[Twisted-Python] schemas (in Zope and Foolscap)

Stephen Waterbury waterbug at pangalactic.us
Thu Jul 19 20:36:36 EDT 2007

Brian Warner wrote:
> [my apologies if this gets duplicated, I've been having SMTP problems]

It appears that this message was relayed twice, once today and once
yesterday, and both times I only received the cc to me, and not the
copy that was addressed to the twisted-python list.  So I'm quoting
the whole message below in case others haven't seen it.

> Stephen Waterbury <waterbug at pangalactic.us> writes:
>> I am in the process of converting some of my code to use zope.schema
>> instead of a similar thing that I independently invented.  Since I am
>> also planning on using Foolscap, and since Foolscap has a schema
>> module, I compared zope.schema and foolscap.schema, and to me they
>> seem to have much overlap in design intent.  In particular, in the
>> zope.schema use cases ...
>> So I was wondering -- would it make sense to use zope.schema
>> in Foolscap?
> Yeah, that sounds quite intriguing. The Foolscap schemas serve a number of
> similar purposes:
> #1: provide a clear place to document a program's remote interfaces. This
>     role is aimed at humans, not programs.
> #2: make it convenient to enforce those interfaces on the receiving end.. by
>     relying upon the guards, the code that actually does stuff is easier to
>     read and easier to analyze for other potential problems
> #3: enforce those interfaces on the sending end, to bring the discovery of
>     programming errors closer to their cause
> #4: enforce those interfaces at the receiving *wire*, to mitigate
>     resource-exhaustion attacks
> #1 is satisfied by pretty much anything, as it's more a set of documentation
> #conventions than anything else. The existing zope.interface.Interface class
> #does this pretty well.
> The rest require that the interfaces be machine-parseable. I enhanced z.i's
> Interface (in foolscap.RemoteInterface) to allow the arguments and return
> values to have meaning.. if zope.schema does something similar, I'd love to
> take a look at it and see if it can capture the same expressiveness.

Yes, that appears to be one of the things zope.schema does.

> I'm undecided as to whether #4 is a good idea or not. It seemed like a good
> idea when I first started, but I've had some smart people tell me it's not
> the best place to attempt to solve DoS problems. Worse yet, the
> implementation is so incomplete that I've personally had to disable schema
> checking on things that would otherwise be useful (in particular I think a
> "PolyConstraint" in which the two branches are both containers fails to match
> the tokens on the wire correctly, even though such a thing would be quite
> useful just at the post-serialization phase). So I'm tempted to drop #4 as a
> design criterion and if z.s can represent enough to make that work, great, if
> not, drop the feature.
>> A nice practical factor is that zope.schema has been packaged as
>> a separate namespace module, similarly to zope.interface.
> I have to admit that I'm slightly hesitant to add a new dependency. But maybe
> it isn't too big or too unwieldy. There doesn't seem to be a debian package
> for it, though..

Right, and for both of those reasons (extra dependency and the debian
packaging lag) I would like to see zope.schema merged into
zope.interface, since zope.schema depends on zope.interface and I don't
think it would be a big burden on zope.interface to carry along
zope.schema even if zope.interface users don't all use it.  I'll do a
little lobbying for that on interface-dev (which has been extremely
quiet lately ... probably a good sign).

> But I'll definitely check it out. Thanks for the suggestion!
>  -Brian

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