[libvirt] Libvirt Java Bindings - adding domain event support - pull request

[Claudio Bley]

At Thu, 27 Feb 2014 03:47:31 +0000,
Chris Ellis wrote:
> 
> Hi all
> 
> I'm new to this list, I've been making use of the Libvirt Java bindings
> recently.  I wanted to make use of domain events yesterday
> so my application can be alerted when the state of a domain changes etc.
> 
> However I quickly discovered that domain events are completely broken in
> the current Java bindings.

> So I have implemented support for domain events in the Java
> bindings.

Searching the mailing list or even asking on the list before
implementing something would have probably saved you some work...

> Currently I've only implemented the following domain
> event IDs:
> 
>  * VIR_DOMAIN_EVENT_ID_LIFECYCLE
>  * VIR_DOMAIN_EVENT_ID_REBOOT
>  * VIR_DOMAIN_EVENT_ID_RTC_CHANGE

As Daniel already pointed out, there's my patch set here
https://www.redhat.com/archives/libvir-list/2014-February/msg00823.html
which contains event support for I/O error, life cycle, PM wakeup, PM
suspend and reboot events (because that's what I needed).

> Implementing these is enough to test that my implementation is sane,
> I'm hoping to implement the majority of the other events
> soon.
> 
> Events can be listened to by, registering via the Connect object, as
> follows:
> 
>  con.domainEventRegisterAny(DomainEventID.VIR_DOMAIN_EVENT_ID_LIFECYCLE,
> new DomainLifecycleEventHandler() {
>    @Override
>    public void onStarted(Connect connection, Domain domain, DomainEventType
> event, DomainEventStartedDetailType detail) throws LibvirtException {
>      System.out.println("Got start event: " + event + "::" + detail + " for
> domain " + domain.getName());
>    }
>  });

As Eric pointed out, there's no need to strictly adhere to the
function naming of libirt's C API.

I dislike overloading though (because it makes the code ambiguous and
doesn't buy you much).

> I've put my clone of the libvirt-java git repository on Github, my
> modifications to the Java binding are in a separate branch and
> should be simple to merge.  The changes can be viewed at:
> 
>   https://github.com/intrbiz/libvirt-java/compare/master...ce-domain-events

I've had a look at it and I see a few similarities to my
implementation. So, we're on the right track, I guess... :-)

For my implementation I've chosen to put the focus on simplicity for
the user and I tried not to limit the usage of the public interface in
any way. In particular this means:

- using the usual Java term "listener" and add/remove(Listener) idiom

- do not give "null" a special meaning when passed as a domain
  parameter of a method of the public API
  (instead, connect.add*Listener(..) registers a listener for all
   domains of a connection, whereas domain.add*Listener(..) only for
   the domain at hand)

- the user is free to implement several listeners in one class,
  reacting on different events in a central place
  (this is not possible with your approach and would not be possible
   when using method overloading)

- keep the /callback/ method's argument list as short as possible
  (e.g. in your DomainLifecycleEventHandler there's an onStarted method
   which takes 5 arguments, but it would actually only require two --
   the domain and the event detail)

- hide the underlying implementation as far as possible
  (the user shouldn't be bothered with callback IDs, event IDs et cetera)

- do not run an event loop in a different thread by default
  (this might not be what the user wants, for several reasons)

- do not specify a throws LibvirtException clause for any of the
  listener methods
  (this is actually forcing the user to handle any such exception
   right on the spot in the listener method itself (or wrap the
   LibvirtException into some kind of unchecked exception which gets
   handled by the JNA callback exception handler))

Another difference to your implementation is the handling of the
different event types for the live cycle events. You have chosen to
branch on the events and implement a method for each type of event.  I
like the idea in general (because it solves the problem of
transporting the event detail in regard to the event type to the
user's code in a type safe way), but to me it seems a bit too
complicated if the user wants to handle some type of events in the
same way. A simple switch statement would be a lot easier instead of
branching on the event types and then merging the code paths together
again. Of course, the user could override the onEvent method in that
case but one has to plan which approach beforehand.

I'm in favor of giving the user only one approach, not both. If it
seems feasible the user could implement the switch on the event type
herself.

But maybe a LifecycleAdapter class could be added for convenience
which does the branching...

[See, nobody knows what I'm talking about without seeing the
 code. That's why we like to have the patches directly send to the
 list.]

> I'm keen to get these enhancements / fixes merged into libvirt-java.

My patches were excepted already, but I'll wait for a short term if
you want to comment on them...!?

The plan would be to get version 1.5.2 out after the dust has settled
and maybe a few people have tested the API.

> I would also like to submit further fixes to race conditions
> in the free() handling.

Patches are very welcome! See the instructions here:
http://libvirt.org/hacking.html#patches which apply to libvirt-java
also, but use "[java]" or "[libvirt-java]" in the subject to indicate
the focus.

Claudio