Hooks and interfaces!

Fri Feb 26 09:50:15 UTC 2016

Hi Tom

First of all, thank you for sharing your experiences. The reactive approach
is quite new and feedback like this is very valuable. I understand the
frustration, I've had similar experiences with the reactive framework. I
think that there is still significant work needed in both `charm build` and
`charms.reactive` to help users debug problems. Sometimes, I've had to dive
in the code to figure out what was happening, and I don't think everyone
will be willing to do that. The Juju team is also working on better
documentation, and I'm positive that it will eventually catch up. Feedback
like yours helps them figure out where the pain-points are.

Now for your questions. I think I have a pretty good grasp of how the
framework operates. For anyone closer to the code, feel free to chime in
and correct me where needed.

Do I need a placeholder in the hooks directory?

Yes! and no... If reactive framework does not get called when a hook runs,
it cannot execute handlers. The placeholder calls the reactive framework,
the reactive framework executes the handler. However! You should not have
to add these placeholders manually. `charm build` creates placeholders for
all relationship hooks of all interfaces that a Charm has. In your first
implementation, *you did not use an interface layer, so `charm build` did
not know what placeholders it had to add*. So now that you added the http
interface layer, these placeholders should be present, and the commented
code should now work. Please note that this is *not recommended.* All
handeling of relationship hooks should happen in the interface layer. This
interface layer then communicates with other layers using states.

I think a lot of people will make similar mistakes. The `charm build` tool
should throw warnings to let you know that something is wrong. Feel free to
submit a feature request for this.

how to find out what charms implement what layers.

I'll make it a bit more specific: *Find out what charms implement what
interfaces.*

The short answer is that `metadata.yaml` specifies what interfaces are
implemented.

*An interface is a protocol* of how two Charms communicate. If two Charms
implement the same interface, they should be able to communicate with each
other. Juju uses metadata.yaml to know if two Charms have the same
interface. Layers are more or less separate from this. *A layer contains an
implementation of an interface protocol.* The layer contains code that
manages the relationship on both sides. This way, you are 100% sure that
each side of the relationship implements the exact same protocol. However,
it is not required that both sides of a relationship use that
implementation. They can have their own implementation. This is fine as
long as the protocol stays the same.

In the Charm store, you can see what interfaces a Charm implements in the
bar on the right. This also shows what other Charms implement that same
interface. https://jujucharms.com/haproxy/trusty/16/

Kind regards
Merlijn Sebrechts

2016-02-26 1:34 GMT+01:00 Tom Barber <tom at analytical-labs.com>:

> Hello folks,
>
> This is not a complaint more a story of how this afternoon has gone.
>
> I'm writing a gitlab charm, in doing so marco suggested that we add some
> hooks to optionally offload some of the services, like web server and
> database elsewhere should the user desire. Very sane plan, I thought.
>
> So today I figured I'd work out how to plugin another webserver over the
> top. The charm is a new layered job that extends the basic layer and not
> much more. Because of a curiosity about getting used to python, I have
> recently been writing code in Juju in Python, I realise this slows me down
> as most examples are in bash but its a good way to learn.
>
> Out of the box Gitlab claims to support Apache2 and Nginx but to me it
> basically looks like a standard reverse proxy although I'm yet to test it
> in anger. So i figured I'd slap apache2 next to it and see how we got on.
> We didn't. For some reason I am yet to fathom this block of now commented
> code never seemed to fire:
> https://github.com/OSBI/layer-gitlab/blob/master/reactive/gitlabhooks.py#L14
>
>
> Am I missing something in reactive charms? If I juju debug-hooks and
> copied the code into the website-relation-changed hook it ran fine.
>
> Anyway, after a few hours messing around trying to figure that out and
> giving up I realised after just randomly googling there was an http
> interface that did a similar job. Low and behold 10 minutes later with
> haproxy and one @when('website.available') method it seemed to be
> configuring itself with haproxy just fine.
>
> So here's the question. I'm all up for reactive charm stuff, but am I
> missing something with hook triggering? Do I need a placeholder still in
> the hooks directory? It appears not, but I'm not sure.
>
> Also, and more importantly going forward after much digging around to
> find interfaces.juju.solution is there, or will there be a way to find out
> what charms implement what layers. For example, I now seem to have found
> out that haproxy loves the http interface, but apache2 is yet to go
> reactive, so I guess I'm stuck with hooks for that if I chose to use it,
> but, without looking at the code, I don't know that. It would be great,
> would it not, to expose that type of stuff. I'm sure its all in the
> pipeline and you'll tell me its coming shortly, but I figure the feedback
> might prove useful to someone. Also from an interface point of view, whilst
> listing the supported interfaces in a charm in the charm store would be
> great, what i would love, if it does not already exist, is for me to say,
> okay, I need a reverseproxy through the http interface, what charms are
> available that allow me to do that? Is it just haproxy, or are there
> others. This information is key to easy (for those outside of canonical)
> charm development.
>
> Thanks and keep up the cool stuff in Juju 2.
>
> Tom
>
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