[ubuntu-cloud] Fwd: #part-handler
smoser at ubuntu.com
Wed Mar 6 13:53:52 UTC 2013
On Tue, 5 Mar 2013, Bo Shi wrote:
> Clint, Scott - sorry about the late reply.
> I did not explain myself particularly well. Here's another attempt.
> The CloudInit documentation section for "Multipart Input" states "[a]
> single format of user data might not be enough to accomplish what you
> want. For example, you may want to insert an upstart job and also run
> a user-data script." The example provided is:
> $ ls
> my-boothook.txt my-include.txt my-user-script.txt
> my-cloudconfig.txt my-upstart-job.txt
> $ write-mime-multipart --output=combined-userdata.txt \
> my-boothook.txt:text/cloud-boothook \
> my-include.txt:text/x-include-url \
> my-upstart-job.txt:text/upstart-job \
> my-user-script.txt:text/x-shellscript \
> Is the order of execution of the various parts deterministic? Can the
> order of execution be controlled in any manner, e.g. can I specify
> that "my-include.txt" depends on "my-user-script.txt" finishing? A
> rather contrived use-case is if my-cloudconfig.txt was used to create
> some users and my-include.txt performed some operation on those users'
> home directories. Another case is described in my original note.
> > To control order in which user-data is processed you should just need to
> > control the order in which it is created.
> Does this mean that the order in which the parts are specified in the
> invokation of "write-mime-multipart" above determines the order in
> which the parts gets executed? Is execution serial (my observations
> lean toward no here)?
I guess I wasn't as clear as I could have been.
a.) the order output by 'write-mime-multipart' is the same as the order
of the command line arguments to it. If you find otherwise, please
let me know, thats a bug.
b.) the order consumed by cloud-init's user-data handling is in-order
that it appears in the user-data. In your example above, cloud-init
* then 'my-include.txt'
* anything included by my-include.txt (and recursively)
c.) "process" above is unfortunately not so clear. Heres a guide:
* archive formats (mime or cloud-config-archive or include) are
exploded when found, and contents are processed at that point.
* cloud-config-data is merged into the existing cloud-config in the
order that it is seen. The merge routine for cloud-config is very
simplistic, and basically only supports overriding of fields.
Josh Harlow has a branch at  to fix a feature request in bug
1023179 to make this more powerful. I'm really looking forward to
* boothook parts are executed immediately as they are processed.
(cloud-config 'bootcmd' are processed slightly later)
* upstart jobs are written to /etc/init immediately as they are
processed. And should the become ready for upstart usage
* part-handlers are consumed immediately as they're seen, and then can
immediately act on subsequent parts.
* user-scripts are processed in the order they're seen, but the
* processing just means shoving them into a file in
/var/lib/cloud/instance/scripts (path from memory, so it could be
wrong). If the part has a filename, that filename is used. If not,
one is generated, and should increase.
Then, later a 'runparts' equivalent is executed on that directory.
runparts executes in C locale sorted order.
The files that cloud-init created will be invoked in the order that
they were written, but if you've provided filenames in the mime or
cloud-config-archive parts then you can change that ordering.
Also, 'runcmd' parts (from cloud-confing) are dropped into a file
named 'runcmd' in that same directory.
That clear things up a bit?
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