Triggering relation events?

Nick Barcet nick.barcet at
Sun Dec 18 09:04:47 UTC 2011

On 12/17/2011 07:26 PM, Juan Negron wrote:
> Hi Nick:
> One way to do that is to put that code in a config-changed hook which
> will get called when you change the variables with juju set.
> This will ensure that every time you change something with juju set, it
> gets propagated via the config-changed hook as the configuration would
> have changed on all of the units of that service.

Thanks Juan, but I do not think this would work, as a in order to copy a 
file with scp or rsync between two units you need to be able to 
establish a conversation between the two host.  Here is an example of 
this for when the ssh keys have not been shared yet:

#    LEADER                      SLAVE
#       <------------------- set scp-hostname
# set scp-ssh-key ----------------->
#                            save shh-keys
#       <------------------- set scp-ssh-key-saved
# do file copy
# set scp-copy-done --------------->

This is only possible to do, AFAIK, in a relation environment where 
changing a relational parameter will trigger a relation-changed event on 
the other side.  When you are in a config-changed event, you not have 
access to relation-[set/get] since you do not have any context for a 
relation, nor can you do a 'juju set <service> <var>=<value>' because 
this can only be used outside of the environment.  This basically 
prevents a two way conversation to occur.


> On Sat, Dec 17, 2011 at 9:24 AM, Nick Barcet <nick.barcet at
> <mailto:nick.barcet at>> wrote:
>     Hello,
>     I've been having fun during my week of vacation contributing a few
>     charms and a ch_peer_copy function for charm-helpers [1] which
>     allowed me to automatically copy the certificates and des_key that
>     are in the "leader" unit of roundcube to all the other units [2].
>       This works great and shows the power of peer relations, but brings
>     a small question:
>     Let's imagine that at some point in the future I need to update this
>     certificate (let's say, when it expires).  I can upload the new
>     certs to the machine using 'juju scp'.  I can update the
>     configuration of my units using 'juju set'. My problem is how do I
>     trigger a change even on all the peer relations so that the
>     certificates are copied again?  I could not find any way to force an
>     event in the doc.  Maybe I am taking the wrong approach to this?
>     [1]
>     <>
>     [2]
>     <>
>     Nick
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