<div dir="ltr">We learn something everyday, the beauty of coding !<div>Thank you again.</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">On 8 August 2013 12:08, Klaus Espenlaub <span dir="ltr"><<a href="mailto:klaus.espenlaub@oracle.com" target="_blank">klaus.espenlaub@oracle.com</a>></span> wrote:<br>
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Hi Max,<div class="im"><br>
<br>
<div>On 07.08.2013 23:10, Maxime Dor wrote:<br>
</div>
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<div dir="ltr">Hi Klaus,
<div><br>
</div>
<div>Thank you very much for the detailed explaination of the
inner-working of the session - I never understood it this way.
Reading back the SDK after your explaination, it sure makes
perfect sense. Never understood it this way before.</div>
<div>I will give the multi-session a try ASAP and see if I can
have this multi-lock heaven. Thank you!</div>
<div><br>
</div>
<div>About the objects, I am glad to hear the hard work has been
done already. I will therefore leverage the built-in timeout
of objects in the vboxwebsrv and not do it on my side. This
will make things *a lot* easier...</div>
<div>Again, thank you for the detailed & clear explaination.
I'll dare to turn on the verbose log and see how it works.</div>
<div><br>
</div>
<div>Max<br>
</div>
<div><br>
</div>
<div>PS : so I don't die stupid, what does MOR stand for? Google
doesn't give me interesting results...</div>
</div>
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<br></div>
Hehe... seems we don't use this acronym anywhere in the API docs.
MOR=managed object reference. SOAP/WSDL isn't really object oriented
(it's belongs to the RPC family, and actually a rather simple minded
one as there's no predefined way to represent references), thus we
had to invent our own clever scheme.<br>
<br>
You use the 'glue' Java webservice bindings, with the consequence
that the MORs are actually not visible anywhere in your code, as
there are abstracted away by the client side wrapper objects - with
the releaseRemote() method doing a direct release of the underlying
MOR. That's where they shine through slightly...<span class="HOEnZb"><font color="#888888"><br>
<br>
Klaus</font></span><div><div class="h5"><br>
<br>
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<div><br>
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<div class="gmail_extra"><br>
<div class="gmail_quote">On 7 August 2013 18:05, Klaus
Espenlaub <span dir="ltr"><<a href="mailto:klaus.espenlaub@oracle.com" target="_blank">klaus.espenlaub@oracle.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">Hi
Maxime,<br>
<div><br>
On 07.08.2013 17:07, Maxime Dor wrote:<br>
> Hi Devs,<br>
><br>
> After playing around with the Java WS API for a
while now, I would<br>
> appreciate some clarification and confirmation on
some assumptions I am<br>
> making for a while now. Here we go :<br>
><br>
> When getting several ISession object accross the
code, I ended up<br>
> realizing that these objects actually point to
the same session on the<br>
> remote site. This also mean that I can only have
one VM locked at the<br>
> same time.<br>
> 1. Is this on purpose to only have one VM
lockable at the same time? Is<br>
> this also the case in the XPCOM binding? Is it
possible to get a<br>
> different session for each call of
IVirtualbox::getSessionObject()<br>
<br>
</div>
Huh? I guess you mean
IWebsessionManager::getSessionObject().<br>
<br>
There's actually documentation for how to get several
sessions from the<br>
webservice, see<br>
<a href="https://www.virtualbox.org/sdkref/interface_i_session.html" target="_blank">https://www.virtualbox.org/sdkref/interface_i_session.html</a>
- and I<br>
really think the ISession documentation is where one
would look for<br>
information how sessions are handled.<br>
<br>
Bottom line: every logon gives one session, and actually
there's no<br>
relevant limit to the number of logons you can do.<br>
<br>
Yes, I agree this could be handled differently, but it's
been like this<br>
since the webservice was useable. It is consistent and
if we'd change<br>
this then the setup code for a client talking to a new
VirtualBox<br>
version would have to be different, before the client
has a chance to<br>
check the API version number. Not nice.<br>
<br>
Oh, and don't mix up logons and HTTP connections. The
latter is quite<br>
meaningless to the webservice, some SOAP clients open a
new connection<br>
for each and every request, others use keepalive (and
vboxwebsrv by<br>
default closes connections after 100 requests, expecting
the client to<br>
open a new one).<br>
<div><br>
> 2. Am I correct to understand that the only way
to get a lock on several<br>
> machines at the same time is to have several
connections, each with its<br>
> session? Or will this lead to issues?<br>
<br>
</div>
That's possible, but separate (HTTP) connections are not
required to get<br>
multiple sessions. Separate connections are useful with
multi-threaded<br>
clients as they eliminate the need to synchronize access
to the<br>
connection by the threads.<br>
<div><br>
> 3. Is there any recommended way (if even
possible) to get several locks<br>
> in a thread-safe way?<br>
<br>
</div>
Most SOAP/WSDL libraries don't handle the use of one
connection by<br>
several threads too well, you definitely have to be
careful and consult<br>
the documentation of the respective implementation.<br>
<div><br>
> ----------------------------------------<br>
><br>
> Related to the objects living in on the remote
side. In the SDK<br>
> documentation, we are informed that we are
supposed to use the<br>
> releaseRemote() method on objects we get, so they
don't build up on the<br>
> "other side", being the webservice server or the
XPCOM(?). While writing<br>
> my code, the way I did at least, I am having a
hard time releasing the<br>
> object at the proper time. Putting the
releaseRemote() a bit too<br>
> defensively only ends up producing exception on
further calls (Object<br>
> reference is not found...), meaning that the
actual remote object<br>
> doesn't exist anymore. Obviously, since I
released it earlier. The fear<br>
> is to release "too late" and leave such objects
orphan of any control.<br>
<br>
</div>
Yes, releaseRemote() is very hard to use properly, if
there's any chance<br>
of concurrent use by another thread (unless you're
setting up the<br>
websession in each thread, then the MORs are not shared
across threads).<br>
Don't worry about this too much, because...<br>
<div><br>
> 1. What is the actual build up rate? Is it
dangerous to keep these<br>
> objets in memory on the other side? Could it
crash the webserver? The<br>
> context would be a single java process connected
one time (possibly<br>
> several times depending on your answer at the
previous section).<br>
<br>
</div>
The build up rate depends on how many different MORs
(which is<br>
approximately equal to the different objects in the API)
are needed by<br>
the client.<br>
<br>
Even if you're never ever releasing anything, there's
the expiration<br>
time-based garbage collector in vboxwebsrv. Its
parameters are tuneable<br>
(see manual) when starting the webservice. By default,
if a MOR is not<br>
used for 300 seconds, it will be thrown away, freeing
the resources<br>
which might be behind it, doing all necessary cleanup.<br>
<br>
Generally, if you use the API in a straightforward way
then there's no<br>
significant waste of memory, neither in vboxwebsrv nor
in VBoxSVC if you<br>
never use releaseRemote.<br>
<br>
The most critical situations are obviously if a lot of
time passes<br>
between the use of a MOR - you have to avoid this or
increase the<br>
timeout parameter.<br>
<div><br>
> 2. If you use the findMachine() method several
times, using the same<br>
> machine UUID, would you end up re-using the same
remote object or is a<br>
> remote object created each time? If the remote
object is the same, I<br>
> could live with the object residing in memory,
this can only make things<br>
> faster at the cost of using memory. Am I right in
this assumption?<br>
<br>
</div>
MORs are reused within one websession (which is actually
the reason for<br>
the "problem" with releaseRemote you mentioned above),
and that means no<br>
matter how often you get a reference to one object in
the API, it will<br>
not need more memory or cleanup effort.<br>
<div><br>
> 3. Is there a auto-release in Java code using the
finalize() method of<br>
> the object? or such cleanup must be done by the
developper?<br>
<br>
</div>
No, there's no auto-release as such (because the client
bindings doesn't<br>
know if a MOR is used somewhere else, too, and we agree
that<br>
releaseRemote is unsafe except in a few very well
defined cases), but<br>
the expiration will take care of the garbage.<br>
<div><br>
> 4. Any recommended way to deal with this? I am
thinking of using a<br>
> helper tatic class to fetch the objects and a
timeout logic to release<br>
> them. Some kind of cache really.<br>
<br>
</div>
That's exactly the strategy implemented in vboxwebsrv if
you don't do<br>
anything :)<br>
<br>
If you're really curious what exactly is going on
(warning: causes<br>
severe log bloat!), you can enable verbose mode of the
webservice, and<br>
have a look what ~/.VirtualBox/vboxwebsrv.log says about
MOR creation<br>
and the corresponding explicit or timeout based
releases. Depending on<br>
how many API calls your client is making there can be
fast log rotation<br>
(but the 10 last logs are kept by default, each either
containing one<br>
day or 100MB of logs by default, everything relevant is
tuneable).<br>
<br>
<br>
Hope this helps,<br>
Klaus<br>
<div><br>
><br>
> -----------------------------------------<br>
><br>
> Thank you in advance for your time and wisdom on
this.<br>
><br>
> Best regards,<br>
> Max</div>
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</div>
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