Re: [libvirt] [RFC] Events API

On Fri, 2008-09-19 at 11:14 +0100, Richard W.M. Jones wrote: > On Thu, Sep 18, 2008 at 12:45:07PM -0400, David Lively wrote: > > I'm a little concerned that a vector of event type names isn't really > > adequate for specifying a filter. Does this need to be more general > > (XPathString exprs??) > > I think I'm with Dan on this one. I would start small -- just domains > coming & going (unless VirtualIron needs other events). Since there > is no limit to the number of API calls we can have in libvirt, add an > API call just for registering for these domain events. Instead of > trying to overload untyped strings with complicated meanings. Okay. I'm fine with a more strongly-typed event protocol. While it's more work to add new classes of events (as compared with extending event XML), that's probably a Good Thing :-)

> > > But my larger concern is that an asynchronous callback mechanism (as > > proposed above) assumes the presence of some thread / process from which > > to make the callbacks. This works fine in the libvirtd context, but > > not outside of it. For instance, we build a "client only" version of > > libvirt with ONLY the remote driver, which currently doesn't require > > pthreads at all. Introducing asynchronous callbacks into the API means > > pthreads is now required for this. > > I'm not quite sure I follow this -- you mean it introduces pthreads > into libvirt or into the caller? As far as I can see, nothing about > this would require threads in either. I meant that if we expected the callbacks to "just happen", libvirt (at least, the libvirtd-less version) would need to spawn a thread to make the callbacks. But this is quite easily avoided by providing a hook that clients are responsible for calling for event delivery, as suggested. I had considered this as an alternative to the file-descriptor interface (but thought the fd interface was convenient with to use w/select() & poll()).

After considering the problems with fds and power-saving and windows compatibility, I agree an event-delivery hook sounds like the best idea, perhaps: int virDeliverEvents(int timeout) where timeout is interpreted as in poll() (i.e., max millisecs to block, 0 means don't block, negative means block forever).

> > The remote protocol allows event messages to be passed back > asynchronously, although the current remote driver wouldn't expect to > receive them which might be a problem for backwards/forwards > compatibility. (Therefore the remote client must tell the remote > server that it can handle asynchronous events, and the remote client > must be prepared to talk to a server which cannot understand > asynchronous events -- there is an internal feature API you can use > for this). Notwithstanding that, you would have to add a client call > to poll for events or (better) to expose the file descriptor so that > callers can use it in select(2)/poll(2). Yeah, I expect the remote implementation will be the worst part! Thanks for the pointers.