[vbox-dev] API questions

Klaus Espenlaub klaus.espenlaub at oracle.com
Wed Aug 7 09:05:25 PDT 2013

Hi Maxime,

On 07.08.2013 17:07, Maxime Dor wrote:
> Hi Devs,
> After playing around with the Java WS API for a while now, I would
> appreciate some clarification and confirmation on some assumptions I am
> making for a while now. Here we go :
> When getting several ISession object accross the code, I ended up
> realizing that these objects actually point to the same session on the
> remote site. This also mean that I can only have one VM locked at the
> same time.
> 1. Is this on purpose to only have one VM lockable at the same time? Is
> this also the case in the XPCOM binding? Is it possible to get a
> different session for each call of IVirtualbox::getSessionObject()

Huh? I guess you mean IWebsessionManager::getSessionObject().

There's actually documentation for how to get several sessions from the 
webservice, see 
https://www.virtualbox.org/sdkref/interface_i_session.html - and I 
really think the ISession documentation is where one would look for 
information how sessions are handled.

Bottom line: every logon gives one session, and actually there's no 
relevant limit to the number of logons you can do.

Yes, I agree this could be handled differently, but it's been like this 
since the webservice was useable. It is consistent and if we'd change 
this then the setup code for a client talking to a new VirtualBox 
version would have to be different, before the client has a chance to 
check the API version number. Not nice.

Oh, and don't mix up logons and HTTP connections. The latter is quite 
meaningless to the webservice, some SOAP clients open a new connection 
for each and every request, others use keepalive (and vboxwebsrv by 
default closes connections after 100 requests, expecting the client to 
open a new one).

> 2. Am I correct to understand that the only way to get a lock on several
> machines at the same time is to have several connections, each with its
> session? Or will this lead to issues?

That's possible, but separate (HTTP) connections are not required to get 
multiple sessions. Separate connections are useful with multi-threaded 
clients as they eliminate the need to synchronize access to the 
connection by the threads.

> 3. Is there any recommended way (if even possible) to get several locks
> in a thread-safe way?

Most SOAP/WSDL libraries don't handle the use of one connection by 
several threads too well, you definitely have to be careful and consult 
the documentation of the respective implementation.

> ----------------------------------------
> Related to the objects living in on the remote side. In the SDK
> documentation, we are informed that we are supposed to use the
> releaseRemote() method on objects we get, so they don't build up on the
> "other side", being the webservice server or the XPCOM(?). While writing
> my code, the way I did at least, I am having a hard time releasing the
> object at the proper time. Putting the releaseRemote() a bit too
> defensively only ends up producing exception on further calls (Object
> reference is not found...), meaning that the actual remote object
> doesn't exist anymore. Obviously, since I released it earlier. The fear
> is to release "too late" and leave such objects orphan of any control.

Yes, releaseRemote() is very hard to use properly, if there's any chance 
of concurrent use by another thread (unless you're setting up the 
websession in each thread, then the MORs are not shared across threads). 
Don't worry about this too much, because...

> 1. What is the actual build up rate? Is it dangerous to keep these
> objets in memory on the other side? Could it crash the webserver? The
> context would be a single java process connected one time (possibly
> several times depending on your answer at the previous section).

The build up rate depends on how many different MORs (which is 
approximately equal to the different objects in the API) are needed by 
the client.

Even if you're never ever releasing anything, there's the expiration 
time-based garbage collector in vboxwebsrv. Its parameters are tuneable 
(see manual) when starting the webservice. By default, if a MOR is not 
used for 300 seconds, it will be thrown away, freeing the resources 
which might be behind it, doing all necessary cleanup.

Generally, if you use the API in a straightforward way then there's no 
significant waste of memory, neither in vboxwebsrv nor in VBoxSVC if you 
never use releaseRemote.

The most critical situations are obviously if a lot of time passes 
between the use of a MOR - you have to avoid this or increase the 
timeout parameter.

> 2. If you use the findMachine() method several times, using the same
> machine UUID, would you end up re-using the same remote object or is a
> remote object created each time? If the remote object is the same, I
> could live with the object residing in memory, this can only make things
> faster at the cost of using memory. Am I right in this assumption?

MORs are reused within one websession (which is actually the reason for 
the "problem" with releaseRemote you mentioned above), and that means no 
matter how often you get a reference to one object in the API, it will 
not need more memory or cleanup effort.

> 3. Is there a auto-release in Java code using the finalize() method of
> the object? or such cleanup must be done by the developper?

No, there's no auto-release as such (because the client bindings doesn't 
know if a MOR is used somewhere else, too, and we agree that 
releaseRemote is unsafe except in a few very well defined cases), but 
the expiration will take care of the garbage.

> 4. Any recommended way to deal with this? I am thinking of using a
> helper tatic class to fetch the objects and a timeout logic to release
> them. Some kind of cache really.

That's exactly the strategy implemented in vboxwebsrv if you don't do 
anything :)

If you're really curious what exactly is going on (warning: causes 
severe log bloat!), you can enable verbose mode of the webservice, and 
have a look what ~/.VirtualBox/vboxwebsrv.log says about MOR creation 
and the corresponding explicit or timeout based releases. Depending on 
how many API calls your client is making there can be fast log rotation 
(but the 10 last logs are kept by default, each either containing one 
day or 100MB of logs by default, everything relevant is tuneable).

Hope this helps,

> -----------------------------------------
> Thank you in advance for your time and wisdom on this.
> Best regards,
> Max

More information about the vbox-dev mailing list