On Fri 02 Dec 2011 09:00:52 AM EST, Dan Kenigsberg wrote:
On Thu, Dec 01, 2011 at 10:13:53PM -0500, Saggi Mizrahi wrote:
I want to suggest a different way of handling constants.py and config.py the current way is clumsy and makes making convoluted code easy.
GLOBAL VARIABLES ARE BAD. It's not something I invented. It's well accepted that proper scoping yields better code.
FOLLOWING UPERCASE MAXIMS BLINDLY IS BAD. (and that's something I have invented.)
The way we use those two files shows how much of a mess globally accessible variable can be.
Not putting paths to executable in constants.py
Instead we should have a module for each utility executable. The module should wrap the utility with a pythonic interface. That way there is no reason for more then one module to have access to a utility's path.
For example:
cat.py
_CAT_PATH = @CAT_PATH@
def cat(files): cmd = [_CAT_PATH] + files out, err = exec(cmd) return out
Now there is no need to know where cat is. You have a proper interface to use. Preventing people from re-wrapping the utility over and over. This in turn means you have a canonical wrapper that is more likely to work.
I agree that our current behavior is clunky. Most external executable should be wrapped by functions that exposes the functionality which we need from them.
I am not sure that having a module per executable makes the most sense; I'd separate modularity based on Vdsm usage of them. Anyway, the only up side of our current constants-aggregating approach, is that the number of files editted by autoconf is limitted.
I'm don't really care about how many utilities are wrapped per module. I just want only one place to reference and actually prompt the utility while all other places should use the wrapper.
P.S. EXT_CAT is a horrible example. It exists only for letting Vdsm read /etc/multipath.conf. This functionality must be wrapped properly and shoved into supervdsm.
cat was given as an example because it's the easiest utility to wrap I could think of that is not echo. It was just used to illustrate the point.
Every other constant
Other constants should be moved to their proper modules and used only when importing the module makes sense this will give you another step where you make sure your decencies are correct.
Configuration
Configuration files are just parameters to the program. They should be treated as such. Their scope should not leave the main class of the project. In our case VDSM is actually 2 projects meshed together like Siamese twins so I think clientIf.py and hsm.py should be the only ones with access to the config. Everything else should expect to get those values in a regular programmatic fashion.
In my opinion these changes will make VDSM a lot more flexible, will make more parts independent and easily separated and at that end help us make VDSM better for generations to come.
In my opinion our use of config.config is perfectly fine. The configuration file is a single global object, and it makes sense to keep its Python representation as a single global object. I'd like keeping it this way because if Vm migration process needs a new parameter (say, when should we decide that migration is not going anywhere) MigrationSourceThread can simply read it from the config. If we take your approach, we would have to trickle the value all the way from clientIF through Vm down to the specific function that needs it. It would clutter the code and would not scale nicely with the number of configuration parameters.
The "I don't want to have to pass the parameter" excuse is always thrown around for all sorts of bad practices (thread local storage anyone?). I would even stress that the pain with trickling down information helps encourage developers to keep the system flat. And the less convoluted your hierarchies are the simpler and more flexible the code is. In the end, having things too accessible (ahm ahm sdc.produce()) usually results in bad code.
You have a ton of patterns to mediate this effect.
def init(moduleConfig): """configures the module. You see this a lot in the C\C++ world but that doesn't mean it's bad"" pass
public Foo(): def __init__(self, param): """Give values in the ctor.""" pass @classmethod def configure(cls, param, param): """Less recommended but possible""" pass def setDefaultSomething(self, value): pass
You can help!
- Start by taking you favorite utility and write a proper safe module
around it.
- When writing new code try and not access the configuration directly.
- Modules are free (really) don't just put stuff in misc\utils try and
put some order to things.
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