[Twisted-Python] Is there a recommended way for a Service to request Application stop?

[Daniel Sutcliffe]

On Oct 10, 2016, at 4:38 PM, Daniel Sutcliffe <dansut at gmail.com> wrote:
> Generically, can I somehow bubble up events through the Service
> hierarchy, or should I communicate with external objects to the
> hierarchy that can bubble down actions from higher up?

On Mon, Oct 10, 2016 at 8:02 PM, Glyph Lefkowitz
<glyph at twistedmatrix.com> wrote:
> Following the
> <https://en.wikipedia.org/wiki/Single_responsibility_principle>, the service
> hierarchy's job is just to make sure everything gets started up and shut
> down together.
>
> It sounds to me like you have a pretty well-defined hierarchy which seems
> like it fits into the service hierarchy because it's roughly parallel in
> terms of which objects participate; however, you have very
> application-specific semantics for this parallel hierarchy.

Maybe this was my conceptual issue; the parallels were close enough
that it just felt right to add the functionality to Twisted's Service
Hierarchy - close enough that I was sucked into thoughts of
modbusPollingClient IS-A Service... but keeping in mind the arguments
of composition vs inheritance, I somehow finished up with
modbusClientService HAS-A pollingLogic - whereas what I think you are
recommending here is that that pollingLogic HAS-A modbusClientService
and knows about a MultiService which it makes use of only to let the
Application have overall control over starting and stopping all the
active Services.

Actually I just read this back through and am not sure this is
actually really what you meant Glyph :-/
I do get that it seems I am grabbing the stick by the wrong end, I
guess I'll just go back to reading more example code to see if I can
find something that resonates with me as being close to what I'm
trying to achieve.

> [...]
> When it's time to "stop" a service, do setServiceParent(None); when it's
> time to "start" it, do setServiceParent(appropriateServiceParent).  This
> should take care of keeping your services in the appropriate state.

I have to admit I had not even thought of using setServiceParent(None)
to bring services down - looking at the source makes this concept
clearer and potentially useful to me though

> BTW, if you have stateful long-running services that have to self-modify
> based on changing circumstances, you might want to also check out
> https://github.com/glyph/automat to see if it can help you ensure that
> everything's in a consistent state.

I'm still really just prototyping at this stage, trying to find an
architecture that fits what I believe should be quite a simple
application that is reasonably well suited to Twisted, but once I've
passed this stage if I do find myself needing a FSM that is beyond
totally simple then I'll certainly give this a look. Thanks.
/dan
-- 
Daniel Sutcliffe <dansut at gmail.com>