I think you can take a look at what aries blueprint does for these
cases.
They create a proxy for each injected service and switch the service
if the
original one goes away. If no service is available then I think it
waits for some time for a new one to come and throws an exception if
a time
out happens.
Perhaps a similar behaviour can also be added for DS. Not sure if it
matches
the DS ideas though.
Christian
On 13.09.2013 12:10, Thomas Diesler wrote:
Thank you all for your replies. We ended up with three measures
#1 assert that the component is still valid on entry of every public
method
(AtomicBoolean set by activate/deactivate)
#2 Use a ValidatingReference to hold unary references to dependent
services
(prevents NPE when a dependent service goes away)
#3 Throw an InvalidComponentException runtime exception on #1 and #2
The idea is that access to a deactivated reference never throws NPE
Access to the public API is prevented from deactivated service
instances
cheers
--thomas
On Sep 11, 2013, at 11:11 AM, Richard S. Hall <[email protected]>
wrote:
Resending my reply from yesterday since my original message didn't
seem to
go through...
----
Yes, you can do some of these sorts of things with iPOJO.
First, iPOJO has the notion of a service-level service dependency as
well as
an implementation-level service dependency (which is the level of DS
dependencies). Second, iPOJO caches services references within a
service
method invocation so that a thread calling a method on a service
will see
the same injected services until the thread exits the invoked service
method.
It doesn't deal with configuration locking (at least not of which I am
aware).
-> richard
On 9/10/13 06:41 , Thomas Diesler wrote:
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
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--
Christian Schneider
http://www.liquid-reality.de
Open Source Architect
http://www.talend.com
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