Re: Services and log management/monitoring

[rendaw <7e9wc56emjakcm@s.rendaw.me>]

On 4/19/19 9:09 PM, Ludovic Courtès wrote:
> Hi,
>
> rendaw <7e9wc56emjakcm@s.rendaw.me> skribis:
>
>> I think fundamentally what I'd most like to know is when should I use a
>> Shepherd service vs a non-Shepherd service.  Maybe it's as simple as: if
>> you don't have any specific requirements always define a Shepherd service.
> It’s hard to answer that question in the abstract.  Do you have an
> example in mind that we could work through?

Yeah, so the basic example I was thinking of is I have an executable
binary (a server for diagnostics) I'd like to run when the computer boots.

>
>> Beyond what a service actually is though I have a few more questions:
>>
>> * Both
>> https://www.gnu.org/software/guix/manual/en/html_node/Service-Composition.html
>> and
>> https://www.gnu.org/software/guix/manual/en/html_node/Shepherd-Services.html#Shepherd-Services
>> appear to show a dependency graph.
> The first page shows a service extension graph.
>
> The second page shows a graph of dependencies among Shepherd services.
>
> These are two different beasts.

Ah okay, that explains a lot - I was completely misinterpreting the
extension graph then.  I'm afraid I don't know what extension means in
this context.  Is it similar to OOP extension, where a "Dog" is-a
"Canine"?  Looking at the graphic, "accounts" depends-on "etc" (the
directory/mount?) makes a lot more sense to me than "accounts" is-a
"etc".  Similarly I would assume that the udev service runs udevd, but
if upower and colord are both "udev" services (because they extend udev)
does that mean if I run both the upower and colord services then my
system will have 3 instances of udevd running?

I'm fairly sure that's not what it means, but if extends isn't an
inheritance relationship and it's not a dependency relationship I'm not
sure what it is.

Is the udev service in the shepherd graph different then the udev
service (-type?) in the service extension graph?  That might be a factor
in my confusion.

>>   Are the dependency graphs Shepherd and non-Shepherd services
>> entirely separate?  Or maybe I'm completely misunderstanding
>> "extension" in this context.  Can an inet service depend on a non-inet
>> service?  Can an inet service depend on a d-bus service?  * Is there a
>> way to hook into service events - that is, run some code when a
>> service starts or stops?
> An inetd service cannot “depend” on a non-inetd service; a D-Bus service
> cannot depend on a non-D-Bus service.  Both D-Bus and inetd have their
> own notion of what a service is, how to start it, etc., which is
> separate from what the Shepherd does.

So suppose I have an mcron job that needs an ssh tunnel managed by
shepherd to be started, or an inetd handler that needs data files on a
network mount to run... is it possible to express those ordering
dependencies?  Or a service that subscribes the server to external
webhooks, and needs the webhook listeners (inetd handlers) to be started
before registering.

I assume the answer to my 2nd question from the previous email - if
there's a way to run code when a service's state changes - is that there
currently isn't such hooking mechanism.

>
> I reckon that calling everything a “service” does not help understand
> all this…

I don't think it's an unreasonable choice of words :) - there's just a
lot that isn't fitting in mentally for me.

And thank you for your patience with the answers, I think I'm drawing
much closer to understanding all this.  Once I've figured it out I'll
definitely write something up in case someone else has the same issues.