On Sun, 04 Nov 2007 17:52:35 +0000, Paul Moore
I know this is a FAQ, but it's not in the FAQ list and I've had no luck finding a good answer via web or list searches (Windows is a lousy search term, and service isn't much use in a Twisted context...)
I want to run a tiny non-authoritative DNS server on my PC. I've never done this before, but twisted names looks ideal. The documentation (http://twistedmatrix.com/projects/names/documentation/howto/names.html) says I just need to do
mktap dns --recursive --cache twistd -f dns.tap
In recent versions of Twisted, you can simplify the above to this: twistd dns --recursive --cache This skips the tap phase: it creates the same application mktap would have created, but instead of putting it in the tap file, it just runs it.
and indeed this works fine. I seem to recall some time ago seeing postings to the effect that tap format files were out of favour, but I don't follow the twisted list much, so I don't know if this is still true. Nevertheless, it works so who cares?
Quite so.
However, I want to run this as a Windows service. Ideally, twistd would have a --install-service flag or something, but it doesn't so I went looking, and found postings covering tap2ntsvc which creates services from tap files, but it seems to have been replaced by ntsvc, which only handles .tac files :-(
It would be very excellent if twistd had a --install-service flag. There is a relatively minimal level of interest in Windows-specific functionality among the currently active Twisted developers, though. This means that it isn't very likely that any of us will undertake to add this feature. That doesn't mean that we'll never implement it, nor that we wouldn't accept a patch which implements it from someone else who is more interested in it than we are. If someone provides the functionality, with unit tests, then we'll gladly incorporate and maintain it. I'm not sure if you would be interested in doing this work or not; I'm directing this as much at you as at all of the other users of Twisted on Windows who I know are out there and mostly remain silently.
I get the impression that the idea is that .tac files are easy to create by hand, but as I know *nothing* about twisted names (and don't really want to - all I want is to run a DNS server!) it's not as easy as it might be...
They are, more or less, although they are harder to make than tap files. ;)
So, can someone tell me:
1. What is the currently favoured form for Twisted application files?
It all depends on the use-case. For all of the services distributed with Twisted itself, tap files offer very little over the persistence-free form of twistd I gave an example of above. Since tap files contain pickled data, they have all the problems of pickle (lack of introspectability, problems with compatibility, security); if these aren't problems, then tap files might be okay. On the other hand, many uses of twistd don't actually need any persistence, so using tap files doesn't actually provide any benefits.
2. How do I make one for twisted names (the equivalent of the mktap line above) and better still how do I generalise this for any mktap command?
See twisted.names.tap.makeService - this is the function `mktap dns´ is calling to create the object it will pickle into the tap file. Your tac file can call it directly with the desired configuration and add the resulting service to an application object. This will give you a tac which does the same thing as your mktap-produced tap file. You can read more about writing tac files here: http://twistedmatrix.com/projects/core/documentation/howto/application.html
3. How do I run it? (twistd, presumably)
As you said below, twistd -noy.
4. How do I make that file run as a Windows service?
I've never used ntsvc, so I can't say if that's right, but presumably it is at least close. There isn't any reason that if ntsvc works with tac files it couldn't also work with tap files, though. So another solution might be to explore adding tap file support to ntsvc. The interface for loading a tac file is really the same as the interface for loading a tap file, in fact, so I am slightly surprised ntsvc doesn't work with tap files.
I believe, from what I've found, that the answers are: 1. .tac 2. Don't know - not sure there's a generic answer. 3. twistd -noy 4. ntsvc
In which case, the documentation really needs to be updated to explain .tac files throughout, instead of mktap. And I need someone to write me a dns.tac file :-(
Ideally, this should be written up as a FAQ or HOWTO somewhere. I'll volunteer to write it up, as long as someone will maintain it - I'm not in a position to do so (I use twisted occasionally, but have no need or desire to follow development versions or discussions on the list). I don't want to bother doing a write-up, if it won't get included in the docs/FAQ and maintained, though...
Anything (accurate, coherent) you want to write up we'd be glad to add to the Twisted repository and include on the website, in releases, etc. Jean-Paul