[Twisted-Python] Deferred getChild (#3621)

[Laurens Van Houtven]

Hi,


I was wondering if anyone had any concrete thoughts about what should happen
with IResource.getChild being able to return Deferreds, or, in general, the
ability to asynchronously get children of a resource. The ticket for this is
#3621 <http://tm.tl/3621>.

My use case for this is txYoga <https://github.com/lvh/txyoga>. Long story
short, it lets you write REST-y webapps. So, you could have something like:

http://www.twistedmatrix.com/labs/glyph

In local jargon, labs is a collection, and glyph is an element in it. Right
now, the only collection that actually exists is an in-memory one, and the
next one I'm going to write is a SQLite one, so blocking on child access
seems relatively reasonable. In general, however, this should be implemented
using Deferreds, since the information about glyph might be stored in a
database far, far away.

The logical way to do that for me seems to be that in getChild, you'd

I have found a mailing list
post<http://www.twistedmatrix.com/pipermail/twisted-python/2002-October/001847.html>(from
2002) where Christopher Armstrong ostensibly solved the problem. I
can't find any replies to that mailing list post.

I remember Glyph saying something about how that could potentially
change/break public API. I understand that reservation, but I don't see how
it'd be that bad. Existing code that always immediately returns a resource
would still work -- it would merely only use a part of the API it's allowed
to use (in this case, it'd ignore the fact that it is allowed to return a
deferred). Since a Deferred isn't an IResource, it wouldn't be a legal value
to return now, anyway.

Obviously, code that previously relied on getChild always returning a child
*right now*, when now that getChild will occasionally return a deferred,
would break. I'm not sure yet if that's a serious problem, or just a breach
of the contract that code previously had. I'm thinking it's the latter, but
maybe my subconscious is cheating to get to the easier answer.

I don't actually call getChild myself in my application code. I call it in
tests, and Twisted calls it for me in production. Maybe I'm missing major
use cases for getChild there? I could definitely see how that works.

Thoughts welcome :)


cheers
lvh
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