Mailinglist Archive: yast-devel (211 mails)

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Re: [yast-devel] Some notes about new storage API
  • From: Ancor Gonzalez Sosa <ancor@xxxxxxx>
  • Date: Tue, 13 Oct 2015 08:55:48 +0200
  • Message-id: <561CAAF4.4040008@suse.de>
On 10/09/2015 02:54 PM, Arvin Schnell wrote:
On Wed, Oct 07, 2015 at 02:12:44PM +0200, Ancor Gonzalez Sosa wrote:

[,,,]

Only some (maybe too Ruby-centric) comments follow.

Casting and Storage.some_method
===============================

Things like this look quite un-ruby:

tmp1 = devicegraph.find_device(42)
assert(Storage.disk?(tmp1))
assert(Storage.to_disk(tmp1))
assert(!Storage.partition_table?(tmp1))
assert_raises(Storage.DeviceHasWrongType {
Storage.to_partition_table(tmp1)
}

I would have expected things like

tmp1.disk?
tmp1.to_partition_table

At least in C++ such a interface looks like a very bad idea since
for *every* new class you have to modify the base class. With
such a design ABI stability is not possible.

Ok.

In fact, I wouldn't expect to have to perform this kind of
casts,

These cast should not be needed often as I already wrote in
http://lists.opensuse.org/yast-devel/2014-12/msg00025.html.

at most I would expect to do more rubist things like

tmp1.is_a?(Storage::Disk)
tmp1.respond_to?(:whatever_method)

I'll do more experiments with this as soon as the prototype is easily
installable. But looks pretty clear that it would be fairly easy to fix
the un-rubism by adding the corresponding methods in the Ruby side in
case we decide we are so purist that we cannot live with the current
API. :-) So nothing to fix in the libstorage (C++) side.

Query interface
===============

The API contains methods like these:

device_graph.find_device(numeric_id)
Storage::BlkDevice.find(device_graph, "/dev/sda")
Storage::Filesystem.find_by_mountpoint(device_graph, "/")
Storage::Filesystem.find_by_label(device_graph, "someLabel")

Questions that come to my mind.

a) Is device_graph always an object representing the whole
graph?

The devicegraph always represents the whole graph.

Can I do things like this to restrict the search to a subtree?

disk1 = Storage::Disk.find(device_graph, "/dev/sda")
sub_graph = disk1.to_device_graph
# or maybe sub_graph = device_graph.sub_graph(disk1.sid)
# or any other thing that makes sense

In theory there are many ways to get a subgraph, look at the
functions in Device.h.

Storage::Filesystem.find_by_mountpoint(sub_graph, "/")

Do you have a real use-case for that? Adding subgraphs looks like
a lot of work so might not be worthwhile.

No concrete example, just wondering. But the idea of restricting the
search to a subtree somehow came naturally to my mind while reading.
Like "look for a filesystem with this property, but only in this volume
group".

Probably not worth the effort, I was just curious.

b) Adding find_by_xx at demand over time doesn't look very future-proof.
I would prefer to see something like Rails's #find_by, i.e.

Storage::Filesystem.find(device_graph, mountpoint: "/")
Storage::Filesystem.find(device_graph, mountpoint: "/", label: "blah")

Too ruby-centric and not easy to implement in a cross-language way, I
guess. But asking does not hurt. :-)

As I already wrote in
http://lists.opensuse.org/yast-devel/2014-12/msg00028.html if
someone takes care of bindings for a target language more things
are possible.

Yes. And it should be fairly easy to implement. But I still wonder about
how flexible and future-proof is to keep adding find_by_xx on demand to
the API.

More flexible solutions would also be more complex to use for sure. I
just wonder if they are worth exploring. One obvious solution (that
would need refinement, of course) if having a search object that accepts
any number of key-pair filters. The most primitive form just to expose
the idea would be something like (in pseudocode):

s = Storage.search(device_graph, "filesystem")
s.add_filter("mountpoint", "/")
s.first()

Or any other idea that can be implemented with a fixed set of classes
and methods instead of extending the existing classes with new custom
methods over time.

c) Just brainstorming. The approach in (b) could be pushed to the limit
to have things like this. But actually I'm not sure if it's a good idea,
just braindump.

device_graph.find(sid: numeric_id)
device_graph.find(type: :filesystem, mountpoint: "/")

Lack of inline documentation
============================

Right now, the code is the documentation. I have to say that the code is
very nicely structured, so it was really easy to me to browse for the
methods I wanted to check (and they were all fairly small and
understandable as well). But it's time to start adding documentation to
the whole thing.

It's a prototype and development was halted abrupt eight months
ago.

Nitpicking: boolean as parameters
=================================

This is something that was already mentioned by Martin in one of the
mails referred at the top. Calls like this are not 100% obvious

sda.descendants(false)

It would be nicer to have something like sda.descendants(:direct) or
sda.descendants(direct: true)

Looks difficult as already mentioned in
http://lists.opensuse.org/yast-devel/2014-12/msg00025.html

Ok.


[...]


Cheers.
--
Ancor González Sosa
YaST Team at SUSE Linux GmbH
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