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From Greg Trasuk <tras...@stratuscom.com>
Subject Re: River 3.0 beta release candidate
Date Wed, 18 Dec 2013 14:01:57 GMT

I feel like we’re going down a rabbit hole here when you start talking about exporting immutable
objects.  Wouldn’t it be kind of silly to export an immutable service?  Isn’t the whole
point that you interact with the service (i.e. alter its state) over a remote interface?

Perhaps it’s better to say that exported services need to be thread-safe (which should be
fairly obvious).  Yes, immutable objects are inherently thread-safe, so for sharing information
inside a VM, it makes some sense to minimize the number of mutable objects you pass around.
 But fundamentally, we’re talking about shared-state systems here.



On Dec 18, 2013, at 7:42 AM, Peter <jini@zeus.net.au> wrote:

> Pat your comment about non final fields is interesting.
> Isn't it also the case that we need to safely publish an effectively immutable object
to share it among threads?  That usually means copying it to a thread safe collection or shared
via a synchronized method, volatile field, or final field in another object?
> So we should also make sure that Jeri uses safe publication during export.
> That would allow a service that has no final fields to start threads, then export from
within a constructor safely, provided all operations on non final fields happen before starting
threads and exporting.
> All our services have final fields, so Starter is more appropriate for River's own services.
> Regards,
> Peter.
> ----- Original message -----
>> Hmm, good point, Startable, makes more sense.
>> An object can be exported using Startable.
>> I think we should have a policy to strongly discourage exporting from
>> constructors.
>> Regards,
>> Peter.
>> ----- Original message -----
>>> As far as I can tell, the special properties of completing a
>>> constructor   in the JLS memory model are:
>>> 1. A happens-before edge from the end of the constructor to the start
>>> of   a finalizer. (17.4.5)
>>> 2. The guarantee that any thread that only sees a reference to an
>>> object   after the end of the constructor will see the correctly
>>> initialized   values of all final fields. (17.5)
>>> The special issue with final fields is that implementations have
>>> freedom   to optimize access to final fields in ways that are not
>>> permitted for   non-final fields. Strategies for thread safety that
>>> work for non-final   fields do not necessarily work for final fields.
>>> The requirement for   final field safety is that the constructor end
>>> before another thread   accesses the newly constructed object.
>>> Calling a start() method after construction if the class implements a 
>>> new interface seems to me to be harmless, backwards compatible, and 
>>> useful. It enables the simplest and most direct way of preventing
>>> access   to the new object by another thread during construction.
>>> The roadmap issue is whether it should be required, and if so the
>>> level   of enforcement. For example, there is no reason to require it
>>> if the   class does not declare any final fields.
>>> Incidentally, and as a detail, "Commission" does not immediately make
>>> me   think of having a start() method that should be called after 
>>> construction. If you do go this way, the name needs thought.
>>> "Startable"   would be more obvious, more memorable, more likely to be
>>> found on   searches, and more compatible with familiar interface names
>>> such as   "Cloneable" and "Iterable".
>>> Patricia
>>> On 12/18/2013 2:18 AM, Peter wrote:
>>>> Well, now seems like a good time to have the conversation.
>>>> Yes there are other ways, but I haven't seen one safe implementation
>>>> yet, so...
>>>> Does someone have a better way to solve this problem, has someone
>>>> already solved this problem I'm unaware of that we can adopt, or is
>>>> there a way that's more satisfactory?
>>>> If not, is there something objectionable with the Commission
>>>> interface and if so, how can we fix it?
>>>> The SEVERE log message is logged by the River start package, other
>>>> containers or frameworks can choose whether or not to do so, but I'd
>>>> encourage them to do something similar, yes we can change it to WARN.
>>>> A much harsher option is to throw an exception during export which
>>>> breaks backward compatibility.
>>>> Regards,
>>>> Peter.
>>>> ----- Original message -----
>>>>> "org.apache.river.api.util.Commission is an interface services
>>>>> should implement"
>>>>> If it's a SHOULD, not a MUST, chucking out a SEVERE is incorrect
>>>>> logger behaviour IMO. You could issue a WARN if you like but for
>>>>> even that I'd say you need to provide a roadmap explaining why the
>>>>> warning and what you intend to do in future and what you expect of
>>>>> service writers such as myself.
>>>>> Commission, at least from my point of view, is your means (maybe
>>>>> the River community's - did you ask us?) for satisfying your needs
>>>>> in respect of the JMM. As we've discussed previously, there are
>>>>> other ways too and they work and they are safe if you know what
>>>>> you're doing. Your contention was that most don't know what
>>>>> they're doing hence, presumably, Commission.
>>>>> So the thing is, you are seemingly on a road to asserting more
>>>>> structure (gosh, a standard?) on the way people write their
>>>>> services. If so, you'd best start flagging that honestly and openly
>>>>> via a roadmap, deprecation and such/whatever rather than sticking
>>>>> out logger messages with no clear guidance and at the cost of a
>>>>> certain amount of nuisance (no admin I know likes SEVERE's being
>>>>> logged for something which isn't critical cos it's noise they don't
>>>>> want in log files).
>>>>> And of course, we all know that when some entity asserts a standard
>>>>> or requirement on others for entry, they may choose not to enter.
>>>>> Does this help your community or hinder it? The answer to that is,
>>>>> it depends. On what? Have you asked or tested? How have you tested?
>>>>> What would be considered validation or lack of support?
>>>>> I am not out to flame or troll rather I want to see this community
>>>>> demonstrating good behaviour and I'm not feeling like what's going
>>>>> on around Commission (what is that big change in version number
>>>>> really saying?) is such.
>>>>> On 18 December 2013 08:52, Peter <jini@zeus.net.au> wrote:
>>>>>> Just to clarify org.apache.river.api.util.Commission is an
>>>>>> interface services should implement, I would encourage all
>>>>>> container projects to pick up the interface and make suggestions
>>>>>> for improvement if there are any issues.
>>>>>> Interface Commission {
>>>>>> void start () throws Exception;
>>>>>> }
>>>>>> It's called after JMM safe construction to allow the service to
>>>>>> start any threads and be exported.
>>>>>> Regards,
>>>>>> Peter.
>>>>>> ----- Original message -----
>>>>>>> The way that services are instantiated and setup is an
>>>>>>> implementation detail.          When I think of compatibility
>>>>>>> think of the API and the lookup methods.          We think of
>>>>>>> compatibility from a client point of view.          From the
>>>>>>> client point of view, using a service looks like this:
>>>>>>> - Use multicast of unicast discovery to find one or more
>>>>>>> ServiceRegistrar instances              - Call lookup(…) on
>>>>>>> or more of these instances to get a set of service candidates

>>>>>>>             - Choose a candidate and prepare() it using a
>>>>>>> ProxyPreparer, to yield a usable service proxy.             
>>>>>>> Make calls on it.          Ideally hang on to this proxy
>>>>>>> instance, so you can skip the discovery and lookup next time
>>>>>>> you need it.              - If the call fails, repeat the
>>>>>>> lookup (and possibly discovery) til you get a proxy that works.
>>>>>>> Nowhere does the client need to know whether the service
>>>>>>> instance is started up using the “com.sun.jini.start”
>>>>>>> mechanism, your Commission interface, some other IOC container
>>>>>>> (Rio, Harvester, Seven or RiverContainer) or some unknown
>>>>>>> mechanism that starts with a static main() method.
>>>>>>> JSK2.0 was 2.0 because of the introduction of the proxy
>>>>>>> verification mechanisms, as well as JERI.          Absent some
>>>>>>> new client usage mechanism, River doesn’t need to go to 3.0.
>>>>>>> Cheers,
>>>>>>> Greg.
>>>>>>> On Dec 17, 2013, at 1:58 PM, Peter <jini@zeus.net.au> wrote:
>>>>>>>> I think changing services to use safe construction techniques
>>>>>>>> is enough to cause the version jump.
>>>>>>>> At this point I've allowed services to continue unsafe
>>>>>>>> construction practices, while logging a SEVERE warning when
>>>>>>>> the Commission interface isn't implemented, rather than fail.
>>>>>>>> This is a fundamental change to the way services are written.
>>>>>>>> Regards,
>>>>>>>> Peter.
>>>>>>>> ----- Original message -----
>>>>>>>>> Assuming that there aren’t major incompatibilities,
I think
>>>>>>>>> that would be a “minor” version change according
to our
>>>>>>>>> versioning policy, so we’d be looking at the “2.3”
>>>>>>>>> rather than a “3.0” release.
>>>>>>>>> I’m still unnerved by the massive amounts of changes
>>>>>>>>> both code and tests in the qa_refactor branch, as well
>>>>>>>>> the apparent instability of the code, although that seems
>>>>>>>>> to be improving.                  In the next few weeks
>>>>>>>>> going to try and setup a cross-test case, to see what
>>>>>>>>> “2.2” tests say about the potential “2.3” release
>>>>>>>>> vice-versa.
>>>>>>>>> I think what I’d really like to see is an incremental
>>>>>>>>> approach where we update limited components of the “2.2”
>>>>>>>>> branch, one at a time. Is there anything that we could
>>>>>>>>> out piecemeal? Maybe it
>>>>>> would
>>>>>>>>> make sense to split out the infrastructure services,
>>>>>>>>> Reggie, Mahalo, and Outrigger into different sub-projects
>>>>>>>>> that could be updated separately?
>>>>>>>>> Any thoughts?
>>>>>>>>> Greg.
>>>>>>>>> On Dec 17, 2013, at 5:03 AM, Peter <jini@zeus.net.au>
>>>>>>>>> wrote:
>>>>>>>>>> When the qa_refactor branch stabilises, I plan to
>>>>>>>>>> trunk and provide a beta release for client compatibility
>>>>>>>>>> testing.
>>>>>>>>>> Changes made have been focused on making our code
>>>>>>>>>> safe, there are significant changes internally, the
>>>>>>>>>> public api remains focused on backward compatibility,
>>>>>>>>>> however it is advisable that client services adopt
>>>>>>>>>> safe construction techniques for services and implement
>>>>>>>>>> the new Commission interface.
>>>>>>>>>> What's a suitable test period for client testing?
>>>>>>>>>> Regards,
>>>>>>>>>> Peter.

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