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From David Jencks <david_jen...@yahoo.com>
Subject Re: [DS] Feedback wanted on some ideas
Date Fri, 12 Oct 2012 00:29:46 GMT

On Oct 5, 2012, at 12:35 PM, Felix Meschberger wrote:

> Hi,
> Am 03.10.2012 um 19:28 schrieb David Jencks:
>> I've had several ideas about DS enhancements, some of which I've implemented, and
would like some feedback about how desirable they are before committing or proceeding with
>> 1.  (FELIX-3692)  If you manually enable/disable components some of the work gets
done asynchronously.  I propose an api for finding out whether this work is done or waiting
for it, something like
>>  boolean tasksCompleted();
>>  void waitForTasksCompleted();
>> on ScrService.   (suggestions for better names welcome :-)  One use would be in our
tests to replace the delay() call.
> -1
> I think such information is without any value. And our own tests being the sole use cases
is a bit weak to add API.

The use cases I have are:

1. shutdown.  We want to make sure the queue is empty before turning off logging. (FELIX-3704)
2. You might want to install a lot of application deployer components and make sure they are
all fully started before starting say a file listener that looks for applications to deploy.
 If some of the first set of components are enabled through code, it's hard to tell if they
are all completely started without knowing exactly what they are.  Similarly you might want
to know if a server is "fully started", i.e. all the components that are enabled have at least
tried to activate.

I'm not 100% sure this is a good idea, but I think it could have some use.

>> 2.  (FELIX-3557) There are several circumstances in which, as the spec warns, you
can't establish a circular dependency between components.  In some of these cases, the order
in which the components are activated determines whether all the references are established.
 This is hard to understand from a users point of view :-).  Sometimes it's possible to detect
these situations and establish the reference asynchronously.  The patch attached to the issue
does this but needs a little more work to only try with services from DS components.
> If I understand the issue/patch correctly, it does something like this: If an optional
service cannot be eagerly bound (to call a bind method taking the service instance) due to
a circularity (exception thrown from the BundleContext.getService method) it will not be bound
but may later be bound if the actual service instance is created. Correct ?
> I think, this sounds good.

OK, I'll commit it.
>> For these two, I'm wondering if they would be useful enough to propose for the DS
1.3 spec.
> Does FELIX-3557 really imply a spec change ? At most it might be kind of an implementation

If you don't think it needs a spec change I won't worry.

>> 3. (re-proposal)  I'd like to propose moving the implementation to java 5 again with
generics etc.  The last time I suggested this there was a lot of pushback on the grounds that
there are a lot of people using DS on limited platforms.  However, none of these alleged :-)
people is using trunk, because for several months the classes pulled from the concurrent library
were wrong and trunk just didn't run on pre-java-5 vms.  Are the compendium 4.3 spec classes
we pull in even compatible with pre-java-5 vms?
> The longer I think about it, the more I have to admit that I agree....

I'm going to change this now because it will make some refactoring easier.  If people don't
like it we can change back after I'm done.

>> 4.  (radical idea I haven't tried yet)  I'm becoming increasingly convinced that
the state objects in AbstractComponentManager mostly cause confusion and make the code more
complicated and less reliable.  
> Well, these objects have been quite simple and easy to understand and worked perfectly
(admittedly with some glitches). Now over time and patches applied, I agree they became quite
complex and the concurrency behaviour became more complex. So for concurrency having immutable
objects is a lot easier to handle than mutable ones.
>> The spec really only describes two states, enabled and disabled.  The variations
on enabled -- whether the component has all its dependencies satisfied, whether the service
is registered, whether there are any implementation objects created -- all seem somewhat orthogonal
and depend very much on the environment  and don't seem to relate well to a single "dimension"
of state.  I'm considering trying to refactor the code that responds to outside actions (activate/deactivate
and dependencies appearing/disappearing) to be more "straight-through" with checks on the
specific aspects of state that they need.  Possibly we would want to put the "dynamic state"
such as dependencies + instances in a single state object, but this is a different approach
to the current state objects which have no internal state.
> I agree, that the states enabled and disabled might be confusing. But I am not sure,
whether those are really states of component instances (or component configurations as the
spec calls them). Rather they are states of the "abstract" component.
> The component instances on the other hand have states like unsatisfied, activating, active,
registered, etc.
> In our implementation the component instance is represented by the AbstractComponentManager
and its extensions while the "abstract" component is represented by combination of the ComponentHolder
and the ComponentMetadata.

I'm going to think about this some more and try some experiments.

thanks for the comments!
david jencks

> Regards
> Felix

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