From: Michael Neumann <mneumann@...>
Date: 2004-11-05T00:24:30+09:00
Subject: Re: [ANN] Transaction::Simple 1.2.0

On Thu, Nov 04, 2004 at 10:08:17AM -0500, Austin Ziegler wrote:
> On Thu, 4 Nov 2004 15:21:49 +0100, Michael Neumann
> >> reasons that I implemented it this way, the most important being
> >> your implementation of Array#take_snapshot: it dups the array:
> >> 
> >> irb#1(main):001:0> arr = %w(a b c d)
> >> => ["a", "b", "c", "d"]
> >> irb#1(main):002:0> arr2 = arr.dup
> >> => ["a", "b", "c", "d"]
> >> irb#1(main):003:0> arr2[0] << "z"
> >> => "az"
> >> irb#1(main):004:0> arr
> >> => ["az", "b", "c", "d"]
> >> irb#1(main):005:0> 
> > 
> > Right! And that's for my purposes a good thing, as I register
> > objects that I want to be "backtrackable" (it's more flexible this
> > way).
> 
> Mmm. More flexible, but more work. When I want a transaction on an
> object, I want its full state kept. This, to me, is useless:
> 
>     a = %w(a b c)         # 'a', 'b', 'c'
>       # take a snapshot
>     a << "d"              # 'a', 'b', 'c', 'd'
>     a[0] << "z"           # 'az', 'b', 'c', 'd'
>       # restore           # 'az', 'b', 'c'

Sure. My problem indeed is completely different. I register callbacks
for components which live in a "tree". Then I want to be able to make
snapshots and go back in time. I can't use marshal (or dup), without
loosing the callbacks (or use an external representation of the
components, e.g. as IOWA does with it's positional component id's).

> Simply taking snapshots of the current object references (which is
> what your snapshot process does) only performs part of the necessary
> behaviour for true transaction support. I actually was trying
> something like this (not as formalised, certainly) with PDF::Writer
> and was running into problems because my objects were changing even
> when I didn't want them to change, which is why I created
> Transaction::Simple.

Well, it's up to you whether they change or not, or whether you register
them for being part of the transaction or not. But sure, deep-cloning is
probably much more user-friendly. 

Regards,

  Michael