Re: [Netconf] Is there a problem with confirmed commits?

Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de> Mon, 14 January 2019 15:40 UTC

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Date: Mon, 14 Jan 2019 16:40:26 +0100
From: Juergen Schoenwaelder <j.schoenwaelder@jacobs-university.de>
To: Jonathan Hansford <jonathan@hansfords.net>
CC: "netconf@ietf.org" <netconf@ietf.org>
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Subject: Re: [Netconf] Is there a problem with confirmed commits?
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It seems the <candidate> datastore should not be allowed to be used as
long as a persistent confirmed commit is still ongoing. I leave it to
Martin to check whether this is said somewhere or an omission.

In general, an application can't assume that <candidate> contains
anything sensible. Hence, the proper way is to lock <candidate> and
then to make sure it contains something sensible, i.e., issuing a
discard_changes. And I think implementations should not allow an
application to obtain a lock on <candidate> while a commit is active.
The text on page 45 already says:

      A lock MUST NOT be granted if any of the following conditions is
      true:

      [...]

      *  The target configuration is <candidate>, it has already been
         modified, and these changes have not been committed or rolled
         back.

I think this covers the case of an ongoing but not completed
persistent confirmed commit, no?

/js

On Mon, Jan 14, 2019 at 03:14:02PM +0000, Jonathan Hansford wrote:
> If a persistent confirmed commit has not timed out, the running
> configuration datastore will be the same as the candidate and
> <discard-changes> won't change its contents. Any edit of candidate will be
> based on the configuration resulting from the persistent confirmed commit.
> 
> If the persistent confirmed commit has timed out, the running configuration
> datastore will have reverted and <discard-changes> will change candidate.
> Any edit of candidate in this case will be based on the configuration prior
> to the start of the persistent confirmed commit.
> 
> ------ Original Message ------
> From: "Juergen Schoenwaelder" <j.schoenwaelder@jacobs-university.de>;
> To: "Jonathan Hansford" <jonathan@hansfords.net>;
> Cc: "netconf@ietf.org"; <netconf@ietf.org>;
> Sent: 14/01/2019 13:50:56
> Subject: Re: [Netconf] Is there a problem with confirmed commits?
> 
> > Hi,
> > 
> > I have not yet understood where you see a problem. In general,
> > <candidate/> contains arbitrary stuff and hence it is the client's
> > responsibility to clear any arbitrary stuff found in <candidate/>
> > after obtaining a lock. If does not really matter whether there has
> > been a failed confirmed commit before or something else. I think the
> > general safe pattern is:
> > 
> > lock(candidate)
> > discard_changes()
> > push_whatever_needed()
> > commit()
> > unlock(candidate)
> > 
> > If you do a confirmed commit and the session disappears, then the lock
> > will disappear as well. But I do not think this creates a race
> > condition, or I am just not yet seeing it. Perhaps it helps to write
> > down the sequence of actions that leads to a race.
> > 
> > /js
> > 
> > On Mon, Jan 14, 2019 at 12:50:38PM +0000, Jonathan Hansford wrote:
> > >  Hi,
> > > 
> > >  No one seems to be responding to my email and proposed erratum around
> > >  the subject of confirmed commits (apart from Martin), but I would really
> > >  like to know it I am missing something here. As far as I can tell,
> > >  session termination during a confirmed commit leads to unpredictable
> > >  behaviour and I would like to know whether anyone is using confirmed
> > >  commits and how (if at all) they address the issues outlined below. My
> > >  assumptions are that locks are used and :writable-running is not
> > >  supported.
> > > 
> > >  If the <candidate> and <running> configuration datastores are locked to
> > >  prevent concurrent access, and a confirmed commit sequence is
> > >  interrupted by the session terminating, the locks will automatically be
> > >  released but the server MUST NOT accept a lock on <running> from any
> > >  session if another session has an ongoing confirmed <commit>.
> > >  Consequently, after session termination no client can acquire a <lock>
> > >  on <running>, not even the one that initiated the confirmed <commit>,
> > >  until after the confirmed <commit> has timed out. However, if the
> > >  confirmed <commit> included the <persist> parameter, the original client
> > >  could still issue a <commit> using the persist-id to complete the
> > >  sequence prior to the timeout, even without a lock.
> > > 
> > >  Of course, the problem now is the race for the new lock on <candidate>.
> > >  If the original client is successful then all is good. But if a new
> > >  client locks <candidate> before the timeout on the confirmed commit,
> > >  whether or not they precede <lock> with <discard-changes>, <candidate>
> > >  will be the same as <running> and the new client will pick up everything
> > >  from the previous session. However, the client won’t be able to lock
> > >  <running> until after the timeout, at which point <running> reverts but
> > >  <candidate> still represents the previous session. If the client tries
> > >  to lock <candidate> after the timeout, <running> will have reverted and
> > >  the lock will only be granted after a <discard-changes> which will cause
> > >  the <candidate> to revert. So, depending on when the lock on <candidate>
> > >  occurs relative to the confirmed commit timeout, the client could be
> > >  editing <candidate> in one of two states. Further, before the timeout on
> > >  the confirmed commit, even if the new client has locked candidate, the
> > >  original client could still issue a confirming commit (they don’t need a
> > >  lock on <candidate> to do so) which would persistently commit any edits
> > >  made by the new client. NOTE: it is not the use of the persist-id that
> > >  introduces this behaviour; a new client would have the same problem even
> > >  if a confirmed commit was not intended to persist beyond a session
> > >  termination.
> > > 
> > >  If the server also supports the :startup capability then, if the session
> > >  termination was due to the server rebooting, the behaviour above would
> > >  be further complicated by <running> now containing the configuration
> > >  from the <startup> configuration datastore.
> > > 
> > >  Am I right?
> > > 
> > >  Jonathan
> > > 
> > >  ---
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> > 
> > >  _______________________________________________
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> > 
> > 
> > --
> > Juergen Schoenwaelder           Jacobs University Bremen gGmbH
> > Phone: +49 421 200 3587         Campus Ring 1 | 28759 Bremen | Germany
> > Fax:   +49 421 200 3103         <https://www.jacobs-university.de/>

-- 
Juergen Schoenwaelder           Jacobs University Bremen gGmbH
Phone: +49 421 200 3587         Campus Ring 1 | 28759 Bremen | Germany
Fax:   +49 421 200 3103         <https://www.jacobs-university.de/>