On Tue 17 Jan 2012 10:05:15 AM IST, Omer Frenkel wrote:
----- Original Message -----
From: "Moti Asayag"<masa...@redhat.com>
To: engine-devel@ovirt.org
Sent: Tuesday, January 17, 2012 9:47:48 AM
Subject: Re: [Engine-devel] a different approach to the command classes
On 01/17/2012 09:05 AM, Livnat Peer wrote:
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.
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.
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...).
i agree as well
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.
In addition, it prevents from independent validations (inside the
same
canDoAction) to report on several validation failures in a single
execution instead of getting a single failure, rerun the command
again,
get another failure and on...
This is the reason why the type of canDoActionMessages is a List.
actually this is how it works today in most cases, i think its not bad,
some checks relay on the fact that if this check is executed,
then it's safe to do things, for example - first check validates vm is not null,
second check use the vm, assuming its not null.
i would go further and make the validation methods return boolean and call them
in that way:
public boolean canDoAction() {
...return
checkTargetDomainHasSpace()&&
checkTargetDomainIsValidTarget()&&
checkSourceDomainIsValidSource()&&
checkSourceAndTargetAreDifferent();
...
}
private void checkTargetDomainHasSpace() {
if(!targetDomainValidator.hasSpace(getParameters().getDiskImageId()))
{
addCanDoActionMessage(VdcBllMessages.ACTION_TYPE_FAILED_DISK_SPACE_LOW);
reutrn false;
}
return true;
}
Livnat
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I agree that it works better mainly in long and complex checks (did
that in setupNetworks). Its better to encapsulate shared validation
code in validator classes instead of using inheritance for that.
It is also easier to test the validator than constructing a command to
test the canDoAction part.
I'll take advantage and a rule of thumb - to make the canDoAction clean
and simple make sure all parameter validation is done
via the validation framework.
code like if (getParameters().getSomeParam != null ) {
addCanDoMsg(msg) } shouldn't be a part of your canDoAction.
Instead use:
class SomeBaseParameters
@NotNull
SomeParam someParam
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