[Engine-devel] a different approach to the command classes
Jon Choate
jchoate at redhat.com
Tue Jan 17 13:37:54 UTC 2012
----- Original Message -----
> From: "Maor" <mlipchuk at redhat.com>
> To: engine-devel at ovirt.org
> Sent: Tuesday, January 17, 2012 4:33:19 AM
> Subject: Re: [Engine-devel] a different approach to the command classes
>
> On 01/17/2012 09:24 AM, Ayal Baron wrote:
> >
> >
> > ----- Original Message -----
> >> On 17/01/12 04:58, Jon Choate wrote:
> >>> The way the command classes are written has bothered me for a
> >>> while.
> >>> While implementing the multiple storage domain features I am
> >>> presented
> >>> with the opportunity to create a new command from scratch. I
> >>> gave
> >>> some
> >>> thought to what I would like the command classes to look like
> >>> while
> >>> balancing that the class must still fit in with the existing
> >>> structure.
> >>> So here is what I came up with. I'd appreciate any feedback.
> >>>
> >>> The Command class encompasses only the rules of what needs to be
> >>> done.
> >>> It relies upon Validator classes to determine if the canDoAction
> >>> conditions have been met.
> >>>
> >>> @Override
> >>> public boolean canDoAction() {
> >>> ...
> >>> checkTargetDomainHasSpace();
> >>> checkTargetDomainIsValidTarget();
> >>> checkSourceDomainIsValidSource();
> >>> checkSourceAndTargetAreDifferent();
> >>> ...
> >>> }
> >>>
> >>> ...
> >>>
> >>> private void checkTargetDomainHasSpace() {
> >>> if(!actionAllowed) return;
> >>>
> >>> if(!targetDomainValidator.hasSpace(getParameters().getDiskImageId()))
> >>> {
> >>>
> >>> addCanDoActionMessage(VdcBllMessages.ACTION_TYPE_FAILED_DISK_SPACE_LOW);
> >>> actionAllowed = false;
> >>> }
> >>> }
> >>>
> >>>
> >>> Each of the checks follows a similar pattern of
> >>> - guard clause to see of we already failed and bail if we did
> >>> - test for failure of the condition
> >>> - add failure message if needed
> >>> - set variable to failed if needed
> >>>
> >>> Storing the success flag in a variable allows us to keep the
> >>> canDoAction
> >>> method readable as a series of commands and to allow it to be
> >>> accessed
> >>> by all the private methods without them having to pass it around.
> >>>
> >>> The execution of the command will follow a similar pattern where
> >>> the
> >>> command class will only know how to describe what needs to be
> >>> done
> >>> and
> >>> to rely on supporting objects to handle the implementation of
> >>> these
> >>> steps. Getting the implementation out of the command classes
> >>> will
> >>> allow
> >>> the commands to share validation and implementation details and
> >>> remove a
> >>> lot of the duplication that currently exists within the commands.
> >>>
> >>>
> >>> How do people feel about this approach?
> >>
> >>
> >> Hi Jon,
> >>
> >> The scope of your proposal is changing the implementation of the
> >> canDoAction method, I think that the title of this mail is a bit
> >> misleading.
> >
> > Actually I think Jon was suggesting to do the same in the command
> > itself.
Yes I am. I just haven't written that code yet!
>
> I think, using shared canDoAction validation between commands might
> be a
> good idea also, it can be seen in operations such as add/update
> commands.
> In most cases, the update validation, is already based on the add
> command validation, sometime it is implemented with
> inheritance/external
> class, in other cases it is just implemented with multiple code.
> >
> >>
> >> Basically what you are suggesting is to split the canDoAction
> >> implementation into methods and then extract them from the command
> >> class
> >> to a shared class so they can be reused.
> >>
> >> In many cases we can use (are using) inheritance for reusing code,
> >> there
> >> are cases where inheritance does not do the work and we can
> >> extract
> >> to
> >> external classes.
> >>
Our overuse of inheritance is one of the things I'm trying to rectify. Inheritance wasn't added
to the language to facilitate reuse. It is there to represent object hierarchies. In some cases
we abuse this to leverage code reuse. This is often a code smell that the code is in the wrong
place to begin with. If both classes need the code and they are not logically related in an object
hierarchy, the code really needs to be outside the classes.
We have cases where the use of inheritance for reuse have gone too far. For instance:
public class RemoveVdsSpmIdCommand<T extends VdsActionParameters> extends AddVdsSpmIdCommand<T>
So this says that a RemoveVdsSmpIdCommand is a type of AddVdsSpmIdCommand? That implies that I can
use a RemoveVdsSmpIdCommand anywhere that an AddVdsSpmIdCommand is expected. Otherwise we have broken
one of the fundamental contracts of object oriented programming.
> >> I think such a change is welcomed but on a needed basis, I think
> >> it
> >> is
> >> overkill for the general use case and will make the code more
> >> cumbersome
> >> (if the original canDoAction was 4-5 lines long...).
>From what I have seen those canDoActions are in the minority. The overall goals are to increase
the readability and maintainability of the code so I would suggest being pragmatic about any approach.
If doing it doesn't help achieve the goal, then don't do it.
One of the ideas I'm trying to convey here is that the command classes should be fairly ignorant.
They should be responsible for knowing the list of steps involved in a workflow, but not the details
of how those steps are carried out. Imo knowing both the steps and their implementation violates the SRP.
> >
> > Jon, I think the burden of proof is on you here to show a real
> > example and how it makes the code clearer (e.g. change 2 commands
> > which share similar checks).
> > Without 'seeing' it I don't think we would be able to appreciate
> > the advantage of your approach.
> >
I need to get the multiple storage domains feature put away and then I will
definitely do some refactoring to illustrate the value. I wholeheartedly agree
that having actual code to kick around is better. I just can't do it right now!
> >>
> >> One thing I don't like in the above suggestion is the way you
> >> validate
> >> that the previous condition succeeded/failed. Having this
> >> condition
> >> at
> >> the beginning of each validation method is not a good approach
> >> IMO.
Can you elaborate on your opposition to the use of guard clauses? They have
been considered a best practice for quite some time.
> >
> > +1, if the previous check failed it should raise an exception, not
> > rely on the next check to bail.
> > It would be easier (less code, clearer) to wrap all the calls with
> > a try catch clause (1 for all the calls), catch the specific
> > exception that says the check failed and return whatever you want
> > to return.
I'm not really a fan of using exceptions for controlling flow of execution. Failing one of these
checks is not an exceptional event. Its an expected part of the workflow and should
be handled as a normal occurrence.
I would argue that wrapping the entire canDoAction method in a try/catch would not make the code clearer:
public boolean canDoAction() {
boolean succeeded = true;
try {
do1();
do2();
do3();
catch(ValidationException e) {
succeeded = false;
}
return succeeded;
}
has a lot more noise than:
private boolean canDoAction = true;
public boolean canDoAction() {
do1();
do2();
do3();
return canDoAction;
}
In the second form, there is no extra indentation or curly braces. The validation steps
look just like a list of steps and imo it makes the code really easy to understand by a maintainer.
> >
> >>
> >>
> >> Livnat
> >>
> >>
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> >>
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