----- Original Message -----
From: "Moti Asayag" <masayag@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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