Re: [Qemu-devel] ANN: QEMU Monitor Protocol git tree

[Anthony Liguori]

Luiz Capitulino wrote:
>  Hi there,
>
> While I was converting command handlers to the QObject style I realized that
> it would be very interesting to have the protocol working up front, so that
> converted handlers could be tested to assure that their data types have been
> chosen correctly.
>
> More importantly, some protocol design decisions can affect handlers signatures,
> so having at least an idea of how the protocol will look like will help us to
> stay on the right track during mass conversion.
>
> So, I have implemented a rudimentary version of the QEMU Monitor Protocol (QMP)
> it has a command-line switch to enable the protocol.
>
> You will find it at:
>
> http://repo.or.cz/w/qemu/qmp-unstable.git
>   

I think this is a pretty reasonable approach to take since qmp is 
relatively isolated from the changes going on in the rest of the tree.

> Now the controversial part: it's json based. ;)
>
> I have chosen json because of the reasons already explained by others in
> the original QMP thread. Basically, json is so simple that if we design
> a small protocol from scratch, chances are it will look like json.
>   

json is not a deal break.  My main concern was our ability to extend 
json and whether supporting stock json libraries was a hard 
requirement.  I also would like to see a C client library since our 
biggest consumer (libvirt) is based in C.

But the wire protocol is the thing I care the least about.  I care more 
about the internal refactoring.

> """
>            QEMU Monitor Protocol Draft Specification - Version 0.1
>
> 1. Introduction
> ===============
>
> This document specifies the QEMU Monitor Protocol (QMP), a JSON-based protocol
> which is available for applications to control QEMU at the machine-level.
>
> To enable QMP support, QEMU has to be run in "control mode". This is done by
> starting QEMU with the appropriate command-line options. Please, refer to the
> QEMU manual page for more information.
>
> This specification is a work in progress and part of it may NOT be available
> in QEMU, the 'Current Implementation' section has details regarding what has
> already been implemented and known problems.
>
> 2. Protocol Specification
> =========================
>
> This section details the protocol format. For the purpose of this document
> "Client" is any application which is communicating with QEMU in control mode,
> and "Server" is QEMU itself.
>
> JSON data structures, when mentioned in this document, are always in the
> following format:
>
>     json-DATA-STRUCTURE-NAME
>
> Where DATA-STRUCTURE-NAME is any valid JSON data structure, as defined by
> the JSON standard:
>
> http://www.ietf.org/rfc/rfc4627.txt
>
> For convenience, json-objects mentioned in this document will have its members
> in a certain order. However, in real protocol usage json-objects members can
> be in ANY order, thus no particular order should be assumed.
>
> 2.1 General Definitions
> -----------------------
>
> All interactions transmitted by Client and Server are json-objects that end
> with CRLF.
>   

CRLF?  Really?

Ignoring the dos-ism, since you can parse JSON with a regexp, why do we 
need explicit message boundaries?

>  Where,
>
> - The "QEMU" member contains the QEMU version
> - The "QMP" member contains the QMP version
> - The "capabilities" member specify the availability of features beyond the
>   baseline specification
>
>   
Version and capability are a bit redundant, no?  Any reason to have 
QEMU's version in the initial hello too?

Regards,

Anthony Liguori