[Engine-devel] Managing async tasks

[Saggi Mizrahi]

This is an addendum to my previous email.

----- Original Message -----
> From: "Saggi Mizrahi" <smizrahi at redhat.com>
> To: "Adam Litke" <agl at us.ibm.com>
> Cc: "Dan Kenigsberg" <danken at redhat.com>, "Ayal Baron" <abaron at redhat.com>, "Federico Simoncelli"
> <fsimonce at redhat.com>, engine-devel at ovirt.org, vdsm-devel at lists.fedorahosted.org
> Sent: Monday, December 17, 2012 2:52:06 PM
> Subject: Re: Managing async tasks
> 
> 
> 
> ----- Original Message -----
> > From: "Adam Litke" <agl at us.ibm.com>
> > To: "Saggi Mizrahi" <smizrahi at redhat.com>
> > Cc: "Dan Kenigsberg" <danken at redhat.com>, "Ayal Baron"
> > <abaron at redhat.com>, "Federico Simoncelli"
> > <fsimonce at redhat.com>, engine-devel at ovirt.org,
> > vdsm-devel at 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 at us.ibm.com> To:
> > > > vdsm-devel at lists.fedorahosted.org
> > > > Cc: "Dan Kenigsberg" <danken at redhat.com>, "Ayal Baron"
> > > > <abaron at redhat.com>,
> > > > "Saggi Mizrahi" <smizrahi at redhat.com>, "Federico Simoncelli"
> > > > <fsimonce at redhat.com>, engine-devel at 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 at us.ibm.com>
> > IBM Linux Technology Center
> > 
> > 
>