objects, and is able to restore the old object from a snapshot. It does
not use Marshal, and maintains __id__ values correctly.
Snapshots are, what you call "transactions".

  s = StateRegistry.new

  v = "good morning...."          # => "good morning...."
  p v.object_id                   # => xxxx

  s.register(v)

  snap1 = s.snapshot

  v.gsub!(/morning/, "evening")
  snap2 = s.snapshot

  v.gsub!(/good/, "superb")
  snap3 = s.snapshot

  snap1.apply
  p v                             # => "good morning...."
  p v.object_id                   # => xxxx

  snap2.apply
  p v                             # => "good evening...."
  p v.object_id                   # => xxxx

  snap3.apply
  p v                             # => "superb evening...."
  p v.object_id                   # => xxxx


Of course this works with any number of registered objects. How a
snapshot is taken or restored is defined per-class/object (methods
take_snapshot and apply_snapshot). 

http://ntecs.de/viewcvs/viewcvs/Wee/trunk/lib/wee/state_registry.rb?rev=289&view=auto
http://ntecs.de/viewcvs/viewcvs/Wee/trunk/lib/wee/snapshot.rb?rev=289&view=auto

I guess this could be used to implement transactions:

  def transaction(*vars)
    s = StateRegistry.new
    vars.each do |v| s.register(v) end
    snap = s.snapshot
    begin
      yield
    rescue Exception
      snap.apply     # restore old snapshot
    end
  end

  v = "good "          # => "good "
  transaction(v) do
    v << "morning"
    raise
  end
  p v                  # => "good " (and same __id__)


If this is not only usable for my framework, then I'll release this as
an extra library. 

Regards,

  Michael