On Feb 25, 2011, at 11:24 AM, Brice Figureau wrote:
> On 25/02/11 18:29, Markus Roberts wrote:
>>
>> There's also a 3rd thing I didn't started working on: allow the code to
>> set probes dynamically (instead of statistically adding them by
>> modifying puppet code). I don't know if ruby versatility can allow this,
>> but I'd love to be able to instance_eval { ... add probes around a
>> method ... } to a given live puppet instance/class fully dynamically
>> (that would be completely crazy).
>>
>>
>> This is in fact fairly easy to do, though you're right that it's
>> completely crazy. :)
>>
>> Suppose we have an array x and we want to do something with it's length
>> (negate it here, but presumably we'd be doing something more useful like
>> recording it or something):
>>
>> irb(main)> x = [1,2,3]
>> => [1,2,3]
>> irb(main)> unless x.respond_to? :brice_wrapped_length
>> irb(main)> class <<x
>> irb(main)> alias brice_wrapped_length length
>> irb(main)> def length
>> irb(main)> -brice_wrapped_length
>> irb(main)> end
>> irb(main)> end
>> irb(main)> end
>> => nil
>> irb(main)> x.length
>> => -3
>> irb(main)> x << :foo
>> => [1, 2, 3, :foo]
>> irb(main)> x.length
>> => -4
>>
>> Note that this only affects the one object; other arrays aren't touched:
>>
>> irb(main)> y = [:a,:b]
>> [:a,:b]
>> irb(main)> y.length
>> 2
>>
>> The unless part is important to prevent instrumenting the same object
>> twice, which will cause it to hang (and eventually stack overflow). To
>> remove the instrumentation, just reverse everything (if instead of
>> unless, alias the other way, and then remove brice_wrapped_length).
>
> Thanks for the example. This is quite interesting and opens the door to
> a lot of strange things :)
>
> I was thinking about allowing the user to describe probes by specifying
> a puppet class and a method through an indirection to control probes
> (like in "Puppet::Indirector::Indirection.find"), then I'd do something
> (completely untested):
>
> method = "find"
> klass_name = "Puppet::Indirector::Indirection"
> require klass_name.split('::').map { |n| n.downcase }.join('/')
>
> klass = klass_name.split('::').inject(Puppet) { |p,c| p.const_get(c) }
> klass.class_eval do
> alias "instr_#{method}" method
> define_method(method, *args) do
> # here call to instrumentation
> send("instr_#{method}", args)
> ensure
> # here wrap up instrumentation
> end
> end
>
> Then I hope, somewhere else, someone calling Puppet::Something.find
> (provided Something uses the indirector of course), then it would call
> my programmatic redifinition of "find"...
>
> This will be a funny week-end if that proves to work :)
I think this basic model is sound, and is roughly what I'd do.
You could also have a builtin system to allow classes to specify which methods
should normally be probed. E.g., create a module that essentially does this:
module Probable
PROBABLE_CLASSES = []
class Probe
def initialize(method, klass, type)
...
end
def enable
...do the alias method switcheroo here
end
end
def define_probe(name, type = :time)
@probes << probe.new(name, self.class, type)
PROBABLE_CLASSES << self.class
end
def self.enable_probes
PROBABLE_CLASSES.each do |klass|
klass.probes.each { |probe| probe.enable }
end
end
end
Obviously not really a great design per se, but as a general idea, it would
allow any class to define what were most likely to be the appropriate probes,
so that turning probes on for the system should result in the best data. I
like the idea of being able to manually enable probes through your command-line
interface ideas, too, but I think the system should ship with a bunch of these
out of the box.
--
Brand's Shortcut:
The only way to predict the future is to make sure it stays
exactly the same as the present.
---------------------------------------------------------------------
Luke Kanies -|- http://puppetlabs.com -|- +1(615)594-8199
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