Juju architecture questions

[Thomas Leonard]

On 2012-09-08 00:55, Clint Byrum wrote:
> Excerpts from Thomas Leonard's message of 2012-09-07 08:05:16 -0700:
>> Is there a diagram somewhere showing how all the different bits of Juju fit
>> together?
>>
>> I found it quite confusing when I read the tutorial for the first time, and
>> there are still bits that I'm not sure about. I tried making a diagram to
>> show the example (wordpress + mysql). It's probably wrong, but maybe someone
>> could correct it?
>>
>> https://docs.google.com/drawings/d/1LVrBjoMivIQZf6TNaquA5dFrZcd2x2Cj_XNB2GBPaA0/edit
>
> Its pretty close. You missed the provisioning agent though.
>
> https://docs.google.com/drawings/d/1Az4cx4w6J3dvJfQmg8ZbrOIx8vSuBFhYcaP0nHYUHEQ/edit
>
> Its existence helps understand a lot of the things you're confused about.

Ah, thanks - that explains a lot! I can't view your document though ("You 
need permission to access this item").

>> Here's my understanding so far (please correct):
>>
>> The Juju admin has the environment.yaml and the data directory. Together,
>> these give access to the system and could be shared with other admins,
>> allowing them to manage the system together.
>>
>> [ which bits of the data directory need to be shared, and which are just log
>> files and temporary files? do the admins need to sync anything manually
>> between their systems? ]
>>
>
> Only environments.yaml must be shared if you are using the official charm store.
> If you are using local: charms, then you need to share the charm repository too.

If this just so they can be upgraded? I assume even local charms are copied 
to the S3 bucket the first time they're deployed?

[...]
>> Adding a relation is done (as always) by updating the Zookeeper
>> configuration. The units for the services at each end of the relation are
>> invoked immediately and simultaneously. Typically, one end will need to run
>> first. e.g. when connecting wordpress to mysql, the mysql hook needs to run
>> first to create the database. If the wordpress hook runs before mysql has
>> updated the configuration it will detect this and exit(0). It will be
>> invoked again once mysql updates the settings. This can lead to race
>> conditions in charms if the case of running in the wrong order is not
>> considered.
>
> Its really easy to avoid these races though, because you simply exit if
> the data you need has not been set yet. The guarantees around the hooks
> allow for you to make the assumption that your hook will be invoked again
> when the data has changed.

I mention this because I got it wrong in my first ever charm. I tested it 
several times and it worked fine for me, but as soon as I gave it to someone 
else, it crashed.


-- 
Dr Thomas Leonard
IT Innovation Centre
Gamma House, Enterprise Road,
Southampton SO16 7NS, UK


tel: +44 23 8059 8866

mailto:tal at it-innovation.soton.ac.uk
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