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From Greg Trasuk <tras...@stratuscom.com>
Subject Dynamic Behavior wrt IOC in services.
Date Mon, 27 May 2013 18:04:13 GMT
(New subject for an interesting point)

Good point, Dennis...

On Mon, 2013-05-27 at 13:30, Dennis Reedy wrote:
> On May 27, 2013, at 103PM, Greg Trasuk wrote:
> 

> Sure, no problem. One big thing to consider wrt container IoC, is that
> the lifecycle of a River service is different then an EJB. With River,
> a service can join and leave the network (advertised and
> unadvertised). Making sure that you consider that has been important
> in my experience. This when tied with how a service deals with other
> services (what I call associations
> http://www.rio-project.org/associations.html), can change the
> lifecycle of a service. So food for thought
> 

I think you're on the right track in Rio, with injecting a dynamic proxy
for associations.  Curiously, this approach is similar to the dynamic
proxies injected under JEE6 CDI for contextual objects.

Truth is, I've always been hesitant about injecting service
dependencies, because it seems to me that resolving the services needs
to be a part of the logic of the service invocation, so that various
failures can be handled appropriately.

Couple of scenarios:

Context - We're talking about a service that depends on one or more
other services.  When you call service A's 'doSomething()' method, it
needs to call methods on services B, C, etc.

Scenario 1- You have a (not very efficient!) policy of looking up every
service dependency every time A.doSomething() is called.  In this case
it might just be reasonable to throw an exception (ServiceNotAvailable
or something similar) and let the client deal with it.  Straight
injection works in that case.  Or you might want to save the data
locally and do some alternate handling.  In that case, perhaps it might
be best to leave the unresolved service references null, so the
A.doSomething() code can handle it.

Scenario 2- You have some service references cached.  When you go to do
A.doSomething(), you find that service B has failed (of course, you find
this out by means of an IOException when calling service B).  What to
do?  In the past, I've handled this by dumping all the caches and
rediscovering the services.  Which means that A.doSomething() has to
take some control of the dependency resolution, which doesn't fit with
dependency injection very well.  In Harvester, I had a resolver object
(I could be wrong, but I think I saw something similar in Rio), so
A.doSomething() would execute the resolution before attempting to use
service dependencies.  Then in case of failure, it could call fail() on
the resolver, which would dump the caches, and attempt resolution again,
then complete the calls.

Another issue I've been pondering is how Jini interacts with dynamic IP
address assignments (i.e. network cards on DHCP).  If the IP address of
a node changes, it's kind of catastrophic, since the IP addresses of any
services it exports are probably baked in to both the serialized
endpoints, and the codebase urls (assuming that we pessimistically
assumed that name-based resolution won't work - arguably the truth in
most networks).  It seems like in that case, the services' states need
to roll back to before the endpoints were exported, then re-export and
re-publish to the registrar.

>From a client point of view, a client would simply see that as a set of
services failing, and then go and rediscover the services.  That's no
problem.  But I wonder how to handle that on the service side.  It's
almost easiest to just shutdown the services and restart.

And what happens if a host has more than one IP address (multi-homed)? 
Again, the services codebases will likely have the IP address baked-in. 
So a client on one interface needs to use the serialized proxy that
corresponds to the correct interface.  Here's a case where using Maven
artifact ids could help handle the codebase issue.  I suspect the
multiple IP addresses could be handled at the JERI level.

Anyway, lots of things to think about.

Greg.

> Regards
> 
> Dennis
> 
> 


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