Re: [Qemu-devel] [PATCH v2 00/25] qmp: add async command type

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Mon, Jan 23, 2017 at 06:27:29AM -0500, Marc-André Lureau wrote:
> ----- Original Message -----
> > On Wed, Jan 18, 2017 at 08:03:07PM +0400, Marc-André Lureau wrote:
> > > Hi,
> > 
> > CCing Jeff Cody and John Snow, who have been working on generalizing
> > Block Job APIs to generic background jobs.  There is some overlap
> > between async commands and background jobs.
> 
> If you say so :) Did I miss a proposal or a discussion for async qmp commands?

There is no recent mailing list thread, so it's probably best to discuss
here:

The goal of jobs is to support long-running operations that can be
managed via QMP.  Jobs can have a more elaborate lifecycle than just
start -> finish/cancel (e.g. they can be paused/resumed and may have
multiple phases of execution that the client controls).  There are QMP
APIs to query their state (Are they running?  How much "progress" has
been made?).

A client reconnecting to QEMU can query running jobs.  This way a client
can resume with a running QEMU process.  For commands like saving a
screenshot is mostly does not matter, but for commands that modify state
it's critical that clients are aware of running commands after reconnect
to prevent corruption/interference.  This behavior is what I asked about
in my previous mail.

Jobs are currently only used by the block layer and called "block jobs",
but the idea is to generalize this.  They use synchronous QMP + events.

Jobs are more heavy-weight than async QMP commands, but pause/resume,
rate-limiting, progress reporting, robust reconnect, etc are important
features.  Users want to be aware of long-running operations and have
the ability to control them.

I suspect that if we transition synchronous QMP commands to async we'll
soon have requirements for progress reporting, pause/resume, etc.  So is
there a set of commands that should be async and others that should be
jobs or should everything just be a job?

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