On 06:00 pm, peter.westlake@pobox.com wrote:
On Wed, Nov 27, 2013, at 17:13, exarkun@twistedmatrix.com wrote:
On 02:58 pm, peter.westlake@pobox.com wrote: ... So would it be possible for
t.a.s.Service.startService to be allowed to return a Deferred? Then the next service would only be started up when the Deferred fired.
Probably not.
There is some discussion on <https://twistedmatrix.com/trac/ticket/5941>.
That's helpful, thanks. I hadn't realized that startService was called before the reactor started.
Instead, I've passed in a Deferred to each service that needs to wait. How does this look?
# -*- mode: python -*- from twisted.application import service from twisted.internet import defer, reactor from twisted.internet.task import deferLater
application = service.Application('Dependencies')
def report(error): print 'ERROR', error reactor.stop()
class Runner(service.Service): def __init__(self, baton): self.baton = baton
def startService(self): print 'startService', self.name, reactor.running self.baton.addCallback(lambda ignore:deferLater(reactor, 1.0, self.realStartService)) self.baton.addErrback(report)
I'd be concerned with the state of `Runner` at this point in the process. What happens if the application gets shut down while that `deferLater` is still pending? `stopService` has a harder job because it might need to deal with a service that is partially initialized or a service that is completely initialized (and "partially initialized" may cover a multitude of different states depending on the complexity of your service).
def realStartService(self): print 'realStartService', self.name, reactor.running
baton = defer.succeed('pass me to each service')
foo = Runner(baton) foo.setName('foo') foo.setServiceParent(application)
bar = Runner(baton) bar.setName('bar') bar.setServiceParent(application)
What about something like this instead? @implementer(IService) class Runner(object): ... @classmethod def loadFromWhatever(cls, name): return deferLater(reactor, Runner, name) def __init__(self, name): self.name = name def startService(self): self.running = True print 'realStartService', self.name, reactor.running def parent(service): application.setServiceParent(service) loading = Runner.initializeFromWhatever("foo") loading.addCallback(parent) loading.addCallback(lambda ignored: Runner.initializeFromWhatever("bar")) loading.addCallback(parent) loading.addErrback(stopTheReactorOrWhatever) The advantage I see of this approach is that a `Runner` never exists in the service hierarchy until it is fully initialized, started, and running. If a `Runner` is only partially ready and the process shuts down then its `stopService` method isn't called because it's not part of the service hierarchy. I could definitely imagine a library to help with this kind of thing. For example, perhaps you want the above encapsulated as: asynchronouslySetUpServices(application, [ lambda: Runner.initializeFromWhatever("foo"), lambda: Runner.initialifrFromWhatever("bar")]) And maybe then you want to add in some logic so that if the application gets shut down while some things are still being initialized then you cancel their Deferred. Then you have good cleanup support for the uninitialized case - without complicating `stopService` (the cleanup logic is isolated in the implementation of Deferred cancellation where it belongs - eg, with this `deferLater`-based asynchronousness it's alreay present since deferLater implements cancellation already). Jean-Paul