[PEAK] Re: trellis.Set.discard

Sergey Schetinin maluke at gmail.com
Tue Oct 21 13:12:54 EDT 2008

> Yep, I just had the same idea this morning, and I think it's a *lot* simpler
> to implement correctly than deterministic merge.  We could allow todo cells
> to have a copy function, that defaults to copy.copy().  When a new rule
> reads the cell's future, it would change_attr to a copy and then return it,
> and we wouldn't need all the savepoint junk.  (Because rules are rolled back
> in their entirety for redo.)

Indeed a very small change!

>> And if we do that it's not a long way from what I propose. Also,
>> making the merge deterministic would allow us to add a number of
>> useful properties. For example, currently if one rule adds an item to
>> the Set and then some other discards it, the outcome depends on the
>> order they run. But if we choose to merge changes ourselves we can
>> make sure that such conflicting changes would be detected (no matter
>> what order they run in), but at the same time, adding then removing an
>> item wouldn't be considered a conflict as long as it's done in the
>> same rule.
> The problem with this is that error detection is deferred, which makes
> debugging even harder than it already is.

I agree that deferred validation is a problem, but I don't think that
it makes debugging harder. In case described in one case we have
non-deterministic behavior and no errors (a nightmare to debug), in
second case we get an error with useless traceback, but with a lot of
useful info available, like what rule wanted to add the item and which
one tried to remove it etc. This isn't the kind of error meant to be
caught and processed in runtime, but it would be very useful in
development. Anyway, probably it's a good idea to implement this
separately instead of stuffing it into todo/future, but merging value
/ todo variant would still be a useful basic building block.

>> Actually, it seems to me that rules should see the data structures in
>> the way they entered the transaction, so if some rule adds a new item
>> to the set, no other rule should be able to remove it before @todo
>> cell gets the value (Set.added for this example).
>> Anyway, ISTM that .futures the way they are don't fit Trellis model very
>> well.
> Futures are just a way to simplify creating trellis-ized data structures, so
> that you don't have to undo-log everything.  Replacing this with a merge
> requirement rather defeats the point; if you want things to happen
> deterministically, you can always use separate data structures and a rule
> that combines them in a specific order.
>> > The ability to have a side-transaction is a good idea, though.  (Really
>> > a
>> > "back transaction", since it is sort of happening in the past.)  I
>> > haven't
>> > thought a lot about the isolation parameters, though.
>> >
>> > Prototyping certainly would be helpful.
>> I don't think I will have anything interesting to show before next
>> week, but so far I'm using this monkeypatch:
> I still need to read back to your original notes on side transactions and
> why they're important, apart from the futures issue.  I think that
> snapshot-copying futures when accessed by a new rule will prevent futures
> from triggering rollbacks of the type you originally reported, so I'm not
> clear at the moment on what else needs to be done with side transactions.

I have searched through my code and I'm not using it right now. I
remember at least two different cases where side_txn made things work,
but after refactoring the code there's no need for it anymore. I think
the use was legitimate, but strangely, I can't reproduce the need in
first attempt. I'll try a little more and will get back to you if I
can reproduce it.

One more issue I can't reproduce right away was that in some cases a
Link is being created with Contant subject.  This happens while
rolling something back, so reproducing it with minimal code is
problematic. I fixed it by adding  "elif name == 'next_listener':
pass" to ConstantMixin.__setattr__, but can this be a rare, but normal
case or does it indicate a problem with my code?

  File "c:\files\checkouts\trellis\peak\events\stm.py", line 498, in atomically
    return super(Controller,self).atomically(self._process, func, args, kw)
  File "c:\files\checkouts\trellis\peak\events\stm.py", line 187, in atomically
  File "c:\files\checkouts\trellis\peak\events\stm.py", line 183, in atomically
    retval = func(*args, **kw)
  File "c:\files\checkouts\trellis\peak\events\stm.py", line 509, in _process
  File "c:\files\checkouts\trellis\peak\events\stm.py", line 322, in _retry
    self.rollback_to(min([self.has_run[r] for r in self.to_retry]))
  File "c:\files\checkouts\trellis\peak\events\stm.py", line 263, in rollback_to
  File "c:\files\checkouts\trellis\peak\events\stm.py", line 142, in __new__
    subject.next_listener = self
  File "c:\files\checkouts\trellis\peak\events\trellis.py", line 233,
in __setattr__
    raise AttributeError("Constants can't be changed", self)

Best Regards,
Sergey Schetinin

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