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From Antti Koivunen <anryo...@users.sourceforge.net>
Subject Re: Avalon 5 Core Interfaces
Date Mon, 27 Jan 2003 23:29:29 GMT
Stephen McConnell wrote:
> 
> 
> Antti Koivunen wrote:
> 

<snip/>

>> Also, although it makes little difference in practise, we have to 
>> acknowledge the following:
>>
>>   if ( context.hasEntry(KEY) ) {
>>       obj = context.get(KEY);  // Could throw NoSuchEntryException
>>                                // if the entry was just removed.
>>   }
>>
>> But this isn't really an issue, so let's include hasEntry for 
>> convenience.
> 
> 
> 
> Its not an issue because the container will never let it happen.  If the 
> context object has released a referecne, and it invoked get a second 
> time, the handler just builds another or returns a pooled reference or 
> whatever.  The thing is that the container makes sure that hasEntry 
> alway corresponds with the reality of a get().

OK, I know this sounds like nitpicking, but how would you ensure this 
implementation-wise (using a ThreadLocal?). Consider the following:

   if ( context.hasEntry( KEY ) ) {
       Thread.sleep(100000);
       // Is the entry still there even if someone decided to do
       // container management and mess up the context?
   }

The Thread.sleep() above could be GC kicking in. The point is that I 
don't want any loose contracts in the core. A call to context.hasEntry() 
cannot indefinitely block management operations on a particular 
component, even if context.get() isn't called.

<snip/>

>> There is a real scalability issue. Think about a deployed enterprise 
>> application; there might be one configuration change a month and 10 
>> 000 calls to suspend() and resume(). 
> 
> 
> 
> In this sort of a scenario I would take a lot more control over 
> container/component contract (via extensions for example) - get the 
> container to provide some specific change handler that provides the 
> details of what features need updating - if any.  I'm also assuming that 
> suspension/resumption will only occur in order to introduce a change of 
> state from the container (i.e. one a month).  I'm not looking at 
> suspend/resume suporting any other semantics.  Keep in mind that 
> suspension of one component implies suspension of all components that 
> are consuming services from the target.  Once the target is resumed, the 
> dependent service can resume - however - the behaviour of the service 
> may have changed as a result of new state.  From this point of view - I 
> don't see 10,000 calls to suspend/resume (or perhaps I don't want to 
> imagine 10,000 calls to suspend/resume given the implications on 
> associated components).

OK, I misunderstood the contract. I thought Handle.access()/release() 
would trigger resume()/suspend() (~ one HTTP request), but obviously 
this is not the case. The semantics you describe above are, in fact, 
asynchronous in nature, and I can see why they're important for 
component assembly. Thanks for clearing this out!


> Looking at this purely for the context point of view - things are 
> manageble, but configuration changes are a lot more complicated.  I've 
> through about a number of scenarios such as a component registering 
> listeners on particular notes - etc.  but it always ends up icky.  A 
> simpler solution could be to simply tag artifacts as modified and then 
> just redo configuration, contextualization, etc.  The component could do 
> something like:
> 
>   public void reconfigure( Configuration config ) throws 
> ConfigurationException
>   {
>       if( config.isModifiedSince( m_timestamp ) )
>       {
>           // configuration contains a modified value so we need
>           // check any volatile children
>       }
> 
>       m_timestamp = new Date();
>   }

I think we could make this even simpler and more generic, e.g.

   void resume( Object[] theKeysOfTheChangedContextEntries );

One of the changed entries could be an instance of Configuration.

(: A ;)
-- 
Antti Koivunen (Mr.) <anryoshi@users.sf.net>


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