Sending binaries over relations

Thu Jan 21 13:54:37 UTC 2016

Hi Mark

I understand your concern. Leaking operational details is something that
should not be done in Juju. Apache proxy relationship cannot be implemented
by Nginx because the relation leaks operational details (apache specific
config files). This is not 'the Juju way'.

The reason for sharing resources actually is interoperability. Say a client
wants to connect to a server, then the client may need libraries to do
that. What libraries will be used is dependent on both the client and the
server. If you ship the libraries with the client, you are essentially
hard-coding the server version. This is also not the Juju way; information
like this should be passed over relations. The problem here is that both
Charms have to make a joint decision about what libraries, if any, to use
and where to get them. No Charm has full knowledge to make that decision.

I've seen two common patterns in the wild for these kind of libraries:

1. The client uses its own libraries compiled for that specific version of
the server.
2. The client uses libraries bundled with the server.

The first case is easy: The client has full knowledge about where to get
the libraries. It only needs to know the server version. The server tells
the client his version and it's done. The second case is a little bit
tougher. The client has knowledge about what libraries it needs, but the
server has knowledge about where to get those libraries. In this case, the
client should request the libraries from the server, and the server should
have a way to send them to the client.

If both cases are supported by the server, you can swap any client in and
out. This also translates nicely to the idea that a Charm gets created by
the person who is an expert in that service. The server experts know where
to get the libraries if the server has to provide them. The client expert
knows how to compile the libraries if the client has to compile them.

What do you think?

*PS: I don't think this should be done on stack level, since the shared
libraries are only relevant to the two related Charms. The relevance of the
libraries is on relation-level so they should be handled by the relations.*

2016-01-21 13:58 GMT+01:00 Mark Shuttleworth <mark at ubuntu.com>:

>
> Two thoughts.
>
> First, I think it's interesting that you see resources as such a dynamic
> thing. I believe the current model accommodates this (we considered
> user-provided resources, and what you are describing are essentially
> charm-generated resources, but they would behave the same way).
>
> Second, I am sceptical about cross-charm resource coordination. We go to
> great lengths to keep a service encapsulated. Each service is generally
> prevented from knowing too much about what is going on in the service
> next door. This is deliberate because it leads to the development of
> services which can more naturally be swapped out, because the web of
> expectations between related services is limited by what they can
> possibly know about each other. What you are describing suggests too
> intimate an awareness between two services of their exact operational
> implementation.
>
> That said, we do have an idea in the "futures" department, which is a
> higher-level charm that would manage a group of charms. While they would
> remain encapsulated, the higher-level charm would have admin-like
> visibility across all of the services it is supervising. Imagine an
> "openstack" charm which can command-and-control events across all the
> constituent services. In THAT case, yes, it would make sense for the
> higher level charm, which we call a "stack", to be able to coordinate
> resources across services under its supervision.
>
> How does that sound?
>
> Mark
>
> On 21/01/16 01:21, Merlijn Sebrechts wrote:
> > Hi John, Mark
> >
> >
> > If I understand you correctly, a Charm will be able to decide what
> version
> > of the resource it needs at runtime? This way, a Charm could tell related
> > Charms what version of the resource they should get? That would solve my
> > use-case almost completely...
> >
> > The only exception being the case where a Charm compiles a library during
> > installation and wants to distribute that binary to its related Charms.
> > This would require the Charm to be able to push the resource to the state
> > server and then distribute a link to that resource over its relations.
> >
> > Using the state server instead of rsync would be a lot better long-term,
> I
> > think. Network spaces and multi-tenancy might make it possible that a
> Charm
> > cannot ssh to a related Charm...
> >
> >
> > Any thoughts on this?
> >
> >
> > Kind regards
> > Merlijn Sebrechts
> >
> > 2016-01-21 7:42 GMT+01:00 John Meinel <john at arbash-meinel.com>:
> >
> >> It does feel like a good fit for resources, with the one caveat that he
> >> wants to maintain a lock-step version of the resource across services.
> >> There is slightly more work with the current designs for resources, in
> that
> >> each charm will think about its version of the resource independently.
> But
> >> we will have the fingerprint information to allow for users to compare
> and
> >> be confident that both services are using the same resource.
> >>
> >> John
> >> =:->
> >>
> >> On Wed, Jan 20, 2016 at 8:53 PM, Mark Shuttleworth <mark at ubuntu.com>
> >> wrote:
> >>
> >>> On 20/01/16 14:24, Merlijn Sebrechts wrote:
> >>>> So my question is: Is there a way to send large binary files between
> >>>> Charms? Or is this problem better solved by using a subordinate
> >>>> kafka-plugin Charm like the Hadoop Charms do?
> >>> It sounds like you want the new "Resources" capability coming in Juju
> 2.0
> >>> :)
> >>>
> >>> For shared large blobs (like a JVM or a big ball of libraries) the
> charm
> >>> can ask the state server to cache the blob and distribute it to all the
> >>> units. There are mechanisms for users to supply the blob if needed,
> too.
> >>>
> >>> Mark
> >>>
> >>> --
> >>> Juju mailing list
> >>> Juju at lists.ubuntu.com
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> >>> https://lists.ubuntu.com/mailman/listinfo/juju
> >>>
> >>
>
>
>
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