> On May 19, 2015, at 05:59, exar...@twistedmatrix.com wrote:
>
> On 12:07 pm, zadka.mo...@gmail.com wrote:
>> Background
>>
>> Currently, "twistd" assumes one-run-one-plugin. It would be nice to load up
>> multiple plug-ins in Twisted, for many reasons. These include: serving the
>> same in-memory content via separate protocols, adding manhole to other
>> plug-ins (so the end-deployer can add it to other things, as opposed to the
>> original implementor, and a catch-all category of "auxiliary services".
>>
>> Auxiliary services are those which are not useful in and of themselves, but
>> add value to a service which does something else of use.
>>
>> Examples of auxiliary services -- a logging service (that connects to some
>> logging protocol on start-up), a metrics service (that sends statistics to
>> a collector like statsd or riemann) or an error-sending service (to
>> something like Sentry).
>>
>> Proposal
>>
>> tl;dr: four new tickets (codenamed SUBCOMMANDS, SERVICES, MANHOLE and
>> PROVIDERS) and one old ticket (3538)
>>
>> SUBCOMMANDS: Add '+' as a special character in t.p.usage.Options. This
>> behavior will be off by default, and controlled by an attribute on the
>> Options instance "allowMultipleSubcommands".
>>
>> The attribute will only be checked when the first sub-command starts, to
>> allow setting it based on global flags. When the option is on, '-+' will be
>> passed as '+' to the Options instance, to allow sending plain '+' to
>> sub-commands.
>
> Having a new, weird, fragile syntax is probably the least interesting part of
> this. I suggest not doing this part - or at the very least, not doing it
> first and not making it a general part of `Options`.
>
> There are lots of other ways to get the service object from more than one
> service plugin. For example, read lines from a file. Or have a variation of
> `--python` or something else similar using the existing option syntax
> `Options` supports.
>
> The more interesting part to get right is the underlying model which you
> discuss elsewhere.
I love new, weird syntaxes, but I have to agree here. Let's do it with a
boring syntax with no compatibility implications first. The boring syntax
would actually be quite similar to the exciting one; a "compose" plugin.
Consider: "twistd compose web --path . + words --irc-port=6789". There's no
need to make this a general feature of Options to do it this way; just have the
"compose" plugin take its argument list and pass it to two discrete other
Options instances as lists of strings, looking for a "+" simply as the place to
split the argument list. You could also find some other way to split the
argument list but "+" doesn't seem particularly obscure in this context to me.
(If there's really a need to pass a literal "+" to a plugin we could add an
escaping syntax as well.)
>> Glyph thinks there's a ticket for it. I couldn't find it in "search for
>> tickets in 'core' whose description mentionds 'command'". Unless anyone can
>> find it, I'll open a new ticket.
>
> I think there is a ticket for being able to use multiple twistd plugins. I
> don't think there's a ticket for a general change to `Options`.
I was referring to the multiple-twistd-plugin ticket.
>> SERVICES (depends on SUBCOMMANDS): In twistd, set the flag aMS if
>> '--allow-multiple-services' is given. Add to the application all services.
>
> If you skip the `SUBCOMMANDS` ticket described above, then you can skip this
> too.
>> 3538 (depends on SERVICES): If '--allow-multiple-services' is given, and
>> '--python <.tac file>' is given, process subcommands as usual.
>
> Or just process the tac file and the subcommand if they're both given -
> without requiring an extra option? The current behavior, "silently ignore
> one of the arguments", doesn't seem particularly worth keeping to me.
It seems halfway plausible to me that someone could have some automation that
chooses a plugin and/or a python file and expects that one or the other will be
ignored. So we should do the usual incompatibility-notification dance but I
don't anticipate any objections.
>> PROVIDERS: Add a function,
>> "providersInHierarchy(IService, IInterface) -> List[IInterface]"
>> that returns all services in the hierarchy which provide the interface.
>> This ticket does not depend on any other tickets.
>
> The first argument needs to be `IServiceCollection` instead of `IService`.
>> MANHOLE (depends on PROVIDERS, SERVICES): Add a built-in twistd plugin
>> named "manhole". The plugin will expose manhole as PB/telnet with a
>> namespace that includes
>> {'services': providersInHierarchy(manholeService, IService)}
>> This ticket technically could only depend on PROVIDERS, but to be useful,
>> it also depends on SERVICES
>
> Manhole is part of Conch now and the telnet manhole is deprecated (and the PB
> manhole really should be deprecated - using a structured protocol for manhole
> isn't a bad idea but the existing implementation is half broken, mostly
> untested, exposes tons of implementation details as part of the public
> interface, etc. If this were a piece of widely used software it would
> probably be worth gradually renovating - but it's basically used by no one so
> starting fresh makes more sense).
>
> So it's part of Conch. And ... it exists already. I'm pretty sure no one
> will object if you add a new name to the default namespace.
>
> Thanks for taking this on.
Yeah. It's great to have you around again, Moshe :).
-glyph
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