[Twisted-Python] twisted.names and multicast DNS
exarkun at divmod.com
Sun Jul 10 14:16:34 EDT 2005
On Sat, 9 Jul 2005 17:08:15 +1000, Tim Allen <screwtape at froup.com> wrote:
>So after two years of leaving it alone, I decided to have another look at
>the multicast DNS module I was writing for Twisted, and lo and behold I
>discover a serious design flaw in my response cache. Here's the basic
>situation I'm dealing with:
>The problem is this: the Record_PTR class (just like all the other Record_*
>classes) in t.names.dns stores the TTL as an instance variable! Therefore,
>when I have an existing record and an updated record, they never match, and
>I wind up with two identical records in my data structure that differ only
>As far as I can tell, I have three options from here:
>1. Write my own versions of all the Record_* classes that don't maintain a
>TTL value, and translate backwards and forwards whenever I need to talk to
>Twisted's DNS layer. I'd need to update this code whenever a new DNS record
>type was added.
>2. Write a comparison function for comparing resource records that compares
>instance variables that aren't named 'ttl'. This would probably need to be
>updated less frequently, but is horribly hacky.
What's hacky about this? It seems like a simple, straightforward solution. Record classes have comparison defined one way, you want to compare them another way. Comparison can only be defined one way using the "==" operator, so clearly one of these requirements has to resort to using something other than the "==" operator.
I think it's even nicer than you suggest, since the attributes which should be compared are all declared on each class. eg, Record_NS.compareAttributes == ('name', 'ttl'). You should be able to write a comparison function that looks at this attribute and simply omits ttl.
Hope this helps,
More information about the Twisted-Python