2:12 p.m.
This is an addendum to my previous email.
----- Original Message -----
From: "Saggi Mizrahi" <smizrahi(a)redhat.com>
To: "Adam Litke" <agl(a)us.ibm.com>
Cc: "Dan Kenigsberg" <danken(a)redhat.com>, "Ayal Baron"
<abaron(a)redhat.com>, "Federico Simoncelli"
<fsimonce(a)redhat.com>, engine-devel(a)ovirt.org, vdsm-devel(a)lists.fedorahosted.org
Sent: Monday, December 17, 2012 2:52:06 PM
Subject: Re: Managing async tasks
----- Original Message -----
> From: "Adam Litke" <agl(a)us.ibm.com>
> To: "Saggi Mizrahi" <smizrahi(a)redhat.com>
> Cc: "Dan Kenigsberg" <danken(a)redhat.com>, "Ayal Baron"
> <abaron(a)redhat.com>, "Federico Simoncelli"
> <fsimonce(a)redhat.com>, engine-devel(a)ovirt.org,
> vdsm-devel(a)lists.fedorahosted.org
> Sent: Monday, December 17, 2012 2:16:25 PM
> Subject: Re: Managing async tasks
>
> On Mon, Dec 17, 2012 at 12:15:08PM -0500, Saggi Mizrahi wrote:
> >
> >
> > ----- Original Message -----
> > > From: "Adam Litke" <agl(a)us.ibm.com> To:
> > > vdsm-devel(a)lists.fedorahosted.org
> > > Cc: "Dan Kenigsberg" <danken(a)redhat.com>, "Ayal
Baron"
> > > <abaron(a)redhat.com>,
> > > "Saggi Mizrahi" <smizrahi(a)redhat.com>, "Federico
Simoncelli"
> > > <fsimonce(a)redhat.com>, engine-devel(a)ovirt.org Sent: Monday,
> > > December 17,
> > > 2012 12:00:49 PM Subject: Managing async tasks
> > >
> > > On today's vdsm call we had a lively discussion around how
> > > asynchronous
> > > operations should be handled in the future. In an effort to
> > > include more
> > > people in the discussion and to better capture the resulting
> > > conversation I
> > > would like to continue that discussion here on the mailing
> > > list.
> > >
> > > A lot of ideas were thrown around about how 'tasks' should be
> > > handled in the
> > > future. There are a lot of ways that it can be done. To
> > > determine how we
> > > should implement it, it's probably best if we start with a set
> > > of
> > > requirements. If we can first agree on these, it should be
> > > easy
> > > to find a
> > > solution that meets them. I'll take a stab at identifying a
> > > first set of
> > > POSSIBLE requirements:
> > >
> > > - Standardized method for determining the result of an
> > > operation
> > >
> > > This is a big one for me because it directly affects the
> > > consumability of
> > > the API. If each verb has different semantics for
> > > discovering
> > > whether it
> > > has completed successfully, then the API will be nearly
> > > impossible to use
> > > easily.
> > Since there is no way to assure if of some tasks completed
> > successfully or
> > failed, especially around the murky waters of storage, I say this
> > requirement
> > should be removed. At least not in the context of a task.
>
> I don't agree. Please feel free to convince me with some exampled.
> If we
> cannot provide feedback to a user as to whether their request has
> been satisfied
> or not, then we have some bigger problems to solve.
If VDSM sends a write command to a storage server, and the connection
hangs up before the ACK has returned.
The operation has been committed but VDSM has no way of knowing if
that happened as far as VDSM is concerned it got an ETIMEO or EIO.
This is the same problem that the engine has with VDSM.
If VDSM creates an image\VM\network\repo but the connection hangs up
before the response can be sent back as far as the engine is
concerned the operation times out.
This is an inherent issue with clustering.
This is why I want to move away from tasks being *the* trackable
objects.
Tasks should be short. As short as possible.
Run VM should just persist the VM information on the VDSM host and
return. The rest of the tracking should be done using the VM ID.
Create image should return once VDSM persisted the information about
the request on the repository and created the metadata files.
Tracking should be done on the repo or the imageId.
The thing is that I know how long a VM object should live (or an Image object).
So tracking it is straight forward. How long a task should live is very problematic and
quite context specific.
It depends on what the task is.
I think it's quite confusing from an API standpoint to have every task have a
different scope, id requirement and life-cycle.
In VDSM has two types of APIs
CRUD objects - VM, Image, Repository, Bridge, Storage Connections....
General transient methods - getBiosInfo(), getDeviceList()
The latter are quite simple to manage. They don't need any special handling. If you
lost a getBiosInfo() call you just send another one, no harm done.
The same is even true with things that "change" the host like getDeviceList()
What we are really arguing about is fitting the CRUD objects to some generic task oriented
scheme.
I'm saying it's a waste of time as you can quite easily have flows to recover from
each operation.
Create - Check if the object exists
Read - Read again
Update - either update again or read and update if update didn't commit the first
time
Delete - Check if object doesn't exist
Each of the objects we CRUD have different life-cycles and ownership semantics.
Danken raised the point that creation has a problem that if it fails there is no way to
get why it failed.
This is why Create method should be minimal. They shouldn't create the object just the
entry in the respective persistent storage.
Even now storage connections are persisted to disk and then the operation returns and the
user polls to see the state of the connection.
The same should be done for everything. Do the minimum required to create the object entry
and mark it as "not usable".
For storage connections it's "connecting"
For VMs it's "preparing for launch"
For new images it's "broken" and in some regards "degraded"
I hope this makes things clearer
>
> > >
> > >
> > > Sorry. That's my list :) Hopefully others will be willing to
> > > add other
> > > requirements for consideration.
> > >
> > > From my understanding, task recovery (stop, abort, rollback,
> > > etc)
> > > will not
> > > be generally supported and should not be a requirement.
> > >
>
> --
> Adam Litke <agl(a)us.ibm.com>
> IBM Linux Technology Center
>
>