You still have the issue that services are transient. You cannot "pin" a set of them for some time duration. A service can represent a remote service, access to which is subject to failure of the network or remote withdrawal of the service.
But I am not totally sure I understood your proposal. -- BJ Hargrave Senior Technical Staff Member, IBM OSGi Fellow and CTO of the OSGi Alliance [email protected] office: +1 386 848 1781 mobile: +1 386 848 3788 From: Thomas Diesler <[email protected]> To: OSGi Developer Mail List <[email protected]> Date: 2013/09/10 07:01 Subject: [osgi-dev] Fabric Service Model - Request for feedback Sent by: [email protected] Hi Folks, in Fabric we have a service model whereby services have interdependencies, are configurable and dynamic by nature - all of which is managed in OSGi with the help of Declarative Services. To illustrate I use a simple example ServiceT { @Reference ServiceA serviceA; @Reference ServiceB serviceB; public doStuff() { // that uses serviceA & serviceB } } The injection is handled by the DS framework - there are various callbacks involved. Lets assume the system is fully configured and a client makes a call on ServiceT ServiceT serviceT = getServiceT(); serviceT.doStuff(); Due to the dynamic nature of OSGi services and their respective configuration ServiceT must deal with the following possible/likely situations #1 An instance of a referenced service is not available at the point of access (i.e. serviceA is null) #2 In the context of a single call the service instance may change (i.e. call may span multiple instances of serviceA) #3 In the context of a single call the configuration of a service instance may change (i.e. serviceA is not immutable, sequential operations on A may access different configurations) In OSGi there is no notion of global lock for service/configurations nor a notion of lock of a given set of services/configurations - I cannot do lock(T, A, B); try { ServiceT serviceT = getServiceT(); serviceT.doStuff(); } finally { unlock(T, A, B); } This code is also flawed because it assumes that the caller of doStuff() is aware of the transitive set of services involved in the call and that this set will not change. As a conclusion we can say that the behaviour of doStuff() is only defined when we assume stability in service availability and their respective configuration, which happens to be true most of the time - nevertheless, there are no guarantees for defined behaviour. How about this … The functionality of A and B and its respective configuration is decoupled from OSGi and its dynamicity A { final Map config; public doStuffInA() { } } B { final Map config; public doStuffInB() { } } ServiceA and ServiceB are providers of immutable instances of A and B respectively. There is a notion of CallContext that provides an idempotent set of instances involved in the call. CallContext { public T get(Class<T> type); } This guarantees that throughout the duration of a call we always access the same instance, which itself is immutable. CallContext also takes care of instance availability and may have appropriate timeouts if a given instance type cannot be provided. It would still be the responsibility of A/B to decide wether an operation is permissible on stale configuration. Changes to the system would be non-trival and before I do any prototyping I'd like to hear what you think. cheers --thomas _______________________________________________ OSGi Developer Mail List [email protected] https://mail.osgi.org/mailman/listinfo/osgi-dev
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