[infinispan-dev] Configuration visitor - Re: [JBoss JIRA] Commented: (ISPN-145) No transport and singleton store enabled should not be allowed

[Mircea Markus]

On Sep 10, 2009, at 4:59 PM, Galder Zamarreno wrote:

>
>
> On 09/10/2009 11:38 AM, Vladimir Blagojevic wrote:
>> Hi Galder,
>>
>> Although I agree with you that reflection is little bit harder to
>> understand the advantage of this visitor pattern is many fold. Say,  
>> down
>> the road, we have multiple of these visitors written, some of them,
>> possibly, even written by third party clients.
>
> Hmmm, can you be more specific? We're talking about here in the  
> scope of
> Configuration validation.
>
>> Our entire configuration
>> tree class hierarchy is statically or compile time tied tied to these
>> visitors and we don't want to go down this path. I'll give it another
>> look, we can go static but we need to be aware of the trade offs.
>
> We've implemented the visitor pattern statically for invocation
> execution (see org.infinispan.commands.Visitor. Mircea, you're the
> original author so correct me if I'm wrong) and I don't we had any
> problems dealing with extending it. In fact, I had to extend it myself
> to cope with keySet and entrySet commands and wasn't that bigger deal
> (https://jira.jboss.org/jira/browse/ISPN-94?page=com.atlassian.jira.plugin.ext.subversion%3Asubversion-commits-tabpanel 
> )
The reflection approach is both good (for the point Vladimir outlined)  
and bad: as code won't fail to compile when we change it (not strongly  
typed) the clients won't even be informed that their visitor's  code  
code is not called/visit method are skipped - this is when we change  
our code. This might result in more time to investigate on their side,  
and more frustration for them. On the other hand the strongly typed  
code would fail-fast, telling the users that something needs to change  
- hence my vote for the strongly typed visitors.
>
>>
>> If I understand you correctly the approach you are suggesting is not
>> modular. It is generally considered not to be a good practice to tie
>> logical processing to data structures themselves when the  
>> likelihood of
>> extending logical processing is significant. The whole reason behind
>> adopting visitor pattern on our configuration tree is the ability  
>> to add
>> new operations to existing configuration object structures without
>> modifying these structures. By using visitor pattern and adhering  
>> to the
>> so called open/closed principle we allow room to do any kind of
>> operations on our configuration object structures.
>
> I see your point of not tying up the processing and the data  
> structure.
>
>> It might seems that
>> validation is the last thing we are going to do - but it is not, for
>> sure. And tomorrow when there is yet another operation we need to
>> complete on configuration tree the implementation will be a breeze.
>
> Hmmm, we could implement a swiss knife so that it can solve all the
> problems in this world but before doing that, I'd like to hear more
> specific examples ;). I mean, we could have a created a super generic
> visitor pattern to deal with configuration validation, with
> invocations...etc, but I'm against that since it's hard to read,  
> hard to
> follow and hard to debug. For reflection related nightmares, simply  
> look
> at JBC 1.x code.
>
> So, to sum up, I see the point of using the visitor pattern but let's
> time things a little bit. We're dealing with configuration validation
> here, so let's stick that for the moment. Also, let's go for a
> statically typed one similar to the one already used for dealing with
> invocations. I'd imagine that applied to this use case, this would be
> like: visitSingletonStoreConfig, visitTransportType...etc.
>
> org.infinispan.config package:
>
> ValidationVisitor {
>    void visitSingletonStoreConfig(SingletonStoreConfig ssc);
>    void visitTransportType(TransportType);
>    ....
>    // instead of traversalCompleted, use more meaningfull name
>    validate()
> }
>
>
> SingletonStoreConfig:
>
> public Object accept(Visitor visitor) throws Throwable {
>    return visitor.visitSingletonStoreConfig(this);
> }
>
>>
>> Cheers,
>> Vladimir
>>
>> On 09-09-10 4:11 AM, Galder Zamarreno wrote:
>>> Hi Vladimir,
>>>
>>> I had a look at the implementation and I'm not sure I understand the
>>> need for the reflection visit calls in  
>>> AbstractConfigurationBeanVisitor.
>>> Using reflection makes harder to follow code and it's slower than  
>>> typed
>>> calls and I'm not sure of the reason to use it here.
>>>
>>> Also, I don't see the need for a standard  
>>> ConfigurationValidatingVisitor
>>> that does such validation. Instead, the way I see it working is
>>> SingletonStoreConfig having some kind of callback method being  
>>> called,
>>> i.e. the traversalCompleted() call and within it,  
>>> SingletonStoreConfig
>>> can, using the ComponentRegistry, retrieve the Transport component  
>>> and
>>> see if it's set or not. Or alternatively, SingletonStoreConfig  
>>> could use
>>> the passed InfinispanConfiguration to do its validation.
>>>
>>> To sum up, I think each AbstractNamedCacheConfigurationBean
>>> implementation should, if it requires to, have the ability to  
>>> validate
>>> the configuration via some kind of callback. Such callback should
>>> probably is possibliy traversalCompleted() itself.
>>>
>>> WDYT?
>>>
>>>
>>
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>
> -- 
> Galder Zamarreño
> Sr. Software Engineer
> Infinispan, JBoss Cache
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