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Re: [Qemu-devel] [PATCH v3 2/3] Add migrate_incoming

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v3 2/3] Add migrate_incoming
Date: Fri, 20 Feb 2015 08:39:48 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

On 02/20/2015 01:18 AM, Markus Armbruster wrote:
> I'd like Eric's opinion on on encoding configuration tuples as URIs
> rather than JSON in QMP.

>> +{ 'command': 'migrate-incoming', 'data': {'uri': 'str' } }
>> +
>>  # @xen-save-devices-state:
>>  #
>>  # Save the state of all devices to file. The RAM and the block devices
> Eric, what's your take on this?
> The general rule in QMP is "no ad hoc encoding of tuples in strings, use
> JSON objects instead".

Yes, it would be nice to be type-safe, and have a QAPI union with a
discriminator of different URI types along with appropriate accompanying
data.  But it is also new code (both in qemu to parse it, and in libvirt
to generate it), and we already have existing code that knows how to
generate the string-encoded tuple for the -incoming command line
argument that can be reused as-is with the new QMP command as proposed.
 So even though it is less safe, I'm okay with this particular command
using an overloaded string argument.

Down the road, if we WANT to add type-safety, we can make this command
take an anonymous union, where a 'str' value is the compact old style,
and where a dictionary value is the new discriminated union style.  So
we aren't completely locked into non-type-safe string forever, but we
probably won't add a type-safe variant until (if) we ever add a new
migration format that just looks too ugly as a URI based on the
parameters it requires.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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