Re: [equinox-dev] When is DS done loading services?

[Stoyan Boshev <s.boshev@xxxxxxxxxxx>]

BJ Hargrave wrote:
>  > > I feel that changing DS to perform service registration synchronous 
> with
>  > > bundle activation will likely result in deadlocks when there are 
> complex
>  > > service dependency graphs.
>  >
>  > This won't be any different from what is done now manually. The
>  > potential for deadlocks are there regardless of DS.
>  >
>
> Perhaps, but at least we are not building it into the DS spec or the DS 
> impl!
>
>  > >
>  > > One possible way to deal with this is through the use of component
>  > > enablement. Using the original example of service A depending upon some
>  > > number of service B, if you can know when all the service Bs are
>  > > available, one can write a component with a dynamic dependency on B.
>  > > When all the Bs are available then enable the component providing
>  > > service A (which are declared disabled in its description). The 
> trick is
>  > > of course to be able to know when all the service Bs are available such
>  > > that we are ready to enable serviceA.
>  >
>  > And we are back to the core problem: we cannot be sure when all service
>  > Bs are available.
>
> How can you ever be sure? :-) Unless there is some definition of the 
> expected set of Bs, it is just random.
>
>  > In code which doesn't use DS, start() method will
>  > serve as a boundary for the service changes - satisfied services will be
>  > registered while the bundle is starting and any services which becomes
>  > satisfied will be registered synchronously (the latter is the current
>  > behavior of DS).
>
> This makes the assumption that bundle will always register their B 
> during BundleActivator execution. I think this is a fairly tall 
> assumption. What if a bundle's B depends upon something which is not yet 
> available? It is very bad form to block the BundleActivator. So such a 
> bundle, if well behaved, would have to return from the BundleActivator 
> w/o having registered B and delay registering B until the dependency is 
> met some time later.
>
>  >  After all the bundles are activated, we can be sure
>  > that all services which doesn't depend on external factors will be
>  > available and the system can be considered ready.
>  >
>
> You assume ("all services which doesn't depend on external factors") 
> that all B's have no dependencies and can be registered at will. That is 
> a pretty big assumption.
>
> Perhaps one thing a DS impl could do, is to register all services for 
> which all dependencies are currently met synchronously during bundle 
> activation. All other services would have to handled asynchronously. 
> This splits the DS work between work that can be quickly done during 
> bundle activation and work that will take time which is done after 
> bundle activation. I don't think any change to the DS spec is needed to 
> allow this behavior.

I totally agree. This is what I actually wanted in the very beginning of 
this thread.



> --
>
> *BJ Hargrave*
> Senior Technical Staff Member, IBM
> OSGi Fellow and CTO of the _OSGi Alliance_ <http://www.osgi.org/>_
> __hargrave@xxxxxx.com_ <mailto:hargrave@xxxxxxxxxx> 	
>
> office: +1 386 848 1781
> mobile: +1 386 848 3788
>
>
>
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dipl. eng. Stoyan Boshev . Department manager
ProSyst Labs EOOD
1606 Sofia, Bulgaria . 48 Vladajska Str.
Tel. +359 2 953 05 88; Fax +359 2 953 26 17
Mobile: +359 88 898 29 17
http://www.prosyst.com . s.boshev@xxxxxxxxxxx
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