[Engine-devel] [vdsm] [RFC] New Connection Management API

Thu Jan 26 18:58:40 UTC 2012

On Thu, Jan 26, 2012 at 10:00:57AM -0500, Saggi Mizrahi wrote:
> <snip>
> Again trying to sum up and address all comments
> 
> Clear all:
> ==========
> My opinions is still to not implement it.
> Even though it might generate a bit more traffic premature optimization is bad and there are other reasons we can improve VDSM command overhead without doing this.
> 
> In any case this argument is redundant because my intention is (as Litke pointed out) is to have a lean API.
> and API call is something you have to support across versions, this call implemented in the engine is something that no one has to support and can change\evolve easily.
> 
> As a rule, if an API call C and be implemented by doing A + B then C is redundant.
> 
> List of connections as args:
> ============================
> Sorry I forgot to respond about that. I'm not as strongly opposed to the idea as the other things you suggested. It'll just make implementing the persistence logic in VDSM significantly more complicated as I will have to commit multiple connection information to disk in an all or nothing mode. I can create a small sqlitedb to do that or do some directory tricks and exploit FS rename atomicity but I'd rather not.

I would be strongly opposed to introducing a sqlite database into vdsm just to
enable "convenience mode" for this API.  Does the operation really need to be
atomic?  Why not just perform each connection sequentially and return a list of
statuses?  Is the only motivation for allowing a list of parameters to reduce
the number of API calls between engine and vdsm)?  If so, the same argument
Saggi makes above applies here.

> The demands are not without base. I would like to keep the code simple under the hood in the price of a few more calls. You would like to make less calls and keep the code simpler on your side. There isn't a real way to settle this.
> If anyone on the list as pros and cons for either way I'd be happy to hear them.
> If no compelling arguments arise I will let Ayal call this one.
> 
> Transient connections:
> ======================
> The problem you are describing as I understand it is that VDSM did not respond and not that the API client did not respond.
> Again, this can happen for a number of reason, most of which VDSM might not be aware that there is actually a problem (network issues).
> 
> This relates to the EOL policy. I agree we have to find a good way to define an automatic EOL for resources. I have made my suggestion. Out of the scope of the API.
> 
> In the meantime cleaning stale connections is trivial and I have made it clear a previous email about how to go about it in a simple non intrusive way. Clean hosts on host connect, and on every poll if you find connections that you don't like. This should keep things squeaky clean.
> 
> ----- Original Message -----
> > From: "Livnat Peer" <lpeer at redhat.com>
> > To: "Saggi Mizrahi" <smizrahi at redhat.com>
> > Cc: vdsm-devel at lists.fedorahosted.org, engine-devel at ovirt.org
> > Sent: Thursday, January 26, 2012 5:22:42 AM
> > Subject: Re: [Engine-devel] [RFC] New Connection Management API
> > 
> > On 25/01/12 23:35, Saggi Mizrahi wrote:
> > > <SNIP>
> > > This is mail was getting way too long.
> > > 
> > > About the clear all verb.
> > > No.
> > > Just loop, find the connections YOU OWN and clean them. Even though
> > > you don't want to support multiple clients to VDSM API doesn't
> > > mean the engine shouldn't behave like a proper citizen.
> > > It's the same reason why VDSM tries and not mess system resources
> > > it didn't initiate.
> > 
> > 
> > There is a big difference, VDSM living in hybrid mode with other
> > workload on the host is a valid use case, having more than one
> > concurrent manager for VDSM is not.
> > Generating a disconnect request for each connection does not seem
> > like
> > the right API to me, again think on the simple flow of moving host
> > from
> > one data center to another, the engine needs to disconnect tall
> > storage
> > domains (each domain can have couple of connections associated with
> > it).
> > 
> > I am giving example from the engine use cases as it is the main user
> > of
> > VDSM ATM but I am sure it will be relevant to any other user of VDSM.
> > 
> > > 
> > > ------------------------
> > > 
> > > As I see it the only point of conflict is the so called
> > > non-peristed connections.
> > > I will call them transient connections from now on.
> > > 
> > > There are 2 user cases being discussed
> > > 1. Wait until a connection is made, if it fails don't retry and
> > > automatically unmanage.
> > > 2. If the called of the API forgets or fails to unmanage a
> > > connection.
> > > 
> > 
> > Actually I was not discussing #2 at all.
> > 
> > > Your suggestion as I understand it:
> > > Transient connections are:
> > >      - Connection that VDSM will only try to connect to once and
> > >      will not reconnect to in case of disconnect.
> > 
> > yes
> > 
> > > 
> > > My problem with this definition that it does not specify the "end
> > > of life" of the connection.
> > > Meaning it solves only use case 1.
> > 
> > since this is the only use case i had in mind, it is what i was
> > looking for.
> > 
> > > If all is well, and it usually is, VDSM will not invoke a
> > > disconnect.
> > > So the caller would have to call unmanage if the connection
> > > succeeded at the end of the flow.
> > 
> > agree.
> > 
> > > Now, if you are already calling unmanage if connection succeeded
> > > you can just call it anyway.
> > 
> > not exactly, an example I gave earlier on the thread was that VSDM
> > hangs
> > or have other error and the engine can not initiate unmanaged,
> > instead
> > let's assume the host is fenced (self-fence or external fence does
> > not
> > matter), in this scenario the engine will not issue unmanage.
> > 
> > > 
> > > instead of doing: (with your suggestion)
> > > ----------------
> > > manage
> > > wait until succeeds or lastError has value
> > > try:
> > >   do stuff
> > > finally:
> > >   unmanage
> > > 
> > > do: (with the canonical flow)
> > > ---
> > > manage
> > > try:
> > >   wait until succeeds or lastError has value
> > >   do stuff
> > > finally:
> > >   unmanage
> > > 
> > > This is simpler to do than having another connection type.
> > 
> > You are assuming the engine can communicate with VDSM and there are
> > scenarios where it is not feasible.
> > 
> > > 
> > > Now that we got that out of the way lets talk about the 2nd use
> > > case.
> > 
> > Since I did not ask VDSM to clean after the (engine) user and you
> > don't
> > want to do it I am not sure we need to discuss this.
> > 
> > If you insist we can start the discussion on who should implement the
> > cleanup mechanism but I'm afraid I have no strong arguments for VDSM
> > to
> > do it, so I rather not go there ;)
> > 
> > 
> > You dropped from the discussion my request for supporting list of
> > connections for manage and unmanage verbs.
> > 
> > > API client died in the middle of the operation and unmanage was
> > > never called.
> > > 
> > > Your suggested definition means that unless there was a problem
> > > with the connection VDSM will still have this connection active.
> > > The engine will have to clean it anyway.
> > > 
> > > The problem is, VDSM has no way of knowing that a client died,
> > > forgot or is thinking really hard and will continue on in about 2
> > > minutes.
> > 
> > > 
> > > Connections that live until they die is a hard to define and work
> > > with lifecycle. Solving this problem is theoretically simple.
> > > 
> > > Have clients hold some sort of session token and force the client
> > > to update it at a specified interval. You could bind resources
> > > (like domains, VMs, connections) to that session token so when it
> > > expires VDSM auto cleans the resources.
> > > 
> > > This kind of mechanism is out of the scope of this API change.
> > > Further more I think that this mechanism should sit in the engine
> > > since the session might actually contain resources from multiple
> > > hosts and resources that are not managed by VDSM.
> > > 
> > > In GUI flows specifically the user might do actions that don't even
> > > touch the engine and forcing it to refresh the engine token is
> > > simpler then having it refresh the VDSM token.
> > > 
> > > I understand that engine currently has no way of tracking a user
> > > session. This, as I said, is also true in the case of VDSM. We can
> > > start and argue about which project should implement the session
> > > semantics. But as I see it it's not relevant to the connection
> > > management API.
> > 
> > 
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-- 
Adam Litke <agl at us.ibm.com>
IBM Linux Technology Center