----- Original Message -----
From: "Juan Hernandez" <jhernand@redhat.com> To: "Daniel P. Berrange" <berrange@redhat.com> Cc: "arch" <arch@ovirt.org>, engine-devel@ovirt.org Sent: Monday, September 24, 2012 6:30:43 PM Subject: Re: [Engine-devel] libosinfo integration
On 09/24/2012 05:24 PM, Daniel P. Berrange wrote:
On Mon, Sep 24, 2012 at 04:51:37PM +0200, Roy Golan wrote:
Here's a draft wiki of the libosinfo integration with the engine http://wiki.ovirt.org/wiki/Libosinfo
please share your comments on this thread and I'll update the wiki accordingly.
As a libosinfo maintainer I strongly discourage you from writing a custom API directly against the XML. While the format is stable from the POV that vendors/users can create new XML docs for new/customized OS variants they have, we do not consider it something for applications to be using directly. The libosinfo library API includes alot of logic beyond simply being an convenient way to process the XML. In particular dealing with the inheritance of data across OS, searching for data across a variety of data sources, relying on GIO for URI resolving, live update of the database, and more.
We're also not really interested in maintaining manually written bindings for any languages. Rather than putting effort into such bindings for Java, time is better spent on finishing the GOBject introspection mapping layer for Java. There was a proof of concept, which was never finished / updated to the final introspection spec, which can be used as a basis for ongoing development.
This work will be applicable way beyond just libosinfo, to a wide variety of useful APIs. Of possible interest to VDSM in the future will be things like the new libvirt-designer & libvirt-builder APIs which are going to provide a bridge between libvirt + libosinfo to simply life for application developers constructing XML for provisioning guests.
I think that the use of JGIR has already been discussed as a way to interface the engine with libvdsm.so:
http://lists.ovirt.org/pipermail/arch/2012-August/000760.html
If I remember correctly the outcome of that discussion was that it shouldn't be used because it is considered dangerous to use native libraries from Java and because JGIR itself is very outdated. I don't 100% agree with the first part, but the second part is completely true.
Anyhow, as it seems to be the right way to interface with libosinfo and future libvirt capabilities, we may want to rethink and put some effort to bring it up to date.
I don't think Java should go native to obtain data, using JNI or whatever native interface that is offered. It can totally break the JVM and cause leaks and stability issues JRE would like to avoid. Having language binding for data access logic seems a bit strange for me, as it should be easy to stabilize a schema either using XML or a set of CSVs, for all to read and use. I do understand why JNI is to be used when OS specific features are to be used, for example unix domain sockets, shared memory, security, but not for data access. The alternatives would be: 1. Run a python program at ovirt-engine start-up, use the python API to fetch the data we need, and generate our own tiny XML of the subset or even load it to the database. We can use the output safely in Java. Disadvantage is that we need to stop/start service or have a "refresh" verb for reload after libosinfo update. 2. When data is requested, execute a python process with cmdline arguments and get result from stdout of the process. The disadvantage is the overhead of clone(). 3. Read the XML directly on runtime, disadvantage is if XML is changed. 4. Use the libosinfo python API during build and distribute an update to the information in versions of ovirt, disadvantage is slow refresh of data, not sure how often this data is updated. 5. Provide a web service that has specific XML-RPC / RESTful API, that can provide data out of the libosinfo, and publish it at internet. Configure ovirt to use it, disadvantage is the need for internet connection. 6. Configure cgi-bin locally at apache where ovirt is running, which provide XML-RPC / RESTful API, and access it locally. In any case I would avoid exiting the JVM to interact with data provider. Regards, Alon.