> On Apr 24, 2020, at 9:59 AM, Jakob Reschke <forums.jakob at resfarm.de> wrote:
> 
> 
> Domain and "data storage" layers are intermixed here, hence the trouble with the concept of read-only. If you don't want such a distinction of layers by using different objects in the first place, then I suggest to at least use separate protocols. So yes, use different managers for different kinds of read-onliness. Your domain-specific layer can still delegate to the VM write barrier if that works well. And different applications can still access the same kind/layer of read-onliness, so Eliot's proposal to have one registry for the VM write barrier handling remains valid.
> 
> I'd say each domain should also have an own exception class in this case. As an implementation detail, it could inherit from the base ModificationForbidden... or hide the concrete class behind an accessor.

No.  That is unnecessary.  The issue is to separate intercepting an attempt to write to a read-only object from what happens in response to that attempt.  And that’s what the ManagedObjects/defaultAction mechanism does.  Attempts to write are caught and other objects handle what happens in response.

Thinking of this issue in the co text of exception handlers doesn’t make sense.  Writes happen all over the system, and not necessarily within the dynamic extent of some computation.  The exception handler is global.  So one won’t be able to establish exception handlers for all cases.  Instead, the global exception system dispatches a per-object-specific response through ManagedObjects.

> 
> 
> 
> Marcel Taeumel <marcel.taeumel at hpi.de> schrieb am Fr., 24. Apr. 2020, 12:10:
>> >  But this means the DB is now occupying the use of the readOnly bit, not available for other applications or use-cases.
>> 
>> Hmmm... ManagedObjects could be partitioned among multiple applications. For example, a game loading some save state could come across some read-only objects and then choses how to manage those. Another application could do this as well, not interfering with the first one. Both might be considered "small DB managers" so to say.
>> 
>> Of course, that check "myDomainObject isReadOnlyObject" would become standard procedure for such DBs, wouldn't it?
>> 
>> Best,
>> Marcel
>>> Am 24.04.2020 05:26:56 schrieb Chris Muller <ma.chris.m at gmail.com>:
>>> 
>>>>>> So if the exception is unhandled, then before it raises an UnhandledError it checks an identity dictionary, which maps object to message, and if the read-only object that was the cause of the error is in the dictionary, instead the exception performs the message with the object as an argument.  So a database layer can add the objects it is managing to the map in ModificationForbidden, and mark them as read-only.  Then any and all attempts at modifying these objects will cause the database man ager to be notified. 
>>>>>>  So very simply we can have a pluggable solution that allows different clients to map the exception into different responses as desired. 
>>>>> 
>>>>> But an object in your db table could easily be an Array literal held by a CompiledMethod, not expected to change, but persistent by reference nonetheless.  So if the application did then modify it accidentally, instead of Forbidden protection, your DB manager would happily modify it.  It's this separation of use-cases is all what the question was about I think, not a big deal, we've lived with unprotected CM literals for this long already, and I've never needed WriteBarrier for anything other than a DB.
>>>> 
>>>> Since objects get added to the collection intentionally the array literal would not be added and hence the manager would not modify it.  
>>> 
>>> How would the manager know not to add it?  It can't be by checking #isReadOnlyObject, since that presents the same paradox -- requiring the manager to know the business of whatever other use-case has it set.  If it assumed it was for CM-literal protection, it wouldn't add it, but what if it was just some debugging or something?
>>>  
>>>> Your example is a straw man.  
>>>> 
>>>>> 
>>>>> However, I still don't see the path for how to use this in a complex multi-db Magma application with oblivious objects (e.g., Morphs).  It's not something any of the GemStone clients I consulted at as developer or DBA ever had to contend with either, but perhaps not something you're targeting.  Squeak is a different animal...  I will stay tuned here and watch for the answers as they unfold...
>>>> 
>>>> The objects that get added to the wrote barrier management collection get added by managers that want to manage specific objects, not arbitrary objects. Can you see that a DB manager would add objects it has materialized from the DB that it wanted to transparently write-back in modification, and no others?
>>> 
>>> Yes, absolutely.  The manager will get every ModificationForbidden signal under its code whether its meant for the DB or not.  Existence in a global table means handle it, otherwise pass it up the stack.  But this means the DB is now occupying the use of the readOnly bit, not available for other applications or use-cases.
>>> 
>>> I hope this critique isn't taken to mean I don't think this isn't a good feature for the VM and image.  I do.  Everything is a balance of features and limitations.  I'm still trying to determine whether Magma can benefit from this, and I have to get deeply critical to find the right decision.  I will be following this feature closely, thanks for your patience with my questions.
>>> 
>>>  - Chris
>> 
> 
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