On Apr 27, 2012, at 3:55 AM, Felix Meschberger wrote: > Hi, > > Am 26.04.2012 um 02:33 schrieb David Jencks: > >> A lot of our confusion thinking about how to fix this is caused by the >> 2-step state changes in felix ds, such as the "activating" state. It seems >> like there are states and state transitions both represented as states. I >> reread the SCR spec and don't see any mention of states like this. Are >> these states entirely a felix invention so we could theoretically change the >> set of states? > > These are our states because they make handling a lot easier -- except for > such special concurrency situations. But then these states are also defined > as part of an API we have already published and to which the Equinox DS > implementation is also adhering. So carefull, when changing.
I don't see how the *ing states, all of which have no implemented methods, make anything easier. All the tests pass if you remove them. They are all transient and with any sort of synchronization would never be seen by anyone. > >> >> Also I'm not sure we're registering the service before activating it.... >> 112.2.2 > > No, we are violating the spec in this perspective -- because IMHO the spec is > not realistic for immediate services to require it: Imagine an immedate > service is registered. A service listener is called and asks for the service. > It gets the incompletely activated service object and boom you go.... > > Also the principle of least surprise would a service to be ready for use when > registered even though according to the spec you might expect a service to > not be ready for use when the consumer is notified. Sounds weird. > > Of course, a proxy might solve that problem, but this sounds a bit too much. > Hence we currently activate and then register -- as do all non-service > factory services. My perspective -- so far untested -- today -- is that it might be a good idea to register and then instantiate an immediate service because it will make immediate and delayed services work more similarly. For a delayed service, we register the service and stop. In the service registration thread , something can get the event and request the service instance immediately, before the registration returns. This can happen for an immediate service too. Here, if the registration results in a request for the service, we just create it like the delayed case. If not, when the registration returns, we can verify that the service wasn't created yet and go ahead and create it. thanks david jencks > > That was the easier message to answer. I will go for the larger one in a > second. > > Regards > Felix > >> >> If an immediate component configuration is satisfied and specifies a >> service, SCR must regis- ter the component configuration as a service in the >> service registry and then activate the component configuration. >> >> >> thanks >> david jencks >> >> On Apr 19, 2012, at 3:19 PM, David Jencks wrote: >> >>> We've run into one definite concurrency problem in SCR and I've been >>> discussing offline with a colleague how to fix it and wanted to get the >>> discussion out in the open. >>> >>> The original symptom was when 2 mandatory service refs were satisfied on >>> different threads at once: the 2nd wasn't recognized so the component never >>> got activated. >>> >>> This is easily solved by synchronizing but this introduces risk of >>> deadlocks (my first attempt, >>> https://issues.apache.org/jira/secure/attachment/12522537/FELIX-3456-1.diff) >>> >>> We tried some partly asynchronous approaches such as >>> https://issues.apache.org/jira/secure/attachment/12523313/FELIX-3456-4.diff. >>> Unless there's a timeout (presumably due to deadlock) this gets all >>> service events processed before the thread exits from its first call into >>> SCR. However this can result in service events getting processed later >>> than one expects possibly on a different thread. On further thought we >>> concluded that a service event must be processed fully before the service >>> registration call returns. We therefore don't think any kind of >>> asynchronous approach will work. >>> >>> We've discovered the anti-circular-dependency clause in the spec (112.3.5) >>> but it appears to be overly biased towards SCR-only graphs of services. We >>> are leaning towards thinking that SCR also needs to consider: >>> >>> - an activate method registers a service that satisfies an optional >>> dependency of a component being activated by scr on the same thread. >>> - the same, except the activate method starts a new thread to register the >>> service and waits for it to complete. >>> >>> Another scenario to consider is >>> >>> components C1 and C2 registering as services, each with an optional dynamic >>> dependency on the other. If one starts, and then the other, there is no >>> problem, they both get references to the other. If they both start at the >>> same time in separate threads (either because they are in different bundles >>> or because they get activated due to mandatory references being satisfied) >>> and register the services while the other is in the Activating state, a >>> simple lock over the service event processing will result in deadlock. >>> Furthermore, to get the correct result, at least one of the services has to >>> be bound while the component to which is is binding is in the Activating >>> state. >>> >>> It looks like the situation can be simplified a bit by considering, for >>> service events, whether the dependency will result in a state change: if >>> it's optional or mandatory but not the only satisfying service, it won't, >>> but if it's mandatory and the first satisfying service, it will. We can >>> calculate this before calling any bind methods or activate methods. After >>> determining this, we know the final state of the component. >>> >>> We're considering whether some kind of 2-stage lock would work: >>> >>> one level can change the state and blocks all other threads >>> the other level can't change the state and lets stuff like service events >>> for non-state-changing service references be processed according to the >>> final state of the component. (e.g. activating will let bind methods be >>> called on the under-configuration object). >>> >>> This does not yet consider bundle event driven state changes or >>> deactivation or delayed component creation or service factories. >>> >>> Comments and more scenarios to consider are more than welcome. >>> >>> thanks >>> david jencks >>> >>> >>> >> >
