+0. Let me explain…
Having a separate interface to say start() is all very well and harmless in
itself, but:
(a) It doesn’t really address the issue of how service instances should be
started. We don’t actually have solid recommendations on this issue, although
there have been attempts in various containers to systematize it.
(b) the proposed warning addresses a case where a service instance is doing
“exporter.export(this);”, which I’d call bad practice in any case. It happens
that ServiceStarter appears to encourage this use case because it works by
calling a constructor. But the called constructor could (and should) construct
an instance of some other class, and then export and register that instance
rather than “this”. That style doesn’t mandate a Startable interface, and
having Startable doesn’t mandate good multi-threaded code. In other words, you
could implement Startable and still have poor thread-safety.
(c) even where there is an apparent problem, the exposure window is very
small. Consider the last bits of the init(…) method in
“com.sun.jini.reggie.ServiceRegistrarImpl”
myRef = (Registrar) serverExporter.export(this);
proxy = RegistrarProxy.getInstance(myRef, myServiceID);
myLocator = (proxy instanceof RemoteMethodControl) ?
new ConstrainableLookupLocator(
unicastDiscoveryHost, unicaster.port, null) :
new LookupLocator(unicastDiscoveryHost, unicaster.port);
/* register myself */
Item item = new Item(new ServiceItem(myServiceID,
proxy,
lookupAttrs));
SvcReg reg = new SvcReg(item, myLeaseID, Long.MAX_VALUE);
addService(reg);
if (log != null) {
log.snapshot();
}
try {
DiscoveryGroupManagement dgm = (DiscoveryGroupManagement) discoer;
String[] groups = dgm.getGroups();
if (groups == null || groups.length > 0) {
throw new ConfigurationException(
"discoveryManager must be initially configured with " +
"no groups");
}
DiscoveryLocatorManagement dlm =
(DiscoveryLocatorManagement) discoer;
if (dlm.getLocators().length > 0) {
throw new ConfigurationException(
"discoveryManager must be initially configured with " +
"no locators");
}
dgm.setGroups(lookupGroups);
dlm.setLocators(lookupLocators);
} catch (ClassCastException e) {
throw new ConfigurationException(null, e);
}
joiner = new JoinManager(proxy, lookupAttrs, myServiceID,
discoer, null, config);
/* start up all the daemon threads */
serviceExpirer.start();
eventExpirer.start();
unicaster.start();
multicaster.start();
announcer.start();
/* Shutdown hook so reggie sends a final announcement
* packet if VM is terminated. If reggie is terminated
* through DestroyAdmin.destroy() this hook will have no effect.
* A timeout on announcer.join() was considered but not deemed
* necessary at this point in time.
*/
Runtime.getRuntime().addShutdownHook(new Thread( new Runnable() {
public void run() {
try {
announcer.interrupt();
announcer.join();
} catch (Throwable t) {
logThrow(Level.FINEST, getClass().getName(),
"run", "exception shutting announcer down",
new Object[]{}, t);
}
}
}));
snapshotter.start();
if (logger.isLoggable(Level.INFO)) {
logger.log(Level.INFO, "started Reggie: {0}, {1}, {2}",
new Object[]{ myServiceID,
Arrays.asList(memberGroups),
myLocator });
}
ready.ready();
}
The window from when the join manager is started up (ignore the
serverExporter.export(). The only way a client gets to this endpoint is
through a proxy downloaded from the registrar) to when the init() method exits
is effectively 9 lines of code, during which time a _lot_ of network i/o would
have to happen in order for the object to be used prior to the constructor
exit. As well, the actual remote calls are filtered through a command
interface and through the tasker object, and the command classes often go
through synchronized() blocks, which (I think) act as happens-before fences.
The whole thing seems to me a little like the old servlet days, when we got the
SingleThreadModel interface and it only served to give naive developers a false
sense of security.
What we need is a discussion on a more developer-friendly service startup
convention to replace ServiceStarter.
Having said that, I’m not overly opposed to having Startable, given that as Dan
suggested, any output from the ServiceStarter is in the form of a suggestion
rather than an error.
Cheers,
Greg.
On Dec 18, 2013, at 7:42 PM, Peter <j...@zeus.net.au> wrote:
> +1 Peter.
>
>
> I've been going over the Exporter implementations and Exporter interface.
>
> The Exporter interface specifies that export and unexport behaviour is
> defined by the implementation.
>
> That means it's up to the implementation to specify whether a happens before
> edge occures before export.
>
> I propose we document in Exporter, that if export is called during object
> construction then that object's implementation cannot safely use final fields.
>
> We should also provide a @see reference to Startable to reccommend its use
> when final fields are desirable and include Startable in the spec.
>
> We don't need to rewrite the example of exporting in net.jini.config as
> export isn't performed during construction.
>
> This is a minor update to the Jini Specification.
>
> The FINE log message is an implementation detail and won't be included.
>
> Regards,
>
> Peter.
>
> ----- Original message -----
>> No, an effectively immutable field is one whose reference doesn't change
>> after the service objects safe publication.
>>
>> An immutable field is final.
>>
>> It would be Jeri's responsibility to safely publish the service during
>> export to ensure a happens before event occurs, ensuring all non
>> volatile fields the service contains are safely published and visible.
>>
>> It's possible to have mutable fields guarded by synchronized methods, or
>> volatile fields.
>>
>> But it isn't possible to have final fields when exporting within the
>> constructor.
>>
>> If the service contains effectively immutable field references, it must
>> be safely published or if another thread accesses those fields through
>> unsynchronized methods, the thread may only see their default value.
>>
>> All our service implementations have final fields. All our services had
>> some unsynchronized access to non volatile, non final fields.
>>
>> Trouble is, when you can't have final fields, it's very difficult to
>> determine which fields are effectively immutable and the original
>> programmers design intent, if there's no comment.
>>
>> Our service implementations all had unsynchronized init() methods called
>> from with the constructor that started threads and exported Some fields
>> were final.
>>
>> For me, a service that contains final fields and is exported after
>> construction is easier to maintain and understand.
>>
>> I suspect, like our Jini service imlementations, most people get it
>> wrong most of the time.
>>
>> I get that others won't want to use this method, however it would be
>> detrimental if we don't have such a method for those who would.
>>
>> Regards,
>>
>> Peter.
>>
>> ----- Original message -----
>>>
>>> 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.
>>>
>>> Cheers,
>>>
>>> Greg.
>>>
>>> On Dec 18, 2013, at 7:42 AM, Peter <j...@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 <j...@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 I 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 one 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 <j...@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” branch rather than a “3.0”
>>>>>>>>>>>> release.
>>>>>>>>>>>>
>>>>>>>>>>>> I’m still unnerved by the massive amounts of changes
>>>>>>>>>>>> to both code and tests in the qa_refactor branch, as
>>>>>>>>>>>> well as the apparent instability of the code,
>>>>>>>>>>>> although that seems to be improving.
>>>>>>>>>>>> In the next few weeks I’m going
>>>>>>>>>>>> to try and setup a cross-test case, to see what the
>>>>>>>>>>>> “2.2” tests say about the potential “2.3” release
>>>>>>>>>>>> and 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 pull out piecemeal? Maybe it
>>>>>>>>> would
>>>>>>>>>>>> make sense to split out the infrastructure services,
>>>>>>>>>>>> like 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 <j...@zeus.net.au>
>>>>>>>>>>>> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>> When the qa_refactor branch stabilises, I plan to
>>>>>>>>>>>>> merge trunk and provide a beta release for client
>>>>>>>>>>>>> compatibility testing.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Changes made have been focused on making our code
>>>>>>>>>>>>> thread safe, there are significant changes
>>>>>>>>>>>>> internally, the public api remains focused on
>>>>>>>>>>>>> backward compatibility, however it is advisable
>>>>>>>>>>>>> that client services adopt new safe construction
>>>>>>>>>>>>> techniques for services and implement the new
>>>>>>>>>>>>> Commission interface.
>>>>>>>>>>>>>
>>>>>>>>>>>>> What's a suitable test period for client testing?
>>>>>>>>>>>>>
>>>>>>>>>>>>> Regards,
>>>>>>>>>>>>>
>>>>>>>>>>>>> Peter.
>>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
